Which graph represents a reaction that is first order with respect to reactant A.

A

Which is correct about reaction mechanisms?

A.     A species that is zero order does not take part in the reaction.

B.     A catalyst does not take part in the reaction.

C.     Reactants in a fast step before the slow step are included in the rate expression.

D.     Reactants in a fast step after the slow step are included in the rate expression.

C

What are correct labels for the Maxwell−Boltzmann energy distribution curves?

D

What are the units of the rate constant for a zero-order reaction?

A.     s

B.     \({{\text{s}}^{ - 1}}\)

C.     \({\text{mo}}{{\text{l}}^{ - 1}}\,{\text{d}}{{\text{m}}^{\text{3}}}\,{{\text{s}}^{ - 1}}\)

D.     \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\,{{\text{s}}^{ - 1}}\)

D

Over 34% confused the units with those of a first order reaction.

Which graph represents a reaction that is second order with respect to X for the reaction X \( \to \) products?

A

One respondent stated that the graph in A looks more like a first order reaction than a second order reaction. In this question, C is ruled out as the rate-concentration plot would represent a clear zero-order reaction. B is also ruled out as a zero-order reaction will involve a straight line. D is also a straight line. Hence, by a process of elimination A must be the answer, as a second-order reaction will involve a curve for a concentration-time plot. It is true to say that a first-order reaction will also involve a curve for a concentration-time plot. In fact a first-order concentration-time curve is an exponential curve and a second-order concentration-time curve is a quadratic curve, which appears somewhat to have greater depth if the two are compared. It can be difficult to distinguish the two plots in fact based on experimental data, but this was not an issue for this question as second order was clearly mentioned in the question and B, C and D could be eliminated also. The question in fact was the fourth easiest question on the paper for candidates, with 86.42% of candidates getting the correct answer A.

The following experimental rate data were obtained for a reaction carried out at temperature T.

\[{\text{A(g)}} + {\text{B(g)}} \to {\text{C(g)}} + {\text{D(g)}}\]

What are the orders with respect to A(g) and B(g)?

C

Consider the following reaction.

\[2{\text{NO(g)}} + 2{{\text{H}}_2}({\text{g)}} \to {{\text{N}}_2}({\text{g)}} + 2{{\text{H}}_2}{\text{O(g)}}\]

A proposed reaction mechanism is:

\[\begin{array}{*{20}{l}} {{\text{NO(g)}} + {\text{NO(g)}} \rightleftharpoons {{\text{N}}_2}{{\text{O}}_2}{\text{(g)}}}&{fast} \\ {{{\text{N}}_2}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{(g)}} \to {{\text{N}}_2}{\text{O(g)}} + {{\text{H}}_2}{\text{O(g)}}}&{slow} \\ {{{\text{N}}_2}{\text{O(g)}} + {{\text{H}}_2}{\text{(g)}} \to {{\text{N}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{O(g)}}}&{fast} \end{array}\]

What is the rate expression?

A.     \({\text{rate}} = k{\text{[}}{{\text{H}}_2}]{[{\text{NO]}}^2}\)

B.     \({\text{rate}} = k{\text{[}}{{\text{N}}_2}{{\text{O}}_2}][{{\text{H}}_2}]\)

C.     \({\text{rate}} = k{{\text{[NO]}}^2}{[{{\text{H}}_2}]^2}\)

D.     \({\text{rate}} = k{{\text{[NO]}}^2}{[{{\text{N}}_2}{{\text{O}}_2}]^2}[{{\text{H}}_2}]\)

A

There were three G2 comments on this question. Two of these comments stated the question was very difficult and one comment stated that the first step should have had an equilibrium sign which is correct. The question itself was answered correctly by only 46.28% and was the third most difficult question on the entire paper.

Bromine and nitrogen(II) oxide react according to the following equation.

\[{\text{B}}{{\text{r}}_{\text{2}}}{\text{(g)}} + {\text{2NO(g)}} \to {\text{2NOBr(g)}}\]

Which rate equation is consistent with the experimental data?

