Lesson 1 of 4
Follow the energy transfer
A temperature change is evidence about the surroundings.
O-Level 6092 (2026) / SEC G3 K324 (2027)
I can explain enthalpy-change sign.Syllabus
Syllabus K324, 9(a). Explain enthalpy-change sign
Follow the energy transfer
A temperature change is evidence about the surroundings.
Make a guess. You are not marked.
A solid dissolves in water and the beaker feels cold to touch. What happened to energy?
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The dissolving took in energy from the water and your hand
The process absorbed energy from its surroundings, so the surroundings cooled. That is an endothermic change.
Define the system as the reacting substances and the surroundings as what exchanges energy with them, such as the water, container and air. An exothermic process transfers energy to its surroundings, often as heat; the measured mixture temperature usually rises. An endothermic process takes energy from the surroundings, so their temperature usually falls.
| Process | Observation | Energy direction |
|---|---|---|
| Aqueous sodium hydroxide reacts with hydrochloric acid | Temperature rises | Reaction releases energy to the surroundings |
| Ammonium nitrate dissolves in water | Temperature falls | Dissolving process takes energy from the surroundings |
NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l) is a neutralisation. Dissolving ammonium nitrate is a physical process with an energy change; an energy transfer does not by itself prove that a new substance has formed.
Enthalpy change, ΔH, records the energy change of the reacting system at constant pressure. For an exothermic reaction products have lower enthalpy than reactants, so ΔH is negative. For an endothermic reaction products have higher enthalpy, so ΔH is positive. The sign describes the system, not the thermometer.
Check your understanding
A reaction raises the temperature of the surrounding solution from 22 degrees C to 29 degrees C. Is it exothermic or endothermic?
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Exothermic, because energy moved from the reacting chemicals to the solution
The surroundings (the water and thermometer) got warmer, so they gained energy from the reaction. The reacting system lost energy, so delta H is negative.