Contents
Return after a break
What can you recall?
deduce
Recall and explain
Supplement. For the reaction X2 + Y2 → 2XY, breaking the bonds in the reactants takes in 600 kJ/mol, and making the bonds in the product gives out 720 kJ/mol. The activation energy is 170 kJ/mol. Which row describes the reaction pathway diagram correctly?
deduce: reach a conclusion from the information given.
| Row | Products' level | Top of the hump | ΔH arrow |
|---|---|---|---|
| A | 120 kJ/mol below the reactants | 600 kJ/mol above the reactants | down, from the reactants' level to the products' level |
| B | 120 kJ/mol below the reactants | 170 kJ/mol above the reactants | down, from the reactants' level to the products' level |
| C | 120 kJ/mol above the reactants | 170 kJ/mol above the reactants | up, from the reactants' level to the products' level |
| D | 120 kJ/mol below the reactants | 170 kJ/mol above the reactants | down, from the top of the hump to the products' level |
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Scope and sources
Cambridge IGCSE Chemistry 0620 · 2026–2028 v2 · Original learning materials.
Bond breaking and bond making (5.1 Supplement 7–8): bond breaking as endothermic and bond making as exothermic, the enthalpy change of a reaction from the energy in and the energy out, and calculating ΔH from bond energies. Exothermic and endothermic reactions, reaction pathway diagrams and activation energy are in the previous module; Hess’s law and energy cycles are not part of 0620.
Apparatus, amounts and times in the experiment are illustrative. Real practical work follows a teacher-approved safety method.
Official Cambridge syllabus