A.     \({\text{rate}} = k{{\text{[B}}{{\text{r}}_{\text{2}}}{\text{]}}^{\text{2}}}{\text{[NO]}}\)

B.     \({\text{rate}} = k{\text{[B}}{{\text{r}}_{\text{2}}}{\text{][NO}}{{\text{]}}^{\text{2}}}\)

C.     \({\text{rate}} = k{{\text{[B}}{{\text{r}}_{\text{2}}}{\text{]}}^{\text{2}}}\)

D.     \({\text{rate}} = k{{\text{[NO]}}^{\text{2}}}\)

C

Consider the following reaction.

\[{\text{5B}}{{\text{r}}^ - }{\text{(aq)}} + {\text{BrO}}_3^ - {\text{(aq)}} + {\text{6}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{3B}}{{\text{r}}_2}{\text{(aq)}} + {\text{3}}{{\text{H}}_2}{\text{O(l)}}\]

The rate expression for the reaction is found to be:

\[{\text{rate}} = k{\text{[B}}{{\text{r}}^ - }{\text{][BrO}}_3^ - {\text{][}}{{\text{H}}^ + }{{\text{]}}^2}\]

Which statement is correct?

A.     The overall order is 12.

B.     Doubling the concentration of all of the reactants at the same time would increase the rate of the reaction by a factor of 16.

C.     The units of the rate constant, \(k\), are \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{{\text{s}}^{ - 1}}\).

D.     A change in concentration of \({\text{B}}{{\text{r}}^ - }\) or \({\text{BrO}}_3^ - \) does not affect the rate of the reaction.

B

The activation energy of a reaction may be determined by studying the effect of a particular variable on the reaction rate. Which variable must be changed?

A.     pH

B.     Concentration

C.     Surface area

D.     Temperature

D

Which statement about a reaction best describes the relationship between the temperature, \(T\), and the rate constant, \(k\)?

A.     As \(T\) increases, \(k\) decreases linearly.

B.     As \(T\) increases, \(k\) decreases non-linearly.

C.     As \(T\) increases, \(k\) increases linearly.

D.     As \(T\) increases, \(k\) increases non-linearly.

D

Which step is the rate-determining step of a reaction?

A.     The step with the lowest activation energy

B.     The final step

C.     The step with the highest activation energy

D.     The first step

C

Consider the following proposed two-step reaction mechanism at temperature T.

     Step 1:     \({\text{2N}}{{\text{O}}_2}{\text{(g)}}\xrightarrow{{{k_1}}}{\text{NO(g)}} + {\text{N}}{{\text{O}}_3}{\text{(g)}}\)     Slow

     Step 2:     \({\text{N}}{{\text{O}}_3}{\text{(g)}} + {\text{CO(g)}}\xrightarrow{{{k_2}}}{\text{N}}{{\text{O}}_2}{\text{(g)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}\)     Fast

Which statements are correct?

I.     The overall reaction is \({\text{N}}{{\text{O}}_2}{\text{(g)}} + {\text{CO(g)}} \to {\text{NO(g)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}\).

II.     Step 1 is the rate-determining step of the reaction.

III.     The rate expression for Step 1 is rate \( = {k_1}{{\text{[N}}{{\text{O}}_2}{\text{]}}^2}\).

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

D

This was reported to be “demanding (makes them think) but good”. It was the sixth hardest question with 63% scoring correctly. The most common error was not to recognize that the overall reaction equation is correct.

When X reacts with Y to give Z, the following graph is plotted. What can be deduced from the graph?

A.     The concentration of X is directly proportional to time.

B.     The reaction is first order overall.

C.     The reaction is zero order with respect to X.

D.     The reaction is first order with respect to X.

C

A

Which combination shows a second-order rate expression with the correct rate constant units?

D

The rate constant for a reaction is determined at different temperatures. Which diagram represents the relationship between the rate constant, \(k\) , and temperature, \(T\) , in K ?

C

This question could be answered if students knew that the relationship between \(k\) and \(T\) is exponential (as stated in A.S. 16.3.1), but it was not necessary that students recalled the Arrhenius equation. Candidates found the question difficult, having only 46% correct answers.

Consider the following reaction between nitrogen monoxide and oxygen.

\[{\text{2NO(g)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2N}}{{\text{O}}_{\text{2}}}{\text{(g)}}\]

The reaction occurs in two steps:

     Step 1: \({\text{NO(g)}} + {\text{NO(g)}} \rightleftharpoons {{\text{N}}_{\text{2}}}{{\text{O}}_{\text{2}}}{\text{(g)}}\)     fast

     Step 2: \({{\text{N}}_{\text{2}}}{{\text{O}}_{\text{2}}}{\text{(g)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2N}}{{\text{O}}_{\text{2}}}{\text{(g)}}\)     slow

What is the rate expression for this reaction?

