Electrochemistry

Slide 1 · Learning Outcomes

What you will be able to do

Electrochemistry links chemical reactions with electricity: cells turn chemical energy into electrical energy, and electrolysis uses electricity to drive reactions.

Define oxidation and reduction in terms of electrons and oxidation numbers.
Use displacement reactions to arrange metals in order of reactivity.
Describe an electrochemical cell and calculate its e.m.f. from standard electrode potentials.
Explain the factors that decide which ions are discharged during electrolysis.
Predict the products of electrolysis of common electrolytes.
Apply Faraday's laws to calculate masses and volumes of products.
Describe applications of electrolysis: electroplating, purification and extraction of metals.

OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons. In cells and in electrolysis, oxidation happens at the anode and reduction at the cathode.

Slide 2 · Redox and oxidation numbers

Who loses, who gains?

In terms of electrons

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Zn loses 2e⁻ → oxidised (it is the reducing agent).
Cu²⁺ gains 2e⁻ → reduced (it is the oxidising agent).

Oxidation number rules

Element on its own = 0
Simple ion = its charge (Na⁺ = +1)
O = −2 (except peroxides, −1); H = +1
Sum in a compound = 0; in an ion = its charge

Oxidation = increase in oxidation number.

Activity: find the oxidation number of the underlined atom

Slide 3 · Displacement and reactivity

Which metal wins?

A more reactive metal displaces a less reactive metal from a solution of its salt, because it loses electrons more readily.

Activity: tap a box to drop the metal into the solution

Tap any box.

Order from the results: Mg > Zn > Fe > Cu > Ag. The more boxes a metal reacts in, the more reactive it is.

Slide 4 · Electrochemical cells

Build a cell

Two different metals in solutions of their ions, joined by a wire and a salt bridge, make a cell. The metal with the more negative E° is the anode (−, oxidised); the other is the cathode (+, reduced).

E.m.f. = E°(cathode) − E°(anode) = E°_right − E°_left

Tap two half-cells from the electrochemical series.

Pick two half-cells.

Salt bridge (filter paper soaked in KNO₃ or KCl): completes the circuit by letting ions move, and keeps the two solutions electrically neutral. E° values are measured against the standard hydrogen electrode (0.00 V) at 25 °C, 1 atm, 1 M solutions.

Slide 5 · Electrolysis

Using electricity to split compounds

Electrolysis is the decomposition of a compound (an electrolyte, molten or in solution) by an electric current. Cations move to the cathode (−) and gain electrons; anions move to the anode (+) and lose electrons.

Which ion is discharged? Three factors

Factor	Rule	Example
1. Position in the electrochemical series	Cations: the lower (less reactive) is discharged first: Cu²⁺ before H⁺ before Na⁺. Anions: OH⁻ before Cl⁻ before SO₄²⁻/NO₃⁻ (never discharged in solution).	Dilute H₂SO₄: H⁺ and OH⁻ discharged.
2. Concentration	A much more concentrated ion may be discharged instead.	Brine (conc. NaCl): Cl⁻ discharged, not OH⁻.
3. Nature of electrode	An active (copper) anode dissolves instead of any anion being discharged.	CuSO₄ with Cu electrodes: anode dissolves.

Order of discharge of anions (in solution): OH⁻ > I⁻ > Br⁻ > Cl⁻ > NO₃⁻, SO₄²⁻.
Cations: Ag⁺ > Cu²⁺ > H⁺ > Pb²⁺ … > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > K⁺ (easiest first).

Slide 6 · Predicting products

Electrolysis simulator

Electrolyte and electrodes

Slide 7 · Quantitative electrolysis

Faraday's laws

Quantity of electricity Q (coulombs) = current I (amperes) × time t (seconds). 1 Faraday = 96 500 C = charge on 1 mole of electrons.

Cu²⁺ + 2e⁻ → Cu so 2 × 96 500 C deposit 1 mole (63.5 g) of copper

Activity: deposition calculator

Current (A)

Time (minutes)

Substance at electrode

Molar gas volume at room temperature and pressure = 24 dm³ (24 000 cm³). For O₂: 4OH⁻ → 2H₂O + O₂ + 4e⁻, so 4 Faradays give 1 mole of O₂.

Slide 8 · Applications of electrolysis

Electrolysis at work

Electroplating

Coating an object with a thin layer of another metal (e.g. silver, chromium, nickel) to prevent corrosion or improve appearance. The object is the cathode; the plating metal is the anode; the electrolyte contains ions of the plating metal.

Purification of copper

Impure copper anode dissolves; pure copper deposits on the cathode. Impurities fall as anode sludge (contains silver and gold). Electrolyte: acidified CuSO₄.

Activity: set up the cells

Tap a card, then tap the correct box.

Anode (+)

Cathode (−)

Electrolyte

Extraction: very reactive metals (Na, Mg, Al) are extracted by electrolysis of molten compounds, e.g. sodium from molten NaCl in the Downs cell. Other uses: making chlorine and sodium hydroxide from brine; anodising aluminium.

Slide 9 · Practice

Worked example and quick checks

Worked example: mass of copper deposited

A current of 2 A was passed through copper(II) sulphate solution for 30 minutes. Calculate the mass of copper deposited. (Cu = 63.5, 1 F = 96 500 C)

Step 1: Q = I × t = 2 × (30 × 60) = 3600 C

Step 2: Cu²⁺ + 2e⁻ → Cu, so 2 × 96 500 = 193 000 C deposit 63.5 g

Step 3: Mass = (3600 × 63.5) ÷ 193 000

Answer: 1.18 g of copper

Your turn

1. Oxidation number of S in SO₄²⁻ = .

2. E.m.f. of a Mg/Cu cell (Mg −2.37 V, Cu +0.34 V) = V.

3. Electrolysis of dilute H₂SO₄ gives oxygen at the anode and at the cathode.

4. Quantity of electricity when 0.5 A flows for 10 minutes = C.

Slide 10 · Quiz

Check your understanding

Part A: Fill in the blanks (auto-marked, 1 mark each)

1. Oxidation is the loss of .

2. The oxidation number of manganese in MnO₄⁻ is .

3. Reduction takes place at the .

4. In an electrochemical cell, electrons flow through the wire from the to the cathode.

5. The standard electrode potential of the hydrogen electrode is V.

6. During electrolysis of brine, the gas produced at the anode is .

7. One Faraday is equal to coulombs.

8. In the purification of copper, impure copper is made the .

9. Two half-cells are connected by a wire and a salt .

10. Quantity of electricity = current × .

Part B: Short answers (smart-marked against key points)

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