IGCSE Chemistry

Everything around you is made of tiny particles 粒子 — these can be atoms 原子, molecules 分子, or ions 离子. The kinetic particle theory 粒子动理论 says these particles are always moving. How close the particles are, how they are arranged, and how they move decides whether matter is a solid 固体, a liquid 液体, or a gas 气体.

Properties you can observe

You do not need a microscope to tell the three states apart. They behave in different ways:

• A solid has a fixed shape and a fixed volume 体积. It does not flow and you cannot compress 压缩 it (squeeze it smaller).
• A liquid has a fixed volume but no fixed shape. It flows and takes the shape of its container. It is almost impossible to compress.
• A gas has no fixed shape and no fixed volume. It flows and spreads out to fill the whole container. A gas is easy to compress.

The table below sums up these properties. Density 密度 means how much mass 质量 is packed into a given volume.

The particle picture

The kinetic particle theory explains these properties by looking at three things: the separation 间距 of the particles (how far apart they are), their arrangement 排列 (the pattern), and their motion 运动 (how they move).

Strong forces of attraction 引力 hold the particles together. In a solid these forces are strong enough to hold every particle in place, so a solid keeps its shape. In a liquid the forces are weaker, so particles can move around. In a gas the particles move so fast that the forces hardly act at all, so the gas spreads out.

Particles are packed and regular in a solid, close and random in a liquid, and far apart in a gas

Ice melting to water: a change of state as the solid warms.

When you heat or cool a substance, it can change from one state to another. You must know the name of each change.

A pure solid melts at one fixed temperature, the melting point 熔点. A pure liquid boils at one fixed temperature, the boiling point 沸点. The same substance freezes at its melting point and condenses at its boiling point.

Heating: solid → liquid → gas; cooling reverses each change

A few substances change straight from solid to gas without melting first. This is called sublimation 升华. In the photo below, warmed solid iodine in a beaker turns directly into a purple gas; the gas then cools on the round flask of ice above and turns back into a solid.

Warmed solid iodine turns straight into a purple gas (sublimation)

Explaining changes of state with the particle theory

Each change of state is really a change in the energy 能量 of the particles.

• When you heat a solid, the particles gain energy and vibrate faster. At the melting point the particles have enough energy to break away from their fixed places and slide around — the solid melts.
• When you heat a liquid, the particles move faster. At the boiling point they have enough energy to fully escape the forces of attraction and become a gas.
• Cooling does the opposite. The particles lose energy, move more slowly, and the forces of attraction pull them back together.

During a change of state the energy goes into breaking the forces of attraction, not into making the particles move faster. This is why the temperature stays the same while a substance is melting or boiling.

Heating and cooling curves

A heating curve 加热曲线 is a graph of temperature against time as you heat a substance steadily. A cooling curve 冷却曲线 is the same graph as the substance cools.

On a heating curve there are two flat (level) parts:

• The first flat part is at the melting point. Here solid and liquid are both present. The heat energy breaks the forces holding the solid together, so the temperature does not rise.
• The second flat part is at the boiling point. Here liquid and gas are both present, and the temperature again stays constant.

A cooling curve is the reverse. It has a flat part at the boiling point (the gas condenses) and a flat part at the melting point (the liquid freezes). As the particles slow down, thermal energy 热能 is released to the surroundings.

Temperature stays constant at the melting and boiling points while forces of attraction are broken

A gas pushes on the walls of its container. This push, spread over the area of the wall, is the pressure 压强 of the gas. Pressure comes from the gas particles hitting the walls.

Effect of temperature

If you heat a fixed mass of gas while keeping the pressure the same, its volume increases.

Using the particle theory: heating gives the particles more kinetic energy 动能, so they move faster. They hit the walls harder and more often. To keep the pressure the same, the gas must take up more space, so the volume gets bigger.

If instead the volume is fixed (a sealed, rigid container), heating the gas makes the pressure rise, because the faster particles collide 碰撞 with the walls harder and more often.

Effect of pressure

If you increase the pressure on a fixed mass of gas while keeping the temperature the same, its volume decreases. Squeezing the gas into a smaller space means the particles hit the walls more often, which is what a higher pressure means.

Diffusion 扩散 is the spreading of particles from a region where they are crowded to a region where they are spread out — that is, from high concentration 浓度 to low concentration. It happens because particles are always moving in random directions.

Diffusion explains why you can smell food from across a room: the smell particles move and mix with the air until they reach your nose. Diffusion happens in gases and in liquids, but not in solids, because solid particles cannot move from place to place.

Rate of diffusion and molecular mass

Lighter gas particles move faster than heavier ones at the same temperature. So a gas with a smaller relative molecular mass 相对分子质量 (a smaller mass for each molecule) has a faster rate 速率 of diffusion.

A classic experiment shows this. Cotton wool soaked in ammonia 氨气 ($\text{NH}_3$) is put at one end of a long glass tube. Cotton wool soaked in hydrogen chloride 氯化氢 ($\text{HCl}$) is put at the other end. Both gases diffuse along the tube and meet to form a white ring of ammonium chloride 氯化铵 ($\text{NH}_4\text{Cl}$).

Ammonia has $M_r = 17$ and hydrogen chloride has $M_r = 36.5$. Ammonia is lighter, so it diffuses faster and travels further along the tube. The white ring forms nearer the hydrogen chloride end.

Ammonia ($M_r=17$) is lighter, so it diffuses faster and further; the white ring forms nearer the HCl end

A pure element (vanadium): elements are the simplest substances.

All substances are made from about 100 simple building blocks. Knowing how they are joined lets you sort every substance into one of three groups.

• An element 元素 is a substance made of only one type of atom 原子. You cannot break it into anything simpler by a chemical reaction. Examples: copper, oxygen, carbon.
• A compound 化合物 is two or more elements chemically joined (bonded) together. The atoms are joined in a fixed ratio. Examples: water, carbon dioxide. A compound has different properties from the elements in it.
• A mixture 混合物 is two or more substances that are just mixed, not chemically joined. The parts keep their own properties and can be separated by physical methods. Example: air.

The key difference: in a compound the elements are bonded and can only be separated by chemical reactions; in a mixture they are not bonded and are easy to separate.

An element has one type of atom; a compound has different atoms bonded in a fixed ratio; a mixture is not bonded

Inside the atom

Every atom has a small, dense centre called the nucleus 原子核. Around it, electrons 电子 move in shells 电子层 (energy levels). The nucleus contains two kinds of particle: protons 质子 and neutrons 中子.

An atom has a tiny nucleus of protons and neutrons, with electrons in shells around it

Each particle has a relative mass and a relative charge 电荷. You must learn these values:

An atom has no overall charge because it has equal numbers of protons ($+1$ each) and electrons ($-1$ each).

