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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.

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 understandingA reaction raises the surrounding solution temperature from 22 to 29 degrees C. Is it exothermic or endothermic?Think it through, then reveal the answer
Exothermic: energy has been transferred from the reaction to its surroundings. A warmer thermometer does not mean the reaction system gained energy overall.
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 example

Interpret 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.

  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 understandingHow would heat loss to the room affect the observed rise in an exothermic experiment?Think it through, then reveal the answer
It usually makes the measured rise smaller than it would be with perfect insulation. It does not turn the underlying reaction into an endothermic one.

Quick revision

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

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

Scope and references

Learning outcomes and sources

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