CAIE A-Level Chemistry 5.1.2 Reaction Pathway Diagrams

Practise drawing and interpreting one-step or multistep profiles with ΔH and forward or reverse activation energies.

Exam points

place products below reactants for exothermic reactions and above them for endothermic reactions
measure ΔH vertically between reactant and product levels and activation energy from reactants to the peak
use one peak per elementary step and identify the rate-determining step from the largest activation barrier

[Maximum number: 3]

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

Magnesium, Mg , burns in oxygen, $\mathrm{O}_{2}$ .

The activation energy, $E_{\mathrm{a}}$ , for this reaction is $+148 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

On Fig. 1.1:
- sketch a reaction pathway diagram for the reaction that occurs when Mg burns in $\mathrm{O}_{2}$
- label the diagram to show the enthalpy change, $\Delta H$ , and the activation energy, $E_{\mathrm{a}}$ , for the reaction.

Fig. 1.1

Correct exothermic reaction pathway diagram is shown in the attached official markscheme figure.

Marking points:
M1 sketch shows an exothermic reaction with a hump, labelled reactants $\mathrm{Mg}+\mathrm{O}_{2}$ and product MgO.
M2 arrow from reactants / $\mathrm{Mg}+\mathrm{O}_{2}$ to product / MgO shown as $\Delta H$ .
M3 arrow showing activation energy / $E_{\mathrm{a}}$ / (+)148.

Figure for Answer to Question 5.1.2—Construct/interpret a reaction pathway diagram question 1 — CAIE A-Level Chemistry AS

All question bank results loaded

5.1.2—Construct/interpret a reaction pathway diagram question 1