1. Does breaking a chemical bond release energy?
- Yes always
- No, it requires energy
- Only in a catalyst
Check the answer and reasoning
No, it requires energy
Bond breaking absorbs energy; bond formation releases it.
CHEM / IGCSE
Energy changes, rates and reversible reactions
Exothermic reactions transfer energy to surroundings; endothermic reactions take it in. Breaking bonds requires energy and making bonds releases it.
Learning objectives
- Explain energy changes, rates and reversible reactions using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Exothermic reactions transfer energy to surroundings; endothermic reactions take it in. Breaking bonds requires energy and making bonds releases it.
Collision theory explains effects of concentration, surface area and temperature. Catalysts provide an alternative pathway of lower activation energy. Dynamic equilibrium balances forward and reverse rates in a closed system; changing conditions can shift composition.
Equations and relationships
mean rate = change in quantity / time
State what quantity is measured.
Worked example
A gas volume rises by 60 cm³ in 20 s. Find mean production rate.
Follow the solution
Divide the volume change by elapsed time: 60/20=3.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Use the evidence activity below; no exact PhET model is claimed.
Check your understanding
2. A gas volume rises by 60 cm³ in 20 s. Find mean production rate.
- 1200 cm³/s
- 1/3 cm³/s
- 3 cm³/s
Check the answer and reasoning
3 cm³/s
Divide the volume change by elapsed time: 60/20=3.
3. Does a catalyst change an equilibrium constant at fixed temperature?
- No
- Yes
- Only if powdered
Check the answer and reasoning
No
It accelerates both directions without changing equilibrium thermodynamics.