Every chemical reaction moves thermal energy. Some push it out into the surroundings and the test tube warms up; others pull it in and the test tube feels cold. Which way it goes is written into the sign of one quantity, the enthalpy change.
An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases. An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases. Extended: the transfer of thermal energy during a reaction is the enthalpy change, ΔH. ΔH is negative for exothermic reactions and positive for endothermic reactions.
Fig. 1: the temperature of the surroundings tells you the direction of energy transfer
Section 01
Measure the temperature change.
Run the neutralisation in the calorimeter and watch the thermometer rise. Then dissolve ammonium nitrate and watch it fall. Record the start and highest (or lowest) temperatures each time.
The surroundings are everything around the reacting particles: the water, the solution, the container, the thermometer and the air.
Temperature rises: energy has left the reacting chemicals and gone into the surroundings. The reaction is exothermic.
Temperature falls: energy has been taken from the surroundings into the reacting chemicals. The reaction is endothermic.
Fair test: use an insulated container such as a polystyrene cup with a lid, so less energy is lost to or gained from the air.
Examiner trap
In an endothermic reaction the chemicals gain energy, but the thermometer shows a fall, because it measures the surroundings, which have lost energy. Do not say an endothermic reaction "gives out cold": cold is not a form of energy.
Stage 1 · Learn
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In an exothermic reaction, thermal energy is:
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When ammonium nitrate dissolves in water the temperature falls from 20 °C to 12 °C. The process is:
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Which of these is an endothermic reaction?
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Which piece of apparatus helps to reduce energy losses when measuring a temperature change?
Section 02
Everyday examples.
Learn a few examples of each type. Most reactions you meet are exothermic.
Exothermic (temperature rises)
Endothermic (temperature falls)
combustion of fuels, such as methane burning
thermal decomposition, such as heating calcium carbonate
neutralisation of an acid by an alkali
photosynthesis
reactions of metals with acids
dissolving ammonium nitrate in water
respiration
reaction of citric acid with sodium hydrogencarbonate
Uses: exothermic reactions in hand warmers and self-heating cans; endothermic reactions in instant cold packs for sports injuries.
Section 03
The enthalpy change, ΔH.
Extended. Chemists put a number and a sign on the energy transfer.
Definition: the enthalpy change, ΔH, is the transfer of thermal energy during a reaction, usually given in kJ/mol.
Exothermic: the chemicals lose energy, so ΔH is negative. Burning methane: ΔH = −890 kJ/mol.
Endothermic: the chemicals gain energy, so ΔH is positive. Decomposing calcium carbonate: ΔH = +178 kJ/mol.
Reversing a reaction reverses the sign: if A → B has ΔH = −100 kJ/mol, then B → A has ΔH = +100 kJ/mol.
Stage 2 · Exam
Exam-style questions
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(Extended) What is the sign of ΔH for an exothermic reaction, and why?
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(Extended) The reaction N2 + 3H2 → 2NH3 has ΔH = −92 kJ/mol. What is ΔH for 2NH3 → N2 + 3H2?
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A student adds magnesium to dilute hydrochloric acid and the temperature rises from 19 °C to 31 °C. What is the temperature change, and what type of reaction is it?
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Which statement about photosynthesis is correct?
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Exothermic and endothermic reactions, mastered.
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