A.     Rate \( = k{{\text{[NO]}}^{\text{2}}}\)

B.     Rate \( = k{\text{[NO][}}{{\text{O}}_{\text{2}}}{\text{]}}\)

C.     Rate \( = k{{\text{[NO]}}^{\text{2}}}{\text{[}}{{\text{O}}_{\text{2}}}{\text{]}}\)

D.     Rate \( = k{\text{[NO][}}{{\text{O}}_{\text{2}}}{{\text{]}}^{\text{2}}}\)

C

There were two comments. One suggested a “quantum change in difficulty” and the other commented that “students are not normally taught how to eliminate the intermediate from the rate expression for the slow step”. In the event, 82.71% of the candidates scored correctly, the sixth “easiest” question on the paper.

Which statement is correct?

A.     The value of the rate constant, k, is independent of temperature and is deduced from the equilibrium constant, K_c.

B.     The value of the rate constant, k, is independent of temperature and the overall reaction order determines its units.

C.     The value of the rate constant, k, is temperature dependent and is deduced from the equilibrium constant, K_c.

D.     The value of the rate constant, k, is temperature dependent and the overall reaction order determines its units.

D

Consider the following reaction mechanism.

\[\begin{array}{*{20}{l}} {{\text{Step 1}}}&{{{\text{H}}_2}{{\text{O}}_2} + {{\text{I}}^ - } \to {{\text{H}}_2}{\text{O}} + {\text{I}}{{\text{O}}^ - }}&{{\text{slow}}} \\ {{\text{Step 2}}}&{{{\text{H}}_2}{{\text{O}}_2} + {\text{I}}{{\text{O}}^ - } \to {{\text{H}}_2}{\text{O}} + {{\text{O}}_2} + {{\text{I}}^ - }}&{{\text{fast}}} \end{array}\]

Which statement correctly identifies the rate-determining step and the explanation?

A.     Step 2 because it is the faster step

B.     Step 1 because it is the slower step

C.     Step 1 because it is the first step

D.     Step 2 because it is the last step

B

What are the units for the rate constant, k, in the expression?

Rate = k [X]²[Y]

A.     mol² dm⁻⁶ s⁻¹

B.     mol⁻¹ dm³ s⁻¹

C.     mol dm⁻³ s⁻¹

D.     mol⁻² dm⁶ s⁻¹

D

Consider the following reaction.

\[{\text{2P}} + {\text{Q}} \to {\text{R}} + {\text{S}}\]

This reaction occurs according to the following mechanism.

\[\begin{array}{*{20}{l}} {{\text{P}} + {\text{Q}} \to {\text{X}}}&{slow} \\ {{\text{P}} + {\text{X}} \to {\text{R}} + {\text{S}}}&{fast} \end{array}\]

What is the rate expression?

A.     rate \( = k{\text{[P]}}\)

B.     rate \( = k{\text{[P][X]}}\)

C.     rate \( = k{\text{[P][Q]}}\)

D.     rate \( = k{{\text{[P]}}^{\text{2}}}{\text{[Q]}}\)

C

What is the effect of increasing temperature on the rate constant, k? 

A.     The rate constant does not change.

B.     The rate constant decreases linearly.

C.     The rate constant increases exponentially.

D.     The rate constant increases proportionally with temperature.

C

What happens when the temperature of a reaction increases?

A.     The activation energy increases.

B.     The rate constant increases.

C.     The enthalpy change increases.

D.     The order of the reaction increases.

B

For the gas phase reaction:

\[{\text{A(g)}} + {\text{B(g)}} \to {\text{C(g)}}\]

the experimentally determined rate expression is: rate \( = k[{\text{A}}]{[{\text{B}}]^2}\)

By what factor will the rate change if the concentration of A is tripled and the concentration of B is halved?

A.     0.75

B.     1.5

C.     6

D.     12

A

Carbon monoxide and nitrogen dioxide react to form carbon dioxide and nitrogen monoxide according to the following equation.

\[{\text{CO(g)}} + {\text{N}}{{\text{O}}_2}{\text{(g)}} \to {\text{C}}{{\text{O}}_2}{\text{(g)}} + {\text{NO(g)}}\]

The reaction occurs in a series of steps. The equation for the rate-determining step is given below.