Proton number and mass number

Two numbers describe an atom:

• The proton number 质子数 (also called the atomic number 原子序数) is the number of protons in the nucleus. It tells you which element the atom is.
• The mass number 质量数 (also called the nucleon number 核子数) is the total number of protons and neutrons in the nucleus.

So the number of neutrons $=$ mass number $-$ proton number.

Electronic configuration

The electrons fill the shells from the inside out. The first shell holds up to 2 electrons; the next shells hold up to 8 each (for the first 20 elements). The electronic configuration 电子排布 lists how many electrons are in each shell, starting from the inside.

For example, an atom with 13 electrons has the configuration $2,8,3$. Sodium (proton number 11) is $2,8,1$. Calcium (proton number 20) is $2,8,8,2$.

The configuration links to the Periodic Table 周期表:

• A Group 族 number (Groups I to VII) equals the number of electrons in the outer shell. So $2,8,1$ is in Group I.
• A Period 周期 number equals the number of shells that hold electrons. So $2,8,1$ has three shells, so it is in Period 3.
• The noble gases 稀有气体 in Group VIII (or 0) have a full outer shell, which makes them very unreactive.

Isotopes 同位素 are atoms of the same element that have the same number of protons but different numbers of neutrons. Because the proton number is the same, they are the same element. Because the neutron number is different, they have different mass numbers.

You write an atom with its mass number on top and proton number below, like $^{12}_{6}\text{C}$. For an ion you add the charge, like $^{35}_{17}\text{Cl}^{-}$.

Isotopes of an element have the same chemical properties. This is because chemical reactions only involve electrons, and isotopes have the same number of electrons and the same electronic configuration. (The extra neutrons change only the mass.)

Calculating relative atomic mass

The relative atomic mass 相对原子质量 ($A_r$) of an element is the average mass of its atoms, taking into account how common each isotope is. The abundance 丰度 is the percentage of each isotope.

For example, chlorine is 75% $^{35}\text{Cl}$ and 25% $^{37}\text{Cl}$:

An ion 离子 is an atom (or group of atoms) that has lost or gained electrons, so it has an electric charge.

• A metal atom loses electrons to form a positive ion, called a cation 阳离子.
• A non-metal atom gains electrons to form a negative ion, called an anion 阴离子.

Atoms do this to get a full outer shell, like a noble gas.

How an ionic bond forms

An ionic bond 离子键 is a strong electrostatic attraction 静电引力 between oppositely charged ions (a $+$ ion and a $-$ ion pull together).

Ionic bonds form between a metal 金属 and a non-metal 非金属. Take sodium chloride, $\text{NaCl}$. Sodium ($2,8,1$) gives its one outer electron to chlorine ($2,8,7$). Now sodium is $\text{Na}^{+}$ ($2,8$) and chlorine is $\text{Cl}^{-}$ ($2,8,8$). Both have full outer shells, and the opposite charges attract.

You can show this with a dot-and-cross diagram: draw each atom's outer-shell electrons as dots for one element and crosses for the other, then show the electron moving from the metal to the non-metal.

Sodium gives its outer electron to chlorine; both reach full outer shells and the opposite charges attract (the blue electron came from sodium)

The structure and properties of ionic compounds

An ionic compound 离子化合物 is not made of separate molecules 分子. The ions pack together into a giant lattice 晶格 — a regular pattern of huge numbers of alternating 交替 positive and negative ions.

Ions pack into a giant lattice: a regular, repeating pattern of alternating positive and negative ions

This structure explains the properties:

A model of a water molecule: atoms share electrons in covalent bonds.

A covalent bond 共价键 forms when two atoms share a pair of electrons. By sharing, each atom gets a full outer shell (a noble gas configuration). Covalent bonds form between non-metal atoms.

Some molecules to know:

• $\text{H}_2$ — two hydrogen atoms share one pair of electrons (a single bond).
• $\text{Cl}_2$, $\text{HCl}$ — one shared pair each.
• $\text{H}_2\text{O}$ — oxygen shares one pair with each of two hydrogen atoms.
• $\text{NH}_3$ — nitrogen shares a pair with each of three hydrogen atoms.
• $\text{CH}_4$ — carbon shares a pair with each of four hydrogen atoms.
• $\text{O}_2$ and $\text{CO}_2$ have double bonds (two shared pairs); $\text{N}_2$ has a triple bond (three shared pairs); $\text{C}_2\text{H}_4$ and $\text{CH}_3\text{OH}$ also use shared pairs.

In a dot-and-cross diagram for a molecule, you draw the outer electrons of each atom and show which pairs are shared in the overlap between the atoms.

In a covalent bond, atoms share pairs of electrons so each reaches a full outer shell

Properties of simple molecular compounds

These substances are made of small, separate molecules.

• They have low melting points and boiling points. The covalent bonds inside each molecule are strong, but the intermolecular forces 分子间作用力 (the forces between one molecule and the next) are weak, so little energy is needed to separate the molecules.
• They are poor conductors of electricity, because the molecules have no overall charge and no free electrons or ions to carry charge.

Some covalent substances are not small molecules. Instead, millions of atoms are joined by covalent bonds into one giant covalent structure 巨型共价结构. The two you must know are both forms of carbon.

Diamond 金刚石: each carbon atom is bonded to four other carbon atoms. This makes a very strong, rigid 3-D network. Diamond is extremely hard, so it is used in cutting tools 切割工具.

Graphite 石墨: each carbon atom is bonded to only three others, forming flat layers 层. There are weak forces between the layers, so the layers can slide over each other — this makes graphite a good lubricant 润滑剂. The fourth outer electron of each carbon is free; these delocalised electrons 离域电子 can move, so graphite conducts electricity and is used as an electrode 电极.

Diamond bonds each carbon to four others (hard); graphite forms flat layers with weak forces between them (slippery)

Silicon(IV) oxide 二氧化硅 ($\text{SiO}_2$) has a giant covalent structure like diamond, so it is also very hard and has a very high melting point.

A metal is a giant structure of positive ions surrounded by a 'sea' of delocalised electrons that are free to move through the whole metal. Metallic bonding 金属键 is the strong electrostatic attraction between these positive ions and the sea of electrons.

A metal is positive ions in a 'sea' of delocalised electrons that are free to move and carry charge

This explains two key properties of metals:

• Good electrical conductivity: the delocalised electrons are free to move and carry charge through the metal.
• Malleability 展性 (can be hammered into sheets) and ductility 延性 (can be pulled into wires): the layers of positive ions can slide over each other without breaking the metallic bond, so the metal changes shape instead of shattering.

A formula 化学式 shows which atoms 原子 are in a substance, and how many of each. There are two kinds of formula you must know.

