This topic accounts for approximately 7% of your exam marks.
stable
Low
Stable7%
Calorimetry calculations and energy profile diagrams appear in nearly every series.
Conservation of energy
Energy cannot be created or destroyed during a chemical reaction — only transferred between the reaction mixture (the system) and its surroundings
When energy leaves the system, the surroundings warm up by the same amount
When energy enters the system, the surroundings cool down by the same amount
A thermometer placed in the reaction mixture is the simplest way to detect this transfer
Exothermic reactions
An exothermic reaction transfers energy out of the system to the surroundings
The temperature of the reaction mixture, together with any solvent it sits in, rises
The chemical energy stored in the products is lower than in the reactants — the difference is what escapes as heat
The enthalpy change is negative: ΔH < 0
Reactions that are always exothermic include:
of fuels
of an acid by a base
The reaction of a reactive metal with water or acid
Many displacement and oxidation reactions
Endothermic reactions
An endothermic reaction transfers energy into the system from the surroundings
The temperature of the reaction mixture falls
The products store more chemical energy than the reactants
The enthalpy change is positive: ΔH > 0
Reactions that are commonly endothermic include:
of metal carbonates
The reaction between citric acid and sodium hydrogencarbonate
The dissolving of some ionic salts (e.g. ammonium nitrate) — useful in cold packs
Photosynthesis (energy supplied by sunlight)
Reading the experimental data
Reaction
Initial T / °C
Final T / °C
ΔT / °C
Type
20 cm³ HCl + 20 cm³ NaOH
18.0
24.5
+6.5
Exothermic (T rose)
5 g KNO₃ stirred into 50 cm³ water
21.0
17.2
−3.8
Endothermic (T fell)
A rise in temperature = exothermic; a fall = endothermic. Mark schemes credit the direction of the temperature change plus the named conclusion
Exam tip
Classifying a reaction as exothermic or endothermic
What comes up: given a reaction or an experiment, state whether it is exothermic or endothermic and justify the answer.
Write (two marks): (1) the temperature of the mixture/surroundings increases (for exothermic) or decreases (for endothermic); (2) so the reaction is exothermic/endothermic. Both the temperature change direction and the correct classification are needed.
Watch out: naming the type without stating the temperature change direction drops the justification mark. Simply writing "heat is released" without tying it to a temperature rise or fall is not enough.