Topic 19.1 - Electrolysis

Question 1

Inert electrodes

Answer 1

Made from an unreactive metal (platinum etc.) or graphite. These are not changed during electrolysis.

Question 2

Electrolysis of water

Answer 2

Occurs in sulfuric acid (H₂SO₄) or sodium hydroxide (NaOH) using inert platinum (Pt) electrodes

H⁺ ions are discharged at the cathode to form hydrogen gas (2H⁺ (aq) + 2e⁻ -> H₂ (g))

OH⁻ ions are discharged at the anode to form water and oxygen gas (4OH⁻ (aq) - 4e⁻ -> 2H₂O (l) + O₂ (g))

Question 3

Electrolysis of aqueous solutions

Answer 3

Contains the ions of the solute, hydrogen ions (H⁺), and hydroxide ions (OH⁻).

Hydrogen ions (H⁺) will discharge at the cathode unless a less reactive metal is present (Sb, As, Bi, Cu, Ag, Pd, Hg, Pt, Au)

hydroxide ions (OH⁻) will discharge at the anode unless halogen ions (excluding fluoride ions (F⁻)) are present in high concentrations.

Question 4

Standard electrode potential

Answer 4

The electrode potential of a redox system to lose or gain electrons when compared to the SHE.

More positive standard electrode potential - preferred product for reactions at the anode, more negative standard electrode potential for reactions at the cathode.

Question 5

Coating a substance in zinc

Answer 5

Galvanising

Question 6

Electroplating

Answer 6

Type of electrolysis where an object is coated in a thin layer of metal.

Cathode - negative electrode, contains the original metal which you want to have electroplated
Anode - positive electrode containing the metal you want to use to electroplate
Electrolyte - Ions of the metal that you are using to electroplate (usually with H₂SO₄²⁻ or NO₃⁻ anions )

Question 7

Voltaic cells

Answer 7

Generates an electromotive force (EMF) resulting in the movement of electrons from the anode (-) to the cathode (-) via the external circuit.

Question 8

EMF

Answer 8

Electromotive force is the standard cell potential value (E^⦵) and the value is determined under standard conditions: 298K, 1 mol dm⁻³, 100kPa

Question 9

E^⦵

Answer 9

Standard electrode potential. The potential voltage of the reduction relative to the SHE.

E^⦵ꜜcell = E^⦵ꜜred - E^⦵ꜜox

Question 10

Cell notation

Answer 10

More positive E⦵ value is on the right (cathode).
Cu²⁺ + 2e⁻ ⇌ Cu (E⦵ = +0.34V)
Zn²⁺ + 2e⁻ ⇌ Zn (E⦵ = -0.76V)

Zn ⋮ Zn²⁺ ll Cu²⁺ ⋮ Cu

Question 11

SHE

Answer 11

Standard hydrogen electrode.

Consists of an inert platinum electrode in contact with 1 mol dm⁻³ hydrogen ion and hydrogen gas at 100kPa and 298K.

Question 12

Gibbs free energy

Answer 12

If E^⦵ꜜcell is positive, the redox reaction will be spontaneous.

Gibbs free energy is negative if it’s spontaneous,
ΔG = -nFE^⦵ꜜcell

Question 13

ΔG = -nFE^⦵ꜜcell

Answer 13

F = charge in coloumbs of 1 mole of electrons (Faradays constant (in DB))
n = amount in moles (moles of electrons transferred)
E^⦵ꜜcell = standard electrode potential (V of cell)