5.2.3 Redox and Electrode Potentials

Oxidising agent

A species that is reduced in a reaction and causes another species to be oxidised

Reducing agent

A species that is oxidised in a reaction and causes another species to be reduced

Oxidation

Loss of electrons; An increase in the oxidation number

Reduction

Gain of electrons; Decrease in the oxidation number

Redox reaction

Electrons are transferred from one species to another. One element is oxidised whilst the other is reduced

Redox reaction example

2CrO4^2- + 2H+ → Cr2O7^2- + H2O is not a redox reaction because chromium is oxidised whereas hydrogen remains the same oxidation state (no element is reduced)

Half Equations for SnO + Zn → ZnO + Sn

Half Equations: Sn^2+ + 2e- → Sn; Zn → Zn^2+ + 2e-

Ionic Equation for SnO + Zn → ZnO + Sn

Sn^2+ + Zn → Sn + Zn^2+

Standard electrode potential

The e.m.f. of a half cell compared with a standard hydrogen half cell measured at 298 K with solution concentration of 1 mol dm-3 and a gas pressure of 100 kPa

Metal rod in own ions solution

An equilibrium is set up between the solid metal and the aqueous metal ions

Half-equation for zinc (s) to zinc (II)

Zn (s) ⇌ Zn^2+(aq) + 2e-

Half-equation for copper (II) to copper (III)

Cu^2+(aq) ⇌ Cu^3+(aq) + e-

Standard hydrogen half cell components

Hydrochloric acid 1 mol dm-3; Hydrogen gas at 100 kPa; Inert platinum electrode

Hydrogen half cell standard usage

Easy to control its purity and reproducibility

Simple salt bridge construction

Soak a piece of filter paper in an aqueous solution of KNO3 or NH4NO3

Necessity of salt bridges

To complete the circuit by connecting the two solutions. This enables charge to be transferred between the half cells. They do not react with the electrodes

Standard hydrogen half cell drawing

Answer

Other standard electrodes usage

They are cheaper/easier/quicker to use and can provide just as good a reference. Platinum is expensive

Eo value more negative meaning

Better reducing agent (easier to oxidise)

Eo value more positive meaning

Indicates stronger oxidising power

Eo value

Indicates oxidising/reducing power; a more positive Eo means a better oxidising agent (easier to reduce).

emf of a cell

Calculated using the formula: Eo_cell = Eo_positive - Eo_negative.

Platinum electrode

Used when both the oxidised and reduced forms of the metal are in aqueous solution.

Why is Platinum chosen?

Because it is inert and a good conductor to complete the circuit.

Predicting if a reaction will occur

Take the 2 half equations, find the species being reduced, calculate its Eo value minus the Eo value of the species being oxidised; if Eo overall > 0.4V, the reaction will occur.

Types of electrochemical cells

The three main types are Non rechargeable cells, Rechargeable cells, and Fuel cells.

Non rechargeable cells

They provide electrical energy until all the chemicals have reacted.

Rechargeable cells

Chemicals in the cell provide electrical energy; when recharging, the reactions of the cells can be reversed.

Examples of rechargeable cells

Nickel and cadmium batteries, Lithium ion batteries, Lithium polymer batteries.

Lithium in laptop batteries

Lithium has low density so the electrode is light and it is very reactive.

Drawbacks of lithium batteries

They are toxic if ingested and rapid discharge of current can cause fire or even explosions.

Fuel cells

They use external supplies of fuel and an oxidant, which need to be continuously supplied.

Modern fuel cell fuels

Based on Hydrogen and hydrogen-rich fuels like methanol.

Reactions in alkaline hydrogen fuel cell

2H2 + 4OH- → 4H2O + 4e- and O2 + 2H2O + 4e- → 4OH-.

Disadvantages of fuel cells

Hydrogen is flammable with a low boiling point, making it hard and dangerous to store and transport; fuel cells have a limited lifetime and use toxic chemicals in their manufacture.

Non-rechargeable cells reason

The reaction of the cell is not reversible; a product is produced that either dissipates or cannot be converted back into the reactants.

Change in e.m.f. over time

The concentrations of the ions change as the reagents are used up.

Keeping e.m.f. constant

Reagents are supplied constantly, so the concentrations of the ions are constant; Eo remains constant.