Energetics and thermodynamics: every key term you need (+ practice quiz)

Enthalpy change

Heat energy transferred at constant pressure. Negative for exothermic reactions, positive for endothermic ones.

Standard conditions

100 kilopascals, a stated temperature (usually 298 kelvin), and all solutions at one mole per cubic decimetre.

Hess's law

The total enthalpy change is independent of the route taken. Lets you calculate values that cannot be measured directly.

Mean bond enthalpy

The average energy to break one mole of a given bond, taken across many compounds — hence calculations using it are approximate.

Calorimetry equation

Energy transferred equals mass of solution times its specific heat capacity times temperature change. Divide by moles for the molar value.

Heat loss in calorimetry

The main source of error, making measured exothermic values less negative. Reduced by insulation or by extrapolating a cooling curve.

Enthalpy of formation

The enthalpy change when one mole of a compound forms from its elements in standard states. Zero for an element by definition.

Enthalpy of combustion

The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions. Always negative.

Lattice enthalpy

The enthalpy change forming one mole of an ionic solid from gaseous ions. Grows more exothermic with higher charge and smaller ionic radius.

Born-Haber cycle

An enthalpy cycle used to find lattice enthalpy indirectly, from atomisation, ionisation, electron affinity and formation values.

Enthalpy of hydration

The enthalpy change when one mole of gaseous ions dissolves in water to give an infinitely dilute solution. Always exothermic.

Entropy

A measure of the disorder of a system. Increases with temperature, on melting or boiling, and when the number of gas molecules rises.

Gibbs free energy

Enthalpy change minus temperature times entropy change. A reaction is feasible only when this value is negative or zero.

Temperature and feasibility

For a reaction with positive entropy change, raising temperature makes the free energy term more negative, so it becomes feasible above a threshold.

Kinetic vs thermodynamic control

A feasible reaction may still be immeasurably slow if its activation energy is high — feasibility says nothing about rate.

Perfect ionic model

Assumes ions are point charges. A difference between theoretical and experimental lattice enthalpy indicates covalent character.

More A-Level Chemistry guides

• Atomic structure and the periodic table
• Bonding, structure and intermolecular forces
• Amount of substance and quantitative chemistry
• Kinetics, equilibria and acids
• Redox, electrochemistry and transition metals