• The molecular formula 分子式 is the actual number of each atom in one molecule 分子. For glucose it is $\text{C}_6\text{H}_{12}\text{O}_6$.
• The empirical formula 实验式 is the simplest whole-number ratio 比例 of the atoms or ions 离子 in a compound 化合物. For glucose it is $\text{CH}_2\text{O}$.

Working out a formula

If you are given a model or diagram, just count the atoms of each element and write them as a formula.

For an ionic compound 离子化合物 you can work out the formula from the charges on the ions. The total positive charge must balance the total negative charge, because the compound has no overall charge. Some common ions:

For example, $\text{Na}^{+}$ and $\text{O}^{2-}$: you need two $\text{Na}^{+}$ to balance one $\text{O}^{2-}$, so the formula is $\text{Na}_2\text{O}$.

For an ionic compound, the ion charges cross over to give the formula (here $\text{Al}_2\text{O}_3$)

An equation shows how reactants 反应物 (the starting substances) change into products 生成物 (the substances made).

• A word equation 文字方程式 uses names: magnesium + oxygen → magnesium oxide.
• A symbol equation 化学方程式 uses formulae and must be balanced 配平 — the same number of each atom on both sides.

A balanced symbol equation has the same number of each atom on both sides

You add state symbols 状态符号 in brackets to show the physical state: $(s)$ solid, $(l)$ liquid, $(g)$ gas, and $(aq)$ for aqueous 水溶液 (dissolved in water).

Ionic equations

An ionic equation 离子方程式 shows only the ions that actually change. Ions that are the same on both sides are spectator ions 旁观离子 and are left out. For example, when an acid reacts with an alkali:

A laboratory balance measures mass — the basis of mole calculations.

The relative atomic mass 相对原子质量 ($A_r$) of an element 元素 is the average mass 质量 of its atoms compared to $\tfrac{1}{12}$ of the mass of one $^{12}\text{C}$ atom.

The relative molecular mass 相对分子质量 ($M_r$) is the sum of the relative atomic masses of all the atoms in the molecule. For ionic compounds we use the relative formula mass 相对式量, found the same way from the formula.

Reacting masses by simple proportion

You can sometimes find a reacting mass without the mole. If 24 g of magnesium makes 40 g of magnesium oxide, then 12 g of magnesium (half as much) makes 20 g of magnesium oxide.

The mole 摩尔 (symbol mol) is the unit for the amount of substance 物质的量. One mole of any substance contains $6.02 \times 10^{23}$ particles 粒子 (atoms, ions or molecules). This number is the Avogadro constant 阿伏伽德罗常数.

The molar mass 摩尔质量 is the mass of one mole, in grams per mole (g/mol). Its number is the same as the $A_r$ or $M_r$. The key relationship is:

Worked example. How many moles are in 36 g of water? Molar mass of water $= 18$ g/mol.

To find the number of particles, multiply the moles by the Avogadro constant.

Volumes of gases

At room temperature and pressure (r.t.p.), one mole of any gas takes up the same volume 体积. This molar gas volume 摩尔气体体积 is $24 \text{ dm}^3$.

Concentration of solutions

The concentration 浓度 of a solution 溶液 can be given in $\text{g/dm}^3$ or in $\text{mol/dm}^3$.

Remember to convert volume: $1 \text{ dm}^3 = 1000 \text{ cm}^3$, so divide a volume in $\text{cm}^3$ by 1000.

Moles sit at the centre: multiply going outwards, divide coming back (concentration is moles ÷ volume)

A titration measures exactly how much acid reacts with a base.

Most calculations follow the same steps: change the known amount into moles, use the balanced equation to find the moles of what you want, then change back into mass, volume or concentration.

Limiting reactant

When two reactants are mixed, often one runs out first. This is the limiting reactant 限量反应物. It decides how much product can form; any other reactant is in excess (left over).

Titration calculation

In a titration 滴定 you measure the volume of one solution that reacts with another. From the volume and concentration you find the moles of one solute 溶质, then use the equation ratio to find the moles, concentration or volume of the other.

Empirical and molecular formulae from data

To find an empirical formula from masses or percentages:

1. Divide each element's mass (or %) by its $A_r$.
2. Divide all the answers by the smallest one to get the simplest ratio.

To find the molecular formula, compare the empirical formula mass with the real $M_r$ and multiply up.

Percentages

• Percentage yield 产率 compares how much product you actually got with the most you could get: $\dfrac{\text{actual}}{\text{theoretical}} \times 100$.
• Percentage composition by mass 质量分数 of an element $= \dfrac{\text{mass of that element in the formula}}{M_r} \times 100$.
• Percentage purity 纯度 $= \dfrac{\text{mass of pure substance}}{\text{mass of impure sample}} \times 100$.

Electrolysis 电解 is the breaking down (decomposition 分解) of an ionic compound 离子化合物 — when it is molten 熔融 or in aqueous 水溶液 solution — by passing an electric current 电流 through it.

An electrolysis cell splits a compound using an electric current.

It only works when the substance is molten or dissolved, because then the ions 离子 are free to move and carry the charge. A solid ionic compound cannot be electrolysed because its ions are locked in place.

Electrolysis of water: gas bubbles off at each electrode and collects in the tubes above, with about twice as much hydrogen as oxygen

The electrolytic cell

The set-up is called an electrolytic cell 电解池. Two electrodes 电极 (solid conductors) dip into the electrolyte 电解质 — the molten or aqueous substance being broken down.

• The anode 阳极 is the positive ($+$) electrode.
• The cathode 阴极 is the negative ($-$) electrode.

The electrodes are often inert 惰性 (they do not react), such as platinum 铂 or carbon/graphite 石墨.

What is formed at each electrode

There is a simple rule:

• Metals 金属 or hydrogen 氢气 are formed at the cathode.
• Non-metals 非金属 (other than hydrogen) are formed at the anode.

For aqueous solutions, the product can depend on whether the solution is dilute 稀 or concentrated 浓. Here are the three Core examples:

How the charge moves

During electrolysis:

• Electrons 电子 move through the wires (the external circuit 外电路) from the power supply.
• At the cathode, positive ions gain electrons. Gaining electrons is reduction 还原.
• At the anode, negative ions lose electrons. Losing electrons is oxidation 氧化.
• Inside the electrolyte, the ions move: positive ions go to the cathode and negative ions go to the anode.

You can write a half-equation 半反应式 for each electrode. For molten lead(II) bromide:

In molten lead(II) bromide, $\text{Pb}^{2+}$ moves to the cathode and $\text{Br}^-$ to the anode, where each is discharged

Electrolysis of copper(II) sulfate

The product at the anode depends on the electrode:

• With inert carbon electrodes: copper 铜 forms at the cathode and oxygen forms at the anode. The blue colour of the solution slowly fades as copper is removed.
• With copper electrodes: copper forms at the cathode, while the anode itself dissolves into the solution. The blue colour stays the same. This is used to purify copper.

