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IGCSE · Practice questions · Reaction pathway diagrams

Draw the hill.

Six original Cambridge-style questions on reaction pathway diagrams: interpreting them, drawing and labelling them, and reading off Ea and ΔH.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Start every arrow at the reactants.

01
Recall
[2 marks]

Describe how a reaction pathway diagram shows whether a reaction is exothermic or endothermic.

  • If the products are at a lower energy than the reactants, the reaction is exothermic. ✓
  • If the products are at a higher energy than the reactants, the reaction is endothermic. ✓
02
Recall
[1 marks]

(Extended) Define activation energy.

  • The minimum energy that colliding particles must have in order to react. ✓
03
Analysis
[4 marks]

(Extended) Sketch and label a reaction pathway diagram for the combustion of methane, CH4 + 2O2 → CO2 + 2H2O, which is exothermic.

  • Energy on the vertical axis, progress of reaction on the horizontal axis. ✓
  • Reactants CH4 + 2O2 at a higher level; products CO2 + 2H2O at a lower level, joined by a curve with a hill. ✓
  • Ea: an upward arrow from the reactants level to the top of the hill. ✓
  • ΔH: a downward arrow from the reactants level to the products level, labelled as negative. ✓
04
Analysis
[3 marks]

(Extended) On a reaction pathway diagram the reactants are at 120 kJ, the highest point is at 300 kJ and the products are at 190 kJ. Calculate Ea and ΔH, and state whether the reaction is exothermic or endothermic.

  • Ea = 300 − 120 = 180 kJ. ✓
  • ΔH = 190 − 120 = +70 kJ. ✓
  • The products are higher than the reactants, so the reaction is endothermic. ✓
05
Explain
[2 marks]

(Extended) State and explain the effect of a catalyst on the activation energy and on the enthalpy change of a reaction.

  • The activation energy is lowered, because the catalyst provides an alternative reaction pathway. ✓
  • The enthalpy change is unchanged, because the energies of the reactants and products are not affected. ✓
06
Explain
[2 marks]

Explain why a mixture of hydrogen and oxygen does not react until a lighted splint is applied, although the reaction is very exothermic.

  • The reaction has an activation energy; at room temperature very few collisions have enough energy to react. ✓
  • The flame supplies the energy needed; the energy released by the reaction then provides the activation energy for further reaction. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.