\({\text{2N}}{{\text{O}}_2}{\text{(g)}} \to {\text{N}}{{\text{O}}_3}{\text{(g)}} + {\text{NO(g)}}\)

What is the rate expression for this reaction?

A.     rate \( = k{\text{[CO(g)][N}}{{\text{O}}_2}{\text{(g)]}}\)

B.     rate \( = k{{\text{[N}}{{\text{O}}_2}{\text{(g)]}}^2}\)

C.     rate \( = k{\text{[N}}{{\text{O}}_3}{\text{(g)][NO(g)]}}\)

D.     rate \( = k{\text{[C}}{{\text{O}}_2}{\text{(g)][NO(g)]}}\)

B

Which is true of an Arrhenius plot of \(\ln k\) (y-axis) against \(\frac{1}{T}\)?

A.     The graph goes through the origin.

B.     The activation energy can be determined from the gradient.

C.     The intercept on the x-axis is the activation energy.

D.     The intercept on the y-axis is the frequency factor, A.

B

The rate expression for a reaction is:

\[{\text{rate}} = k{\text{[X][Y]}}\]

Which statement is correct?

A.     As the temperature increases the rate constant decreases.

B.     The rate constant increases with increased temperature but eventually reaches a constant value.

C.     As the temperature increases the rate constant increases.

D.     The rate constant is not affected by a change in temperature.

C

Which statement describes the characteristics of a transition state relative to the potential energy of the reactants and products?

A. It is an unstable species with lower potential energy.

B. It is an unstable species with higher potential energy.

C. It is a stable species with lower potential energy.

D. It is a stable species with higher potential energy.

B

The following data were obtained for the reaction between gases A and B.

Which relationship represents the rate expression for the reaction?

A.     \({\text{rate}} = k{{\text{[B]}}^{\text{2}}}\)

B.     \({\text{rate}} = k{{\text{[A]}}^{\text{2}}}\)

C.     \({\text{rate}} = k{\text{[A]}}\)

D.     \({\text{rate}} = k{\text{[B]}}\)

D

Which is the first step in the CFC-catalysed destruction of ozone in UV light?

A.     CCl₂F₂ → CClF₂⁺ + Cl^–

B.     CCl₂F₂ → •CClF₂ + Cl•

C.     CCl₂F₂ → CCl₂F⁺ + F^–

D.     CCl₂F₂ → •CCl₂F + F•

B

Which graph best represents a second-order reaction?

B

One respondent stated that graphs A and B look too similar so students might not be able to clearly see that the answer is B. The initial gradient of curve B is clearly greater so candidates should have recognized the faster decrease in initial concentration for a second order reaction. Curve A clearly shows a constant half-life so candidates should have recognized that it refers to a first order reaction. 60.65% of the candidates chose the correct answer B.

Consider the rate expression:

\[{\text{Rate}} = k{\text{[X][Y]}}\]

Which change decreases the value of the rate constant, \(k\)?

A.     Increase in the reaction temperature

B.     Decrease in the reaction temperature

C.     Increase in the concentration of X and Y

D.     Decrease in the concentration of X and Y

B

The rate expression for the reaction X (g) + 2Y (g) → 3Z (g) is 

rate = k[X]⁰ [Y]²

By which factor will the rate of reaction increase when the concentrations of X and Y are both increased by a factor of 3?

A. 6

B. 9

C. 18

D. 27

B

The table gives rate data for the reaction in a suitable solvent.

C₄H₉Br + OH⁻ → C₄H₉OH + Br⁻

Which statement is correct?

A.     The rate expression is rate = k [C₄H₉Br] [OH⁻].

B.     The rate increases by a factor of 4 when the [OH⁻] is doubled.

C.     C₄H₉Br is a primary halogenoalkane.

D.     The reaction occurs via S_N1 mechanism.

D

Which pair of statements explains the increase in rate of reaction when the temperature is increased or a catalyst is added?

C

X and Y react according to the equation \({\text{2X}} + {\text{Y}} \to {\text{Z}}\). The reaction can be described by the following mechanism:

\({\text{X}} + {\text{X}} \to {{\text{X}}_2}\)     slow

\({{\text{X}}_2} + {\text{Y}} \to {\text{Z}}\)     fast

What is the order of the reaction with respect to X and Y?

C

What happens to the rate constant, k, and the activation energy, \({E_{\text{a}}}\), as the temperature of a chemical reaction is increased?