Interpret enthalpy changes and standard terms
Connect reaction heat transfer to the sign of ΔH and define standard, reaction, formation, combustion and neutralisation enthalpy changes.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Report signs, states, units and significant figures consistently. Standard conditions use 298 K and 101 kPa in this syllabus. Written practice is self-checked
Before you begin
- Rearrange a formula, balance a chemical equation and use temperature differences.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What sign does an exothermic reaction have for ΔH?
What you will learn
- Distinguish exothermic and endothermic changes from the sign of ΔH
- Define standard conditions and standard symbol
- Define reaction, formation, combustion and neutralisation enthalpy changes
The sign of ΔH follows the system's energy change
Enthalpy change, ΔH, describes energy transferred at constant pressure for the reaction as written. If energy is released to the surroundings, the reaction is exothermic and ΔH is negative. If energy is taken in, the reaction is endothermic and ΔH is positive.
The sign refers to the reacting system. A temperature rise in the surroundings usually indicates that the reaction released energy, so ΔH is negative; a temperature fall usually indicates that it absorbed energy, so ΔH is positive.
Standard conditions and named enthalpy changes
For this syllabus, standard conditions are 298 K and 101 kPa, shown by the standard symbol ⦵. State the physical states and conditions when writing a thermochemical equation.
ΔHᵣ is the enthalpy change for a reaction as written. ΔHf is formation of one mole of a compound from its constituent elements in their standard states. ΔHc is complete combustion of one mole of a substance in oxygen. ΔHneut is the enthalpy change when an acid and alkali react to form one mole of water.
Pause and explain
What amount of product is specified in the definition of ΔHneut?
Worked example
A reaction warms its surroundings. State whether it is exothermic or endothermic and give the sign of ΔH for the reacting system.
Show the worked solution
- Energy leaves the reacting system and enters the surroundings.
- The reaction is exothermic.
- For the system, the enthalpy change is negative.
Answer The reaction is exothermic and ΔH is negative.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A reaction takes in 18 kJ from its surroundings. Classify the reaction and state ΔH for the system.
Give me a hint
Track energy from the surroundings into the reacting system.
Compare my reasoning
- The system absorbs energy.
- That is endothermic.
- Absorbed energy gives a positive enthalpy change.
The reaction is endothermic and ΔH = +18 kJ for the reaction amount stated.
Look for these in your work
- Direction of energy transfer correct
- Endothermic classification correct
- Positive sign stated
Define ΔHf and standard conditions for this syllabus.
Give me a hint
Include the amount formed, element reference states and stated conditions.
Compare my reasoning
- One mole of the compound is formed.
- The starting substances are its constituent elements in their standard states.
- Use 298 K and 101 kPa; mark standard values with ⦵.
ΔHf is the enthalpy change when one mole of a compound forms from its constituent elements in their standard states. Here standard conditions are 298 K and 101 kPa, shown by ⦵.
Look for these in your work
- One mole specified
- Elements in standard states specified
- Standard conditions and notation correct
Distinguish ΔHc and ΔHneut by naming the process and the one-mole basis for each.
Give me a hint
For combustion, identify what is burned; for neutralisation, identify what is formed.
Compare my reasoning
- Combustion is complete burning of one mole of a substance in oxygen.
- Neutralisation forms one mole of water from acid and alkali.
- Both are enthalpy changes for their stated processes.
ΔHc is the enthalpy change for complete combustion of one mole of a substance in oxygen. ΔHneut is for acid–alkali reaction forming one mole of water.
Look for these in your work
- Complete combustion included
- One mole of substance included
- One mole water included
Common mistakes
- Giving a positive sign for energy released by the reacting system
- Defining formation for an amount other than one mole
- Defining combustion without complete burning in oxygen
- Omitting the one mole of water in neutralisation enthalpy
What can you explain now?
What sign does an exothermic reaction have for ΔH?
Compare with the explanation
Negative
The reacting system releases energy.
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Make it stick
What does the sign of ΔH say about energy transfer, and what is the one-mole basis of each named standard enthalpy?
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