Electroplating

Electroplating 电镀 means covering a metal object with a thin layer of another metal. This improves its appearance and its resistance to corrosion 腐蚀 (rusting and wearing away).

To electroplate an object:

• the object to be coated is made the cathode;
• the plating metal is made the anode;
• the electrolyte is a solution containing ions of the plating metal.

To electroplate, make the object the cathode and the plating metal the anode, in a solution of the plating-metal ions

A fuel cell 燃料电池 uses hydrogen and oxygen to make electricity directly. The only chemical product is water.

A hydrogen–oxygen fuel cell turns chemical energy straight into electricity, with water as the only product

It is useful to compare a fuel cell with a normal petrol (gasoline) engine in a vehicle.

Advantages of the fuel cell: the only product is water, so it does not pollute the air, and it changes chemical energy into electricity efficiently. Disadvantages: hydrogen is hard to store and transport safely, and producing the hydrogen can still use energy from fossil fuels.

Burning wood is exothermic, releasing heat to the surroundings.

In every chemical reaction, energy is transferred. The reaction is either exothermic or endothermic, depending on which way the energy moves.

• An exothermic reaction 放热反应 gives out thermal energy 热能 to the surroundings 环境. So the temperature of the surroundings goes up.
• An endothermic reaction 吸热反应 takes in thermal energy from the surroundings. So the temperature of the surroundings goes down.

Examples of exothermic reactions are combustion 燃烧 (burning a fuel) and neutralisation 中和 (an acid reacting with an alkali). An example of an endothermic reaction is the thermal decomposition 分解 of a compound (breaking it down using heat).

An exothermic reaction warms the surroundings; an endothermic reaction cools them

The amount of thermal energy transferred in a reaction is called the enthalpy change 焓变. It is written as $\Delta H$ and has these signs:

• $\Delta H$ is negative for an exothermic reaction, because energy leaves the chemicals.
• $\Delta H$ is positive for an endothermic reaction, because energy is taken in.

Particles do not react every time they meet. The activation energy 活化能 ($E_a$) is the smallest amount of energy that colliding 碰撞 particles 粒子 must have before they can react. It is like a hill the particles must get over before the reaction can happen.

A reaction pathway diagram 反应进程图 shows how the energy changes as a reaction happens. Energy is on the vertical axis, and the progress of the reaction is on the horizontal axis.

• The line starts at the energy level of the reactants 反应物.
• It rises over a 'hill' — the height of this hill is the activation energy $E_a$.
• It then falls or rises to the energy level of the products 生成物.
• The gap between the reactant level and the product level is the enthalpy change $\Delta H$.

For an exothermic reaction, the products are lower than the reactants, so energy is given out and $\Delta H$ is negative.

For an endothermic reaction, the products are higher than the reactants, so energy is taken in and $\Delta H$ is positive.

Exothermic reactions end lower than they start ($\Delta H<0$); endothermic reactions end higher ($\Delta H>0$)

Breaking and making bonds transfers energy — the flame supplies it here.

A reaction involves breaking the bonds in the reactants and making new bonds in the products.

• Bond breaking 断键 takes in energy, so it is an endothermic step.
• Bond making 成键 gives out energy, so it is an exothermic step.

The bond energy 键能 is the energy needed to break one mole of a particular bond. You can use bond energies to find the enthalpy change:

If more energy is given out making bonds than is taken in breaking bonds, the reaction is exothermic ($\Delta H$ negative). If less energy is given out, it is endothermic ($\Delta H$ positive).

Worked example

For the reaction $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$, use these bond energies (in kJ/mol): H–H $= 436$, Cl–Cl $= 242$, H–Cl $= 431$.

Bonds broken: one H–H and one Cl–Cl $= 436 + 242 = 678$.

Bonds made: two H–Cl $= 2 \times 431 = 862$.

The answer is negative, so this reaction is exothermic.

Breaking bonds takes energy in ($+678$); making bonds gives more out ($-862$); so $\Delta H = -184$ kJ/mol

Iron rusting is a chemical change — it forms a new substance.

A physical change 物理变化 does not make a new substance. The substance only changes its state or shape, and the change can usually be reversed. Melting ice and dissolving sugar are physical changes.

A chemical change 化学变化 (a chemical reaction) makes one or more new substances and is usually hard to reverse. Signs of a chemical change include a colour change, a gas being given off, an energy change, or a precipitate 沉淀 (a solid) forming.

The rate of reaction 反应速率 tells you how fast the reactants 反应物 change into products 生成物.

Collision theory

Collision theory 碰撞理论 explains what controls the rate. For a reaction to happen, the particles 粒子 must collide 碰撞, and they must collide with enough energy. The least energy they need is the activation energy 活化能 ($E_a$). A faster rate happens when the particles have successful collisions more often.

What changes the rate

More particles in the same volume collide more often, so the rate is faster

Breaking a solid into a powder exposes much more surface, so the rate is faster

Catalysts

A catalyst speeds up a reaction but is not used up — it is unchanged at the end. It works by lowering the activation energy. Enzymes 酶 are biological catalysts (catalysts that work in living things).

A catalyst gives a lower activation energy so more collisions succeed; $\Delta H$ is unchanged

Measuring the rate

You can follow a reaction over time in these ways:

• Change in mass: stand the flask on a balance. If a gas escapes, the mass falls. Record the mass at regular times.
• Volume of gas: collect the gas in a gas syringe 注射器 and read its volume at regular times.
• Formation of a precipitate: in a reaction that turns cloudy, time how long it takes for a mark under the flask to disappear.

A gas syringe collects the gas given off; read its volume at regular times to follow the rate

On a graph of product against time, the line is steepest at the start (fastest rate), becomes less steep as reactants are used up, and goes flat when the reaction has finished.

The rate is fastest at the start (steepest) and the line levels off when the reaction finishes

Some reactions are reversible 可逆反应: the products can react to form the reactants again. We show this with the symbol $\rightleftharpoons$.

Changing the direction

A clear example uses hydrated 水合 and anhydrous 无水 compounds:

• Blue hydrated copper(II) sulfate, when heated, loses its water to become white anhydrous copper(II) sulfate. Adding water turns it blue again.
• Pink hydrated cobalt(II) chloride loses water when heated to become blue anhydrous cobalt(II) chloride. Adding water turns it pink again.

Adding water to the anhydrous solid (and seeing the colour return) is used as a test for water.

Equilibrium

In a closed system 密闭系统 (where nothing enters or leaves), a reversible reaction reaches equilibrium 平衡 when:

• the rate of the forward reaction 正反应 equals the rate of the reverse reaction 逆反应, and
• the concentrations of the reactants and products are no longer changing.

