Enthalpy change: the definition and what the sign means
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
An enthalpy change is the heat energy transferred in a reaction at constant pressure. It has the symbol delta H and the units kJ per mole.
Words that must appear
- Heat energy transferred
- At constant pressure
The sign
Exothermic: delta H is negative, heat is released and the surroundings warm up. Endothermic: delta H is positive, heat is taken in and the surroundings cool down.
Do not confuse with
Temperature change. Enthalpy change is the heat transferred, in kJ per mole.
Common questions
What is an enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What is the sign of delta H for an exothermic reaction?
Negative.
What are the units of enthalpy change?
kJ per mole.
Trayna does this automatically. You enter your subjects, your exam dates and how long you have each day, and it builds the route from today to those exams, then adapts it as you go. Maths follows Edexcel; Chemistry, Physics and Biology follow AQA.
Get Trayna on the App Store · on Google Play
Free for 14 days, then from £9.99 a month. Cancel any time.