Every reaction either releases energy to the surroundings or takes it in. Chemists measure that exchange as an enthalpy change, and a handful of precise definitions let you compare one reaction with another.
An enthalpy change, ΔH, is the heat energy exchanged with the surroundings at constant pressure. Exothermic reactions release heat, so ΔH is negative; endothermic reactions absorb heat, so ΔH is positive. Standard values (⊖) are for 298 K, 101 kPa and 1 mol dm−3 solutions, with substances in their standard states. Learn four definitions exactly: reaction (the molar quantities in the equation), formation (1 mol of compound from its elements), combustion (1 mol burned completely in oxygen) and neutralisation (1 mol of water formed).
Switch between reactions in the enthalpy profile simulator. Watch whether the products finish above or below the reactants, and how the activation energy hump changes when a catalyst is added.
| Exothermic | Endothermic | |
|---|---|---|
| sign of ΔH | negative | positive |
| products on diagram | below reactants | above reactants |
| surroundings | warm up | cool down |
| examples | combustion, neutralisation, respiration | thermal decomposition of CaCO3, photosynthesis |
The activation energy, Ea, is the minimum energy colliding particles need to react. It is drawn from the reactants up to the top of the hump, for exothermic and endothermic reactions alike.
Open the definitions tab and match each equation to its definition. The equation must show exactly one mole of the right substance, in its standard state.
| Enthalpy change | Definition (standard conditions) | Example equation |
|---|---|---|
| formation, ΔHf⊖ | 1 mol of a compound formed from its elements in their standard states | 2C(s) + 3H2(g) + ½O2(g) → C2H5OH(l) |
| combustion, ΔHc⊖ | 1 mol of a substance burned completely in oxygen | CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) |
| neutralisation | 1 mol of water formed when an acid reacts with a base | H+(aq) + OH−(aq) → H2O(l) |
| reaction, ΔHr⊖ | the molar quantities shown in a given equation react | N2(g) + 3H2(g) → 2NH3(g) |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
For an exothermic reaction, ΔH is:
Standard conditions for enthalpy changes are:
What is the standard enthalpy change of formation of O2(g)?
The activation energy on a reaction pathway diagram is measured from:
Most marks lost on definitions are lost in the equation, not the words. Check four things every time.
The standard enthalpy change of formation of water and the standard enthalpy change of combustion of hydrogen are the same equation, H2(g) + ½O2(g) → H2O(l), so they have the same value, −286 kJ mol−1. Writing 2H2 + O2 → 2H2O doubles the value and fits neither definition.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which equation represents the standard enthalpy change of formation of ethanol?
Which enthalpy change has the same value as the standard enthalpy change of formation of CO2(g)?
Why is the enthalpy change of neutralisation of any strong acid with any strong base about −57 kJ mol−1?
Thermal decomposition of calcium carbonate has ΔH = +178 kJ mol−1. Which statement is correct?
This skill is now recorded on your profile. Keep the chain going.