2.2.7 (HL)—Energy profiles

In a multistep profile, peaks are transition states and valleys between peaks are intermediates. Each step has its own Ea; the largest barrier from its preceding intermediate identifies the rate-determining step.

$ΔH = energy(products) − energy(reactants)$

For each step, measure Ea upward from its own preceding reactant or intermediate valley to the next peak. The rate-determining barrier is the largest of those step barriers, which need not be the peak with the greatest absolute height above the page baseline.

Representative question

[Maximum number: 4]

Sketch an energy profile for the two-step reaction, labelling reactants, intermediate and products, activation energies, $E_{\mathrm{a}}$ , and overall enthalpy change, $\Delta H$ . Assume that the reaction is exothermic.

reaction coordinate

products lower than reactants

intermediate at an energy minimum

$E_{\mathrm{a}}$ of both steps correctly marked

$\Delta H$ from reactants down to products

Accept any combination of peak heights.

Accept double-headed arrows or lines with no heads.

M1 and M4 may be awarded for a curve with a single maximum.

Ignore the energy position of the intermediate.

Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.

Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.

2.2.7 (HL)—Energy profiles

Multistep Energy Profiles

Constructing a Two-Step Energy Profile

Assessment in practice

Rate and Mechanisms Summary

Question 1