Changing the position of equilibrium

The position of equilibrium 平衡位置 tells you whether there are more reactants or more products. You can move it:

• Temperature: heating moves the equilibrium in the direction that takes in heat (the endothermic 吸热反应 direction); cooling moves it in the direction that gives out heat (the exothermic 放热反应 direction).
• Pressure (gases): more pressure moves the equilibrium to the side with fewer gas molecules.
• Concentration: adding more of a substance moves the equilibrium to the other side, to use it up.
• A catalyst does not move the position of equilibrium. It only helps the reaction reach equilibrium faster.

The Haber process

The Haber process 哈伯法 makes ammonia:

• The hydrogen 氢气 comes from methane 甲烷 (natural gas); the nitrogen 氮气 comes from the air.
• Typical conditions: a temperature of $450\,{}^{\circ}\text{C}$, a pressure of about $200$ atm ($20\,000$ kPa), and an iron 铁 catalyst.

The Contact process

The Contact process 接触法 turns sulfur dioxide into sulfur trioxide:

• The sulfur dioxide comes from burning sulfur 硫 (or roasting sulfide ores); the oxygen 氧气 comes from the air.
• Typical conditions: a temperature of $450\,{}^{\circ}\text{C}$, a pressure of about $2$ atm ($200$ kPa), and a vanadium(V) oxide 五氧化二钒 catalyst.

Why these conditions are chosen

The conditions are a compromise 折中:

• A higher pressure would give more product, but very high pressure is dangerous and expensive, so a medium pressure is used.
• A lower temperature would give more product (both forward reactions are exothermic), but the reaction would be too slow, so a fairly high temperature is used to keep a good rate.
• The catalyst speeds up the reaction without changing the position of equilibrium, which lowers cost.

Oxidation 氧化 and reduction 还原 always happen at the same time, in what is called a redox reaction 氧化还原反应. ('Redox' is short for reduction–oxidation.)

There are three ways to describe oxidation and reduction:

• Oxygen: oxidation is the gain of oxygen; reduction is the loss of oxygen.
• Electrons 电子: oxidation is the loss of electrons; reduction is the gain of electrons.
• Oxidation number 氧化数: in oxidation the oxidation number goes up; in reduction it goes down.

A useful memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain — of electrons.

Oxidation number rules

The oxidation number is shown by a Roman numeral, as in iron(II) and iron(III). The rules are:

• An element that is not combined has an oxidation number of $0$.
• A single-atom ion has an oxidation number equal to its charge (so $\text{Na}^{+}$ is $+1$).
• The oxidation numbers in a compound add up to $0$.
• The oxidation numbers in an ion add up to the charge on the ion.

Oxidising and reducing agents

• An oxidising agent 氧化剂 oxidises another substance, and is itself reduced.
• A reducing agent 还原剂 reduces another substance, and is itself oxidised.

Some redox reactions show clear colour changes:

• Acidified potassium manganate(VII) 高锰酸钾 is purple. When it acts as an oxidising agent it is reduced, and the purple colour fades to colourless.
• Potassium iodide 碘化钾 is colourless. When it is oxidised, red-brown iodine 碘 is formed.

Potassium manganate(VII) is deep purple; as it is reduced (or diluted) the purple fades to colourless

An acid 酸 is a substance that forms hydrogen ions ($\text{H}^{+}$) when dissolved in water. Acids have three typical reactions:

• with metals 金属: acid + metal → a salt 盐 + hydrogen 氢气
• with bases: acid + base 碱 → a salt + water (this is neutralisation 中和)
• with carbonates 碳酸盐: acid + carbonate → a salt + water + carbon dioxide 二氧化碳

Indicators

An indicator 指示剂 is a dye that changes colour to show whether a solution is acidic or alkaline.

Aqueous solutions of acids contain hydrogen ions 离子 ($\text{H}^{+}$). Aqueous solutions of alkalis contain hydroxide ions ($\text{OH}^{-}$).

Litmus paper: blue litmus turns red in an acid, and red litmus turns blue in an alkali

Bases are oxides 氧化物 or hydroxides 氢氧化物 of metals. An alkali 可溶性碱 is a base that dissolves in water (a soluble base).

Bases have two typical reactions:

• with acids: base + acid → a salt + water (neutralisation again)
• with ammonium salts 铵盐: this releases ammonia gas.

An acid is a proton 质子 donor — it gives away $\text{H}^{+}$ ions (a hydrogen ion is just a proton). A base is a proton acceptor.

How strong an acid is depends on how much of it splits into ions in water:

• A strong acid 强酸 is completely dissociated 电离 (fully split into ions) in water. Hydrochloric acid 盐酸 is strong:

• A weak acid 弱酸 is only partly dissociated. Ethanoic acid 乙酸 is weak, so the equation uses the reversible arrow:

Note: 'strong' and 'weak' are about dissociation, not about being dilute or concentrated.

A strong acid is fully split into ions; a weak acid stays mostly as molecules (only partly dissociated)

Litmus and indicators show whether a solution is acidic or alkaline.

The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. A higher hydrogen ion concentration means a lower pH.

You can find pH using universal indicator 通用指示剂, which turns different colours:

Universal indicator turns red in a strong acid, green at neutral (pH 7), and purple in a strong alkali

When an acid and an alkali react, the $\text{H}^{+}$ and $\text{OH}^{-}$ ions join to make water:

Oxides can be sorted by how they behave:

• Acidic oxides are oxides of non-metals, such as $\text{SO}_2$ and $\text{CO}_2$.
• Basic oxides are oxides of metals, such as $\text{CuO}$ and $\text{CaO}$.
• Amphoteric 两性 oxides react with both acids and bases to make a salt and water. $\text{Al}_2\text{O}_3$ and $\text{ZnO}$ are amphoteric.

So metal oxides tend to be basic and non-metal oxides tend to be acidic.

Whether a salt can be made by a certain method depends on whether it is soluble 可溶 (dissolves) or insoluble 不溶 (does not dissolve).

Solubility rules

Making a soluble salt

You react an acid with one of these:

• an alkali, using titration 滴定 (since both are solutions, you must measure the exact volumes);
• an excess 过量 of a metal, an insoluble base, or an insoluble carbonate.

When you use an excess of a solid, you then filter 过滤 to remove the leftover solid. Finally you evaporate 蒸发 some water and let the solution crystallise 结晶 to get the salt.

Making a soluble salt from an insoluble solid: react with excess, filter off the excess, evaporate, then crystallise

Making an insoluble salt

An insoluble salt is made by precipitation 沉淀: mix two solutions that each contain one of the needed ions, and the insoluble salt forms as a solid. You then filter, wash and dry it.

Water in salts

• A hydrated 水合 substance is chemically joined with water.
• An anhydrous 无水 substance contains no water.

The water molecules inside hydrated crystals are called the water of crystallisation 结晶水. For example, $\text{CuSO}_4 \bullet 5\text{H}_2\text{O}$ has five water molecules for each formula unit.

The periodic table arranges the elements in order of atomic number.

The Periodic Table 周期表 arranges all the elements 元素 in order of increasing proton number 质子数 (the atomic number 原子序数). The horizontal rows are called periods 周期 and the vertical columns are called groups 族.

A few key patterns:

• Across a period, the elements change from metals 金属 on the left to non-metals 非金属 on the right.
• The group number tells you the charge of the ions 离子 that the elements form. Group I forms $+1$ ions, Group II forms $+2$ ions, and Group VII forms $-1$ ions.
• Elements in the same group have similar chemical properties. This is because they have the same number of outer-shell electrons 电子 (the same outer electronic configuration 电子排布).

Because of these patterns, you can use an element's position to predict its properties.

Groups are the columns and periods are the rows; metals lie to the left of the staircase, non-metals to the right

Group I elements are the alkali metals 碱金属: lithium 锂, sodium 钠 and potassium 钾. They are soft 柔软 metals (you can cut them with a knife).

Potassium reacts violently with water, giving off hydrogen that burns with a lilac flame — reactivity increases down Group I

Every Group I atom has one electron in its outer shell, which is why they react in similar ways

Going down the group, there are clear trends 趋势:

You can use these trends to predict the properties of other Group I elements. For example, rubidium (below potassium) would be even more reactive and have an even lower melting point.

Group VII elements are the halogens 卤素: chlorine 氯气, bromine 溴 and iodine 碘. They are diatomic 双原子 non-metals (each molecule is made of two atoms, such as $\text{Cl}_2$).

Their appearance at room temperature and pressure:

Going down the group, the density increases but the reactivity decreases (the opposite trend to Group I).

Group I gets more reactive down the group, while Group VII gets less reactive down — opposite trends

Displacement reactions

A more reactive halogen pushes out (displaces) a less reactive halide 卤化物 ion from its solution. This is a displacement reaction 置换反应.

For example, chlorine is more reactive than bromine, so chlorine displaces bromine from potassium bromide:

The transition elements 过渡元素 are the block of metals in the middle of the Periodic Table. Compared with Group I metals, they:

• have high densities;
• have high melting points;
• form coloured 有色 compounds 化合物;
• often act as catalysts, both as elements and in compounds.

They can also have variable 可变 oxidation numbers 氧化数. For example, iron 铁 forms both iron(II) and iron(III) compounds.

The Group VIII noble gases 稀有气体 are unreactive 不活泼, monatomic 单原子 gases (they exist as single atoms, not as molecules).

They are unreactive because they already have a full outer shell of electrons. This makes them stable, so they do not need to gain, lose or share electrons.

Physical properties

Metals 金属 and non-metals 非金属 behave very differently:

Chemical properties

Metals react in three main ways:

• with dilute acids 酸 → a salt 盐 + hydrogen 氢气
• with cold water or steam 蒸汽 → a metal hydroxide (or oxide) + hydrogen
• with oxygen 氧气 → a metal oxide 氧化物

A metal is chosen for a job because of its physical properties.

• Aluminium 铝 is used to make aircraft because of its low density 密度 (it is light).
• Aluminium is used for overhead electrical cables because of its low density and good electrical conductivity.
• Aluminium is used for food containers because it resists corrosion 腐蚀.
• Copper 铜 is used for electrical wiring because of its good electrical conductivity and its ductility (it can be drawn into wires).

An alloy 合金 is a mixture 混合物 of a metal with one or more other elements.

• Brass 黄铜 is a mixture of copper and zinc 锌.
• Stainless steel 不锈钢 is a mixture of iron 铁 with other elements such as chromium 铬, nickel 镍 and carbon 碳.

Alloys are usually harder and stronger than the pure metals, which makes them more useful. For example, stainless steel is used for cutlery because it is hard and does not rust.

Why alloys are harder. In a pure metal, the atoms 原子 are all the same size, so the layers 层 can slide over each other easily. In an alloy, the different-sized atoms stop the layers from sliding, so the alloy is harder and stronger.

In an alloy, different-sized atoms stop the layers sliding past each other, so the alloy is harder than the pure metal

The reactivity series 金属活动性顺序 lists metals in order of how reactive they are. Carbon and hydrogen are included for comparison:

potassium 钾, sodium 钠, calcium 钙, magnesium 镁, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold

(most reactive at the top, least reactive at the bottom)

Metals above carbon are extracted by electrolysis; those below carbon can be extracted by heating with carbon

The higher a metal is, the more easily it forms positive ions 离子. This explains its reactions:

• potassium, sodium and calcium react with cold water.
• magnesium reacts with steam (but only very slowly with cold water).
• magnesium, zinc and iron react with dilute hydrochloric acid; copper, silver and gold do not.

Displacement reactions

A more reactive metal will displace a less reactive metal from a solution of its ions (a displacement reaction 置换反应). For example, zinc displaces copper from copper(II) sulfate solution, because zinc is more reactive than copper.

The special case of aluminium

Aluminium seems less reactive than its position suggests. This is because it is covered by a thin, strong oxide layer 氧化层 that stops other substances reaching the metal underneath.

Corrosion slowly eats away unprotected metal.

Rusting 生锈 is the corrosion of iron and steel. Two things are needed for rusting: oxygen (from the air) and water. The rust 铁锈 formed is hydrated iron(III) oxide.

Sea water and air have badly rusted this iron chain — the rust is hydrated iron(III) oxide

Rusting needs both water and oxygen — the nail only rusts in the tube that has both

Stopping rust

Barrier methods 隔离法 keep oxygen and water away from the iron:

• painting, greasing (covering with oil), and coating with plastic.

Galvanising 镀锌 means coating iron with a layer of zinc. This works in two ways:

• it is a barrier (the zinc keeps out air and water);
• it gives sacrificial protection 牺牲保护. Because zinc is more reactive than iron, the zinc loses electrons 电子 and corrodes instead of the iron — even if the surface is scratched.

How a metal is taken from its ore 矿石 depends on its place in the reactivity series. Metals below carbon can be extracted by heating with carbon (which removes the oxygen). Metals above carbon are too reactive for this and must be extracted by electrolysis 电解.

Iron from the blast furnace

Iron is extracted from its ore hematite 赤铁矿 (iron(III) oxide) in a blast furnace 高炉:

• Carbon (coke) burns in hot air to give carbon dioxide and lots of heat.
• This carbon dioxide 二氧化碳 reacts with more carbon to form carbon monoxide 一氧化碳.
• The carbon monoxide reduces the iron(III) oxide to iron (this is reduction 还原).
• Limestone 石灰石 (calcium carbonate) breaks down in the heat to form calcium oxide 氧化钙.
• The calcium oxide reacts with sandy impurities to form slag 炉渣, which is removed.

In the blast furnace, carbon monoxide reduces the iron ore, and limestone removes sandy impurities as slag

Aluminium by electrolysis

Aluminium is extracted from its ore bauxite 铝土矿 (purified to aluminium oxide) by electrolysis:

• The aluminium oxide is dissolved in molten cryolite 冰晶石 to lower its melting point and save energy.
• At the cathode 阴极, aluminium ions gain electrons to form aluminium metal.
• At the anode 阳极, oxygen is formed. This oxygen reacts with the hot carbon anodes and burns them away, so they must be replaced regularly.

Testing for water

Two chemical tests show that water is present:

• Anhydrous 无水 cobalt(II) chloride turns from blue to pink when water is added.
• Anhydrous copper(II) sulfate turns from white to blue when water is added.

These tests only show that water is there. To show that water is pure, you test its melting point 熔点 and boiling point 沸点: pure water melts at exactly $0\,{}^{\circ}\text{C}$ and boils at exactly $100\,{}^{\circ}\text{C}$. Any dissolved substance changes these values.

This is why distilled water 蒸馏水 is used in chemistry instead of tap water — it has far fewer chemical impurities 杂质.

What is in natural water

Water from rivers, lakes and the sea is not pure. It may contain dissolved oxygen 氧气, metal compounds, plastics, sewage 污水, harmful microbes 微生物, and nitrates 硝酸盐 and phosphates 磷酸盐 (which come from fertilisers and detergents).

Some of these are helpful:

• dissolved oxygen lets aquatic life 水生生物 (fish and plants) breathe;
• some metal compounds give essential minerals 矿物质.

Others are harmful:

• some metal compounds are toxic 有毒 (poisonous);
• some plastics harm aquatic life;
• sewage carries microbes that cause disease;
• nitrates and phosphates cause deoxygenation 缺氧 (loss of oxygen) in the water, which harms aquatic life.

Treating drinking water

To make water safe to drink, the water supply is treated in steps:

• sedimentation 沉降 and filtration 过滤 remove solid bits;
• passing it through carbon 碳 removes bad tastes and smells;
• chlorination 氯消毒 (adding chlorine) kills harmful microbes.

Water is made safe to drink in steps: settle out big bits, filter, remove tastes with carbon, then chlorinate

Fertilisers 肥料 are added to soil to help plants grow. Ammonium salts and nitrates are common fertilisers.

NPK fertilisers contain the three elements plants need most: nitrogen 氮气, phosphorus 磷 and potassium 钾.

Burning fossil fuels pollutes the air and changes the climate.

Clean, dry air is approximately:

• 78% nitrogen ($\text{N}_2$)
• 21% oxygen ($\text{O}_2$)
• the rest is a mixture of noble gases 稀有气体 and carbon dioxide ($\text{CO}_2$).

Clean, dry air is about 78% nitrogen, 21% oxygen, and 1% other gases (noble gases and carbon dioxide)

Air pollutants and their sources

Effects of these pollutants

• Carbon dioxide 二氧化碳 and methane are greenhouse gases 温室气体: more of them causes global warming 全球变暖, which leads to climate change 气候变化.
• Carbon monoxide is a toxic gas.
• Particulates increase the risk of respiratory 呼吸 (breathing) problems and cancer 癌症.
• Oxides of nitrogen cause acid rain 酸雨, photochemical smog 光化学烟雾 and breathing problems.
• Sulfur dioxide causes acid rain.

Reducing these problems

To slow climate change: plant trees, farm fewer animals, burn fewer fossil fuels, and use more hydrogen and renewable energy 可再生能源 such as wind and solar power.

To reduce acid rain: fit catalytic converters 催化转化器 in cars, use low-sulfur fuels, and use flue gas desulfurisation 烟气脱硫 with calcium oxide 氧化钙 to remove sulfur dioxide from waste gases.

Inside a catalytic converter is a honeycomb coated with catalyst metals; the many tiny channels give a huge surface area

How greenhouse gases warm the Earth

Greenhouse gases such as carbon dioxide and methane let sunlight through, but they absorb the thermal energy 热能 given off by the warm Earth, and send some of it back down. This reduces the thermal energy lost to space, so the Earth gets warmer.

Greenhouse gases let sunlight through but trap some of the heat the Earth gives off, so the Earth warms up

In a car engine, the high temperature makes nitrogen and oxygen from the air react to form oxides of nitrogen. A catalytic converter removes them, for example:

Photosynthesis

Photosynthesis 光合作用 is the opposite of combustion — it removes carbon dioxide from the air. Plants use light energy and chlorophyll 叶绿素 to turn carbon dioxide and water into glucose 葡萄糖 and oxygen:

There are several ways to write an organic molecule:

• The molecular formula 分子式 shows how many of each atom there are, for example $\text{C}_2\text{H}_6$.
• The displayed formula 结构式 shows every atom and every bond drawn out in full.
• The structural formula 结构简式 shows how the atoms are arranged without drawing every bond, for example $\text{CH}_3\text{CH}_2\text{OH}$.
• The general formula 通式 works for a whole family, for example $\text{C}_n\text{H}_{2n+2}$ for alkanes.

A homologous series 同系物 is a family of compounds with the same functional group 官能团 — the atom or group of atoms that gives the family its chemical properties. Members of a series have the same general formula, differ by a $\text{CH}_2$ unit each step, and have similar chemical properties with a gradual change in physical properties.

The alkanes are a homologous series: each member has one more $\text{CH}_2$ unit (general formula $\text{C}_n\text{H}_{2n+2}$)

A saturated 饱和 compound has only single carbon–carbon bonds. An unsaturated 不饱和 compound has one or more carbon–carbon bonds that are not single (such as a C=C double bond).

A saturated compound has only single C–C bonds; an unsaturated one has a C=C double bond

Structural isomers 同分异构体 are compounds with the same molecular formula but different structural formulae. For example, $\text{C}_4\text{H}_{10}$ can be a straight chain or a branched chain.

Structural isomers: butane and 2-methylpropane share the formula $\text{C}_4\text{H}_{10}$ but have different structures

The end of the name tells you the family:

The start of the name tells you the number of carbon atoms: meth- = 1, eth- = 2, prop- = 3, but- = 4. For longer alkenes and alcohols, a number shows where the functional group is, for example but-1-ene and but-2-ene, or propan-1-ol and propan-2-ol.

An oil refinery separates crude oil into useful fuels by fractional distillation.

The three fossil fuels 化石燃料 are coal 煤, natural gas 天然气 and petroleum 石油 (crude oil). Methane 甲烷 is the main part of natural gas.

A hydrocarbon 碳氢化合物 is a compound made of hydrogen and carbon only. Petroleum is a mixture of many different hydrocarbons.

Fractional distillation

Petroleum is separated into useful fractions 馏分 by fractional distillation 分馏. The mixture is heated, and the different hydrocarbons turn to gas and rise up a tall fractionating column 分馏塔. The column is hot at the bottom and cool at the top, so each fraction turns back to liquid at a different height.

At an oil refinery, crude oil is separated in the tall fractionating columns you can see rising above the plant

Fractions separate by boiling point: small molecules leave the cool top, thick bitumen stays at the hot bottom

Going from the bottom to the top of the column, the fractions have:

• shorter chain length 链长 (smaller molecules);
• higher volatility 挥发性 (they turn to gas more easily);
• lower boiling points;
• lower viscosity 黏度 (they flow more easily).

Alkanes 烷烃 have only single covalent bonds, so they are saturated hydrocarbons. They are generally unreactive. Their two important reactions are:

• Combustion 燃烧: they burn in plenty of oxygen to give carbon dioxide and water.
• Substitution with chlorine.

In a substitution reaction 取代反应, one atom (or group of atoms) is replaced by another. Alkanes react with chlorine only in ultraviolet light 紫外线 — this is a photochemical reaction 光化学反应, where the light provides the activation energy 活化能. For example:

Alkenes 烯烃 have a carbon–carbon double bond (C=C), so they are unsaturated hydrocarbons.

Cracking

Large alkane molecules are not very useful. Cracking 裂化 breaks them into smaller, more useful molecules — smaller alkanes and alkenes — using a high temperature and a catalyst 催化剂. Cracking also makes hydrogen 氢气 and provides alkenes for making plastics.

Reactions of alkenes

The C=C double bond makes alkenes reactive. They take part in addition reactions 加成反应, where two molecules join to form a single product.

• Test for unsaturation: shake the compound with bromine 溴 water (which is orange). An alkene turns the bromine water colourless; an alkane does not change it.
• With hydrogen and a nickel 镍 catalyst, an alkene becomes an alkane.
• With steam 蒸汽 and an acid catalyst, an alkene becomes an alcohol.

Alcohols 醇 contain the –OH functional group. The most important one is ethanol 乙醇. There are two ways to make ethanol.

In fermentation, yeast 酵母 turns glucose 葡萄糖 into ethanol and carbon dioxide.

Ethanol burns well (combustion), so it is used as a fuel. It also dissolves many substances, so it is used as a solvent 溶剂.

Carboxylic acids 羧酸 contain the –COOH functional group. Ethanoic acid 乙酸 is the one to know. Like other acids, it reacts with:

• metals 金属 → a salt + hydrogen;
• bases 碱 → a salt 盐 + water;
• carbonates 碳酸盐 → a salt + water + carbon dioxide.

The salts formed are called ethanoates.

Ethanoic acid can be made by the oxidation 氧化 of ethanol, either using acidified potassium manganate(VII) 高锰酸钾, or by bacteria during the making of vinegar 醋.

When a carboxylic acid reacts with an alcohol (using an acid catalyst), it forms an ester 酯.

A polymer 聚合物 is a very large molecule built from many small molecules called monomers 单体.

Addition polymerisation

In addition polymerisation 加聚, many unsaturated monomers join together with no other product. For example, many ethene monomers join to make poly(ethene). The small part that repeats along the chain is the repeat unit 重复单元.

In addition polymerisation the C=C bonds open up and many ethene monomers join into a long poly(ethene) chain

Condensation polymerisation

In condensation polymerisation 缩聚, monomers join and a small molecule (usually water) is lost each time a bond forms. This makes two important types:

• Polyamides 聚酰胺 are made from a dicarboxylic acid and a diamine. Nylon 尼龙 is a polyamide.
• Polyesters 聚酯 are made from a dicarboxylic acid and a diol. PET is a polyester, and it can be broken back into its monomers and re-made.

Plastics and the environment

Plastics 塑料 are made from polymers. Because many plastics do not break down, getting rid of them is a problem:

• they build up in landfill 填埋 sites;
• they collect in the oceans and harm sea life;
• burning them can make toxic gases.

Proteins

Proteins 蛋白质 are natural polyamides. They are made from amino acid 氨基酸 monomers joined in long chains.

Apparatus

You should know which piece of apparatus to use for each measurement:

Key words

These words are used throughout practical chemistry:

• A solvent 溶剂 is a substance that dissolves a solute.
• A solute 溶质 is the substance that dissolves in the solvent.
• A solution 溶液 is a mixture of one or more solutes dissolved in a solvent.
• A saturated solution 饱和溶液 holds the maximum amount of solute that will dissolve at a given temperature.
• A residue 残渣 is the substance left behind after evaporation, distillation or filtration.
• A filtrate 滤液 is the liquid that has passed through a filter.

A titration 滴定 finds the exact volume of one solution that reacts with another.

• Use a volumetric pipette to measure a fixed volume of one solution into a flask.
• Add a few drops of a suitable indicator 指示剂.
• Add the other solution from a burette, slowly, until the colour just changes.

The end-point 终点 is the moment the indicator changes colour, which shows the reaction is exactly complete. You read the burette to find the volume added.

Solution is run from the burette into the flask until the indicator just changes colour (the end-point)

A real titration: solution from the burette is added to the flask, where the indicator has turned orange

Paper chromatography 纸色谱法 separates a mixture of soluble substances. You put a spot of the mixture near the bottom of the paper, then stand the paper in a solvent. As the solvent rises up the paper, the substances move different distances, so they separate.

As the solvent rises, the substances travel different distances and separate; $R_f$ is the spot distance ($a$) divided by the solvent distance ($b$)

On real chromatograms, each substance in the mixture rises a different distance, leaving a separate coloured spot

• For coloured substances you can see the spots directly.
• For colourless substances you must spray a locating agent 显色剂 to make the spots show up.

The finished paper is a chromatogram 色谱图. You can use it to:

• identify an unknown substance by comparing it with known substances;
• tell if a substance is pure (a pure substance gives only one spot).

You can also calculate the $R_{\text{f}}$ value of a spot:

You choose a method based on the mixture:

Filtration: the insoluble solid stays on the filter paper (residue) and the liquid passes through (filtrate)

Simple distillation: the solvent boils off, the condenser cools it back to a liquid, and the pure distillate is collected

Fractional distillation adds a fractionating column, so liquids with different boiling points separate cleanly

You can check the purity of a substance using its melting point 熔点 and boiling point 沸点: a pure substance melts and boils at sharp, fixed temperatures, while impurities lower the melting point and raise the boiling point.

Tests for anions

An anion 阴离子 is a negative ion.

Tests for cations

A cation 阳离子 is a positive ion. Add aqueous sodium hydroxide, or aqueous ammonia, and look at the precipitate 沉淀 formed.

Tests for gases

Flame tests

A flame test 焰色试验 identifies some metal cations by the colour they give to a flame: