| Date | May 2017 | Marks available | 1 | Reference code | 17M.2.hl.TZ2.5 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | State | Question number | 5 | Adapted from | N/A |
Question
Nitrogen dioxide and carbon monoxide react according to the following equation:
NO2(g) + CO(g) \( \rightleftharpoons \) NO(g) + CO2(g) ΔH = –226 kJ
Experimental data shows the reaction is second order with respect to NO2 and zero order with respect to CO.
State the rate expression for the reaction.
The following mechanism is proposed for the reaction.
\[\begin{array}{*{20}{l}} {{\text{Step I}}}&{{\text{N}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {\text{N}}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{NO(g)}} + {\text{N}}{{\text{O}}_{\text{3}}}{\text{(g)}}} \\ {{\text{Step II}}}&{{\text{N}}{{\text{O}}_{\text{3}}}{\text{(g)}} + {\text{CO(g)}} \to {\text{N}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {\text{C}}{{\text{O}}_{\text{2}}}{\text{(g)}}} \end{array}\]
Identify the rate determining step giving your reason.
State one method that can be used to measure the rate for this reaction.
Sketch the relationship between the rate of reaction and the concentration of NO2.
The Arrhenius equation, \(k = A{e^{ - \frac{{Ea}}{{RT}}}}\), gives the relationship between the rate constant and temperature.
State how temperature affects activation energy.
Markscheme
«rate =» k [NO2]2
Accept rate = k [NO2]2[CO]0.
[1 mark]
«step» I AND CO does not appear in the rate law expression
OR
«step» I AND only «2 molecules of» NO2 appears in rate expression
Do not allow ECF from (i).
[1 mark]
«IR or UV-vis» spectroscopy
OR
colorimetry
OR
colour change «over time»
Accept GC/gas chromatography.
[1 mark]
Curve must go through origin.
[1 mark]
activation energy is independent of temperature
Accept “no relationship”.
[1 mark]
