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K326 / K328 / 2027
Chemical Energetics

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Chemical Energetics

Explain temperature changes and the energy cost of rearranging particles.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

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?

  1. The dissolving gave out energy, so the water lost heat
  2. The dissolving gave out cold energy to the water
  3. The dissolving took in energy from the water and your hand
Show the answer

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.

Two useful examples
ProcessObservationEnergy direction
Aqueous sodium hydroxide reacts with hydrochloric acidTemperature risesReaction releases energy to the surroundings
Ammonium nitrate dissolves in waterTemperature fallsDissolving 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.

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?

  1. Exothermic, because energy moved from the reacting chemicals to the solution
  2. Endothermic, because the reaction mixture gained energy and grew warmer
  3. Neither, because a temperature rise of 7 degrees C is too small to judge
Show the answer

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.

02

Investigate an energy change

Make the measurement large enough to interpret fairly.

Measure the initial temperatures, mix the specified quantities in an insulated cup, stir consistently and record the highest or lowest temperature reached. A lid reduces energy transfer to the air; the probe should measure the mixture rather than rest on the cup. Use the same volumes, concentrations and apparatus when comparing repeats.

Worked exampleInterpret a cooling experiment

Water starts at 24 degrees C. After a salt dissolves, the lowest temperature is 18 degrees C, before slowly returning towards 24.

Try it first. Then show the worked stepsHide the worked steps
  1. The initial fall is evidence that the dissolving process absorbed energy.
  2. The change in the measured temperature is 18 - 24 = -6 degrees C.
  3. The later warming occurs as the cooler solution gains energy from the room.
Answer

Dissolving is endothermic. The later return towards room temperature does not change that classification.

A smaller observed temperature change may result from heat exchange with the room, a different amount of reacting substance or a different amount of water being warmed. It need not mean a different reaction type. Report measurements and limitations rather than claiming that temperature change alone is an energy value in joules.

Check your understanding

How would heat loss to the room affect the measured temperature rise in an exothermic experiment?

  1. The measured rise would be smaller than the true rise.
  2. The measured rise would be larger than the true rise.
  3. The reaction would turn endothermic in an open cup.
Show the answer

The measured rise would be smaller than the true rise.

Some of the energy released escapes to the room instead of warming the solution, so the thermometer under-reads. The reaction itself is still exothermic.

Quick revision

Revisit the essentials, then return to an explanation when you need it.

Exam definitions: learn these in mark-scheme words
TermDefinition to write
Exothermic reactionA reaction that transfers energy, usually as heat, to the surroundings, so the temperature of the surroundings rises.
Endothermic reactionA reaction that takes in energy, usually as heat, from the surroundings, so the temperature of the surroundings falls.

Exothermic: energy out to surroundings. Endothermic: energy in from surroundings. Use the initial change and account for later heat exchange.

Scope

Learning outcomes

9. Chemical Energetics (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 9(a) Explain exothermic processes

    • Energy transferred to surroundings
    • Temperature increase
    • NaOH with HCl

    Follow the energy transferInvestigate an energy change

  2. 9(b) Explain endothermic processes

    • Energy absorbed from surroundings
    • Temperature decrease
    • Ammonium nitrate dissolving

    Follow the energy transferInvestigate an energy change