IB Chemistry HL 2.2.7 Hl Energy Profiles Questions

Use real HL exam questions to construct and interpret multistep energy profiles, showing the rate-determining activation energy and transition state.

Exam points

Construct or identify an energy profile for a reaction, placing reactants and products at the correct relative potential energies and showing the overall enthalpy change.
Label transition state(s), intermediate(s) and the activation energy of each elementary step, identifying the rate-determining step from the largest barrier.
Calculate or interpret forward and reverse activation energies from profile energy levels and use the multistep profile to evaluate the proposed mechanism.

[Maximum number: 2]

The reaction between hydrogen and nitrogen monoxide is thought to proceed by the mechanism shown below.

\begin{array}{ll} 2 \mathrm{NO}(\mathrm{~g}) \rightleftharpoons \mathrm{N}_{2} \mathrm{O}_{2}(\mathrm{~g}) & \text { fast equilibrium } \\ \mathrm{N}_{2} \mathrm{O}_{2}(\mathrm{~g})+\mathrm{H}_{2}(\mathrm{~g}) \rightarrow \mathrm{N}_{2} \mathrm{O}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) & \text { slow reaction } \\ \mathrm{N}_{2} \mathrm{O}(\mathrm{~g})+\mathrm{H}_{2}(\mathrm{~g}) \rightarrow \mathrm{N}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) & \text { fast reaction } \end{array}

The enthalpy change for the reaction between nitrogen monoxide and hydrogen is -664 kJ and its activation energy is 63 kJ .

Sketch the potential energy profile for the overall reaction, using the axes given, indicating both the enthalpy of reaction and activation energy.

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