Electrolysis

- Electrolysis is the breaking down (decomposition) of a substance using electricity.
- Electrolyte: Molten/dissolved ionic compound.
→ This decomposes when an electric current passes through it, causing ions to move.

- The cations in the electrolyte move towards the cathode (negative electrode), and the anions in the electrolyte move to the anode (positive electrode).
- This process allows the cations to gain electrons and reduce while the anions lose electrons and oxidise (OILRIG).
→ This creates a flow of charge through the electrolyte.
- Anions gain/lose electrons; they form uncharged substances and are discharged from the electrode.

Ionic Half Equations:

- The purpose of this experiment is to show what happens at each electrode in electrolysis.
- At the cathode, the product is connected to the e- terminal on the RHS.
- At the anode, the product goes without e- on the RHS.
- e.g., 2Br^- → Br₂ + 2e-
Cu²⁺ + 2e^- → Cu

Definitions:

- Electrode: Conductor that passes electrons into the electrolyte or collects electrons from the electrolyte.
→ Usually made of carbon or platinum, they conduct electricity and are quite chemically inert; they won't partake in reactions.
- An electrolyte is a substance that undergoes electrolysis.
- Conductor: A substance that allows an electric current to flow through it (an insulator is the opposite).

Molten Electrolysis:

- Ions can freely flow in molten ionic compounds, allowing for electrolysis.
- They're usually broken up into their elements.
- First, you need to melt ionic compounds, e.g., PbBr₂. Using a Bunsen burner, ions can move.

- The lead collects on the electrode and eventually drops off, so a silvery-grey liquid is seen forming at the cathode.
- These pair up to form Br₂ molecules, which are discharged.
- Oxidation occurs.
- 2Br^- → Br_2(g) + 2e^-
- Orange bubbles/gas are given off at the anode.
- In molten electrolysis, the product at the anode is always diatomic.

- Cations are attracted to the cathode, and anions to the anode; ∴ opposite charges attract.

Aqueous Electrolysis:

- Only one element will discharge at the anode or cathode.
- Cathode: The least reactive (reactivity series) ion will be discharged.
→ If the ion is above hydrogen in the reactivity series, then hydrogen gas will be produced.
→ If the metal ion is less reactive than hydrogen, a solid layer of the pure metal is produced.
- Anode: If a halide ion is present, it'll be discharged; else OH^- will be discharged, forming oxygen.

E.g., Solution of NaCl:

- Contains 4 ions: H⁺, OH^-, Na⁺, Cl^-
- Na⁺ > H⁺ ∴ H⁺ is discharged at the cathode.
- Cl^- is present ∴ Cl^- is discharged at anode.
- Cathode: 2H⁺ + 2e^- → H₂ (bubbles of gas)
- Anode: 2Cl^- → Cl₂ + 2e^- (bubbles of gas and bleaches red litmus paper)

Other Products:

- If oxygen is formed (in solution)
- 4OH^- → 2H2O + O2 + 4e^- (bubbles of gas)
- I₂ ​ +2e^- →2I^- (Brown colour appears around the electrode)
- If Cu²⁺ → 2e^- → Cu (pink-brown metal deposited at electrode)

- If oxygen isn't in solution
→ O^2- → O₂ + 4e^-

How to Set Up an Electrochemical Cell (for aq. solution):

- An electrochemical cell is a circuit made up of an anode, cathode, electrolyte, power source, and wires connecting electrodes.

1. Get 2 inert electrodes (e.g., graphite/platinum).
2. Clean the electrode surfaces using emery paper.
3. Now, don't touch the surfaces of electrodes with your hands, as you could transfer grease back onto strips.
4. Place both electrodes into a beaker filled with your electrolyte.
5. Connect electrodes to a power supply using crocodile clips and wires.
6. When you turn the power supply on, a current will flow through the cell.

- Halogens gain an electron when they react.
- As the halogen atoms get bigger, the distance between the nucleus and outer shell electrons increases.
- There is increased shielding due to more shells down the gp, so the gained electron gets a weaker electrostatic attraction to the nucleus.
- This makes the halogens less reactive as the group descends.

OILRIG → oxidation is the loss
OILRIG → or gain of electrons

Redox:

- Oxidation and reduction at the same time.

Ionic Equations:

Cl_2(aq) + 2KBr_(aq) → 2KCl_(aq) + Br_2(aq)
from: Cl_(l) + 2KBr → 2KCl + Br₂

Halogen Displacement Reactions:

- Carried out in solution.
- We mix halogen solutions with solutions containing a halide ion.