Electrolysis
Electrolysis
- Electrolysis uses electricity to break down a molten or aqueous electrolyte.
- A half-reaction happens at each electrode.
- You can predict the products and calculate amounts.
The two electrodes
- at the cathode (negative): positive ions gain electrons (reduction).
- at the anode (positive): negative ions lose electrons (oxidation).
Practice
At the cathode (negative electrode) in electrolysis:
Positive ions are attracted to the cathode and gain electrons (reduction); the anode does oxidation.
Predicting the products
- a molten electrolyte → metal at the cathode, non-metal at the anode.
- an aqueous one also has water: a reactive metal gives hydrogen instead at the cathode; the anode usually gives oxygen, but a concentrated halide gives the halogen.
Practice
Electrolysing an aqueous solution of a reactive metal salt gives, at the cathode:
For a reactive metal, water is reduced to hydrogen at the cathode instead of the metal.
Calculations
- the Faraday constant $F = Le$ (charge on one mole of electrons). Charge passed $Q = It$.
- moles of electrons $= Q / F$.
- use the half-equation to find moles of product.
- find the mass ($\times M_r$) or gas volume.
Practice
The charge passed during electrolysis is given by:
Q = It; then moles of electrons = Q/F, and the half-equation gives the moles of product.
Practice
The Faraday constant F is:
F = L × e, the charge carried by one mole of electrons.
You've got it
Key idea
- cathode (−): reduction (gain electrons); anode (+): oxidation (lose electrons)
- molten → metal + non-metal; aqueous → hydrogen for reactive metals, halogen from concentrated halides
- $Q = It$; moles of electrons $= Q/F$ ($F = Le$); then use the half-equation for the product