IGCSE Chemistry: Chemical Energetics Practice Questions
An exothermic reaction transfers energy to the surroundings, so the temperature rises and the enthalpy change is negative. An endothermic reaction takes energy in from the surroundings, so the temperature falls and the enthalpy change is positive.
Topic 5 of Cambridge IGCSE Chemistry 0620 is short and almost entirely predictable. Bond energy calculations follow one rule, and the sign convention decides several marks. The questions below drill the calculation and the reasoning behind bond breaking and bond making.
What you need to know for Chemical Energetics
- ExothermicEnergy is transferred from the reaction to the surroundings. The temperature of the surroundings rises and the enthalpy change is negative. Combustion, neutralisation and most oxidation reactions are exothermic.
- EndothermicEnergy is taken in from the surroundings. The temperature of the surroundings falls and the enthalpy change is positive. Thermal decomposition and photosynthesis are endothermic.
- Bond breaking and bond makingBreaking bonds always requires energy, so it is endothermic. Making bonds always releases energy, so it is exothermic.
- Bond energy calculationEnthalpy change = total energy to break the bonds in the reactants minus total energy released when the bonds in the products form. A negative answer means exothermic.
- Reaction pathway diagramFor an exothermic reaction the products are drawn lower than the reactants. For an endothermic reaction the products are drawn higher. The hump between them is the activation energy.
- Activation energyThe minimum energy that colliding particles must have for a reaction to occur. A catalyst provides an alternative pathway with a lower activation energy.
IGCSE Chemistry Chemical Energetics questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Hydrogen reacts with chlorine: H2 + Cl2 gives 2HCl. Bond energies in kJ per mole are H-H 436, Cl-Cl 242 and H-Cl 431. Calculate the enthalpy change and state whether the reaction is exothermic or endothermic.
Show the worked answer
- Bonds broken: one H-H bond and one Cl-Cl bond, so 436 + 242 = 678 kJ taken in.
- Bonds formed: two H-Cl bonds, so 2 x 431 = 862 kJ released.
- Enthalpy change = energy in minus energy out = 678 minus 862.
- Enthalpy change = minus 184 kJ per mole. The value is negative, so more energy is released forming bonds than is used breaking them, and the reaction is exothermic.
Explain why breaking a chemical bond is always an endothermic process, while forming a bond is always exothermic.
Show the worked answer
- A chemical bond exists because there is a strong force of attraction holding the atoms together.
- To separate the atoms, that attraction must be overcome, which requires energy to be supplied from outside. Any process that takes energy in is endothermic.
- When a bond forms, the atoms move to a lower energy arrangement as the attraction takes effect.
- The surplus energy is released to the surroundings, so bond formation is exothermic. Whether a whole reaction is exothermic or endothermic depends on which of the two totals is larger.
Describe how you would draw a reaction pathway diagram for an exothermic reaction, and label the two features that must be shown.
Show the worked answer
- Draw energy on the vertical axis and progress of reaction on the horizontal axis.
- Draw the reactants as a horizontal line, then a curve rising to a peak and falling to a lower horizontal line for the products. The products must be lower than the reactants because energy has been released.
- Label the vertical distance from the reactants line up to the peak as the activation energy, the minimum energy needed for a successful collision.
- Label the vertical distance from the reactants line down to the products line as the enthalpy change, which is negative for an exothermic reaction. Draw the arrow pointing downwards.
A student dissolves a salt in water in a polystyrene cup and the temperature of the solution falls from 21.0 to 15.5 degrees Celsius. State whether the process is exothermic or endothermic, explain your answer, and explain why a polystyrene cup is used.
Show the worked answer
- The temperature of the solution falls by 5.5 degrees Celsius.
- A fall in temperature means energy has been taken from the water by the dissolving process, so the process is endothermic.
- The enthalpy change is therefore positive.
- Polystyrene is a poor conductor of thermal energy, so it reduces energy transfer between the contents and the surroundings. This makes the measured temperature change closer to the true value. A lid would reduce losses further.
Common mistakes in this topic
- Losing the negative sign on an exothermic enthalpy change.
- Subtracting bonds broken from bonds formed rather than the other way round.
- Confusing the system with the surroundings. An exothermic reaction makes the surroundings hotter.
- Drawing products above reactants for an exothermic reaction.
- Saying a catalyst changes the enthalpy change. A catalyst lowers the activation energy only.
Exam tips
- Write the two totals separately and label them broken and formed before subtracting. That structure earns method marks.
- Count every bond in the molecule. Water has two O-H bonds, methane has four C-H bonds.
- Multiply by the large number in front of the formula. 2HCl means two H-Cl bonds.
- Check the sign against the chemistry. Combustion is always exothermic, so a positive answer for a combustion reaction means an error.
- In practical questions, name insulation and a lid as improvements, and explain what each reduces.
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Chemical Energetics FAQs
How do I calculate enthalpy change from bond energies?
Add the bond energies of all the bonds broken in the reactants, then add the bond energies of all the bonds formed in the products. Subtract the second total from the first. A negative answer means the reaction is exothermic, and a positive answer means it is endothermic.
Is bond breaking exothermic or endothermic?
Bond breaking is always endothermic, because energy must be supplied to overcome the force of attraction holding the atoms together. Bond making is always exothermic, because energy is released as the atoms move to a lower energy arrangement. Whether the overall reaction is exothermic depends on which total is larger.
What is activation energy?
Activation energy is the minimum energy that colliding particles must have for a reaction to take place. On a reaction pathway diagram it is the height from the reactants line up to the peak. A catalyst provides an alternative route with a lower activation energy, so more collisions are successful.
Why does the temperature fall in an endothermic reaction?
An endothermic reaction takes energy in from its surroundings. In a solution the surroundings are the water, so energy is removed from the water and its temperature falls. The reaction itself is gaining energy, which is why the enthalpy change is positive.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.