Redox, electrochemistry and transition metals: every key term you need (+ practice quiz)

Oxidation state

The charge an atom would have if all bonds were ionic. Rises on oxidation, falls on reduction — the basis of redox half-equations.

Half-equation

Shows either the oxidation or the reduction separately, with electrons included. Combine them so electrons cancel exactly.

Standard electrode potential

The voltage of a half-cell measured against the standard hydrogen electrode under standard conditions.

Electrochemical cell EMF

The difference between the two electrode potentials. A positive value means the cell reaction is feasible as written.

Salt bridge

Completes the circuit by allowing ion flow between half-cells without letting the solutions mix.

Fuel cell

Generates a voltage continuously from a supplied fuel and oxidant, with water as the only product in the hydrogen case.

Complex ion

A central metal ion surrounded by ligands bonded through dative covalent bonds from lone pairs.

Ligand

A molecule or ion donating a lone pair to a metal ion. Monodentate donate one pair, bidentate two, multidentate several.

Chelate effect

Multidentate ligands replace monodentate ones because the large positive entropy change makes the substitution strongly favourable.

Origin of colour

Ligands split the d sub-shell. Electrons absorb visible light to jump the gap, and the transmitted complementary colour is what we see.

Variable oxidation state

Transition metals have 4s and 3d electrons of similar energy, so different numbers can be lost with comparable ease.

Heterogeneous catalyst

In a different phase from the reactants. Works by adsorbing them onto active sites, weakening bonds before desorbing the products.

Catalyst poisoning

Impurities bind irreversibly to active sites, blocking them. Why leaded petrol destroys a catalytic converter.

Ligand substitution

One ligand replaces another. The coordination number and shape may change if the incoming ligand is much larger.

Redox titration

Uses a colour change from the reagent itself rather than an indicator — manganate(VII) turning from purple to colourless at the end point.

Autocatalysis

A product of the reaction catalyses it, so the rate rises as the reaction proceeds before falling as reactants run out.

More A-Level Chemistry guides

• Atomic structure and the periodic table
• Bonding, structure and intermolecular forces
• Amount of substance and quantitative chemistry
• Energetics and thermodynamics
• Kinetics, equilibria and acids