This module covers how energy moves in chemical reactions: whether a change releases or absorbs energy, how an energy profile shows it, what activation energy is, and how bond energies let you estimate the energy change. It connects particles and bonding to the behaviour you observe, and it supports later work on rates and equilibrium.
Check the current Cambridge IGCSE Chemistry 0620 syllabus for the exact wording of each content point in your exam year. Our Chemistry learning guide shows where this module sits among the others.
What should you already know?
You should be able to write and balance simple formulae and equations, and know that a covalent molecule such as H₂ or H₂O is held together by bonds. The formulas and equations module covers that. You only need basic subtraction and multiplication for the calculations.
An orienting example
The values here are invented for practice. Bond energies supplied: H–H 436, Cl–Cl 242, H–Cl 431 (kJ/mol). Estimate the energy change for H₂ + Cl₂ → 2HCl.
Step 1, broken: 1 H–H and 1 Cl–Cl need 436 + 242 = 678 kJ/mol.
Step 2, made: 2 H–Cl release 2 × 431 = 862 kJ/mol.
Step 3, subtract: broken − made = 678 − 862 = −184 kJ/mol.
Interpretation: more energy is released than absorbed, so the reaction is exothermic and ΔH is negative. On an energy profile the products would sit below the reactants.
Check: 436 + 242 = 678 and 2 × 431 = 862, so the answer is −184 again.
That example used the idea of release and absorption, the profile picture and bond breaking versus making. The module teaches each one separately.
In which order should you study it?
- Distinguish energy released from energy absorbed: the language of exothermic and endothermic.
- Interpret an energy profile: reading levels and the sign of ΔH.
- Explain activation energy on a diagram: the barrier, and what a catalyst changes.
- Estimate an energy change using supplied bond energies: the calculation.
- Separate bond breaking from bond making: the explanation behind the sign.
Then work through the mixed practice set. A steady pace is one lesson a day with the practice set at the weekend.
Which traps catch most students here?
- Reversing the subtraction and calculating made minus broken.
- Forgetting coefficients when counting bonds.
- Using the peak to work out ΔH instead of the two flat levels.
- Claiming a catalyst changes ΔH when it only changes the activation energy.
- Saying exothermic reactions need no starting energy.
Each lesson shows at least one of these and corrects it.
How should you use the practice set?
Attempt each question on paper before opening the answer, with the units written. If you want more counting support, the equation balance reasoning trainer checks atoms step by step, and the mole and equation-ratio tutor supports the amount work around it.
When you get something wrong, go to the lesson named in the routing notes, then retry a fresh question a few days later.
If you want teaching beyond self-study, online one-to-one Chemistry tuition means an experienced teacher reads your written working and finds the habit behind the error.