| Date | May 2015 | Marks available | 2 | Reference code | 15M.2.hl.TZ2.2 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Evaluate | Question number | 2 | Adapted from | N/A |
Question
Nitrogen(II) oxide reacts with hydrogen according to the equation below.
\[{\text{2NO(g)}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{(g)}} \to {{\text{N}}_{\text{2}}}{\text{(g)}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O(g)}}\]
A suggested mechanism for this reaction is:
Step 1: \({\text{NO}} + {{\text{H}}_{\text{2}}} \rightleftharpoons {\text{X}}\) fast
Step 2: \({\text{X}} + {\text{NO}} \to {\text{Y}} + {{\text{H}}_{\text{2}}}{\text{O}}\) slow
Step 3: \({\text{Y}} + {{\text{H}}_{\text{2}}} \to {{\text{N}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}}\) fast
Define the term rate of reaction.
Explain why increasing the particle size of a solid reactant decreases the rate of reaction.
Identify the rate-determining step.
A student hypothesized that the order of reaction with respect to \({{\text{H}}_{\text{2}}}\) is 2.
Evaluate this hypothesis.
Markscheme
change in concentration of reactant/product with time / rate of change of concentration;
Accept “increase” instead of “change” for product and “decrease” instead of “change” for reactant.
Accept “mass/amount/volume” instead of “concentration”.
Do not accept substance.
surface area decreases;
frequency/probability of collisions decreases;
Accept number of collisions per unit time decreases.
step 2 \({\text{/ X}} + {\text{NO}} \to {\text{Y}} + {{\text{H}}_{\text{2}}}{\text{O /}}\) slow;
invalid / unlikely as order most likely one (with respect to hydrogen);
\({\text{rate}} = k{{\text{[NO]}}^{\text{2}}}{\text{[}}{{\text{H}}_{\text{2}}}{\text{] / }}{{\text{H}}_{\text{2}}}\) only involved once in the formation of the intermediate before the slow step / OWTTE;
Award M2 only if M1 is correct.
Examiners report
Although most candidates were able to define the rate of reaction, some of weaker candidates gave imprecise answers which did not refer to concentration of the reactants or products and the “the time for the reaction to go to completion” was not an uncommon response. Most candidates realized that the surface area would decrease but, as in previous sessions, lost marks as they did not refer to the reduced “frequency” of collisions. Most candidates were able to identify the rate determining step and correctly state that the reaction would be first order with respect to hydrogen however only a minority could explain their answer in sufficient detail i.e. that \({{\text{H}}_{\text{2}}}\) was involved only once in the formation of the intermediate before the rate determining step.
Although most candidates were able to define the rate of reaction, some of weaker candidates gave imprecise answers which did not refer to concentration of the reactants or products and the “the time for the reaction to go to completion” was not an uncommon response. Most candidates realized that the surface area would decrease but, as in previous sessions, lost marks as they did not refer to the reduced “frequency” of collisions. Most candidates were able to identify the rate determining step and correctly state that the reaction would be first order with respect to hydrogen however only a minority could explain their answer in sufficient detail i.e. that \({{\text{H}}_{\text{2}}}\) was involved only once in the formation of the intermediate before the rate determining step.
Although most candidates were able to define the rate of reaction, some of weaker candidates gave imprecise answers which did not refer to concentration of the reactants or products and the “the time for the reaction to go to completion” was not an uncommon response. Most candidates realized that the surface area would decrease but, as in previous sessions, lost marks as they did not refer to the reduced “frequency” of collisions. Most candidates were able to identify the rate determining step and correctly state that the reaction would be first order with respect to hydrogen however only a minority could explain their answer in sufficient detail i.e. that \({{\text{H}}_{\text{2}}}\) was involved only once in the formation of the intermediate before the rate determining step.
Although most candidates were able to define the rate of reaction, some of weaker candidates gave imprecise answers which did not refer to concentration of the reactants or products and the “the time for the reaction to go to completion” was not an uncommon response. Most candidates realized that the surface area would decrease but, as in previous sessions, lost marks as they did not refer to the reduced “frequency” of collisions. Most candidates were able to identify the rate determining step and correctly state that the reaction would be first order with respect to hydrogen however only a minority could explain their answer in sufficient detail i.e. that \({{\text{H}}_{\text{2}}}\) was involved only once in the formation of the intermediate before the rate determining step.
