Energetics & Enthalpy

Enthalpy diagrams, Hess's Law & bond enthalpies — A-Level Chemistry

Enthalpy Profile Diagrams

An enthalpy diagram shows the energy change during a reaction. The y-axis represents enthalpy (H), and the x-axis represents the progress of the reaction.

Hess's Law

The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This allows us to calculate ΔH values that can't be measured directly.

A + B C + D ΔH (Route 1) Elements ΔH°f (A+B) ΔH°f (C+D) ΔH = ΣΔH°f(products) − ΣΔH°f(reactants) Route 1 = Route 2 Can also use: ΔH = ΣΔH°c(reactants) − ΣΔH°c(products) Note: combustion formula has REVERSED signs vs formation formula

Quick Calculator — Hess's Law (using formation enthalpies)

ΔH°reaction = ΣΔH°f(products) − ΣΔH°f(reactants)

kJ/mol
kJ/mol
Exam tip: The most common error is getting the signs wrong for combustion cycles. Remember: for formation, it's products minus reactants. For combustion, it's reactants minus products (reversed). Draw the cycle diagram to keep track.

Bond Enthalpy Calculations

Bond enthalpy is the energy required to break one mole of a specific covalent bond in the gaseous state. Breaking bonds is endothermic (energy absorbed); making bonds is exothermic (energy released).

ΔH = Σ(bonds broken) − Σ(bonds made)

Bond Enthalpy Calculator

kJ/mol
kJ/mol
Exam tip: Bond enthalpies are averages, so calculations using them give approximate ΔH values. Formation/combustion data gives more accurate values because they're measured for specific compounds. If asked "why is there a difference?" — this is the answer.

Key Definitions

These must be learned precisely — examiners penalise vague or incomplete definitions.

Exothermic reactionA reaction that transfers energy to the surroundings (ΔH is negative). Temperature of surroundings increases.
Endothermic reactionA reaction that absorbs energy from the surroundings (ΔH is positive). Temperature of surroundings decreases.
Standard enthalpy of formation (ΔH°f)The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions (298K, 100kPa).
Standard enthalpy of combustion (ΔH°c)The enthalpy change when one mole of a substance is completely burned in excess oxygen, under standard conditions.
Bond enthalpyThe energy required to break one mole of a specific covalent bond in the gaseous state, averaged over many different compounds.
Activation energy (Ea)The minimum energy that colliding particles must have in order to react. Shown as the "hump" on an enthalpy profile diagram.
Enthalpy of neutralisationThe enthalpy change when one mole of water is formed by the reaction of an acid with an alkali, under standard conditions.
Standard conditions298K (25°C), 100 kPa, solutions at 1 mol dm⁻³. All substances in their standard states.