Exothermic and endothermic energy transfers
Use the direction of thermal energy transfer and the reactant–product levels to classify a reaction.
Core and Extended · Chemistry 0620, 2026–2028. Enthalpy signs, activation energy and bond-energy calculations follow in Extended lessons.
Before you begin
- Distinguish a reaction mixture from its surroundings.
- Read a diagram with an energy axis and reactant and product levels.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is a temperature increase?
What you will learn
- Describe exothermic and endothermic reactions through thermal energy transfers.
- Interpret the relative reactant and product energy levels on reaction pathway diagrams.
Name the surroundings when describing a temperature change
An exothermic reaction transfers thermal energy to its surroundings, so their temperature increases when the transfer is observed without another process masking it. An endothermic reaction takes in thermal energy from its surroundings, so their temperature decreases. The surroundings can include the solution and apparatus used to monitor the change; make the stated boundary clear instead of saying energy appears or disappears.
A temperature rise in a well-defined classroom reaction setup supports an exothermic classification; a temperature fall supports an endothermic one. A hot flame supplied from outside is not by itself proof that a reaction is exothermic. A reaction can need an initial input to start and still transfer energy out overall. Interpret the direction of net transfer rather than the fact that some heating was used.
Compare the starting and finishing energy levels
On a reaction pathway diagram, the vertical axis is energy and the horizontal axis shows progress through the reaction, not elapsed time. An exothermic diagram ends with products at a lower energy level than reactants; the difference corresponds to energy transferred out. An endothermic diagram ends with products at a higher level; energy has been taken in overall.
The curve can rise over a peak before reaching its final level in either type of reaction. For Core interpretation, identify reactants, products and their relative levels before classifying the net energy transfer. The peak's numerical activation energy and the sign of the enthalpy change are Extended topics and are labelled separately in the next lesson.
Pause and explain
Which level comparison identifies an exothermic pathway?
Worked example
A reaction pathway diagram starts with reactants at a schematic energy level of 80 units and ends with products at 30 units. Classify the reaction and predict the thermal transfer to the surroundings.
Show the worked solution
- The product level is lower than the reactant level.
- The decrease in chemical energy corresponds to thermal energy transferred to the surroundings.
- The reaction is exothermic; under the stated simple observation model, the surroundings become warmer.
Answer Exothermic: thermal energy transfers to the surroundings.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
In a simple reaction measurement, surroundings rise from 21 °C to 29 °C. State the temperature change, classify the reaction and give the energy direction.
Give me a hint
A rise indicates the surroundings received thermal energy in the stated model.
Compare my reasoning
- The observed rise is 29−21=8 °C.
- Thermal energy transfers from the reaction to the surroundings.
- The reaction is exothermic.
8 °C rise; exothermic; thermal energy transfers to the surroundings.
Look for these in your work
- The observed change is calculated with the correct direction.
- The classification includes the energy transfer.
A pathway starts at 40 schematic energy units and finishes at 65. Interpret the levels and classify the reaction without using the peak height.
Give me a hint
Compare only products with reactants for the net transfer.
Compare my reasoning
- Products are 25 schematic units above reactants.
- Energy has been taken in overall.
- The reaction is endothermic, taking thermal energy from the surroundings.
Endothermic: products higher than reactants and net thermal energy taken in.
Look for these in your work
- The two endpoint levels determine the classification.
- Schematic units are not turned into a measured enthalpy value.
A reaction needs a spark to start, then makes its surroundings warmer. A student calls it endothermic because the spark added energy. Explain why the overall classification is exothermic.
Give me a hint
Starting input and net energy transfer are different questions.
Compare my reasoning
- The spark provides an initial input to start the reaction.
- The observed overall transfer warms the surroundings.
- Therefore the net reaction is exothermic; needing an initial input does not change that classification.
Initial input can start an exothermic reaction; the net transfer here is to the surroundings.
Look for these in your work
- Starting energy is kept separate from net transfer.
- The observed overall direction supports the classification.
Common mistakes
- Calling any reaction started by heating endothermic.
- Treating the horizontal pathway axis as a time measurement.
- Comparing the peak with products while ignoring the starting reactant level.
What can you explain now?
A pathway diagram ends with products above reactants. Classify the reaction and state the direction of thermal transfer.
Compare with the explanation
Endothermic; thermal energy is taken in from the surroundings.
The higher product level means energy is absorbed overall, which can cool the surroundings in the simple observation model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch one exothermic and one endothermic pathway with labelled endpoints. Explain their thermal-transfer directions and why a starting spark does not determine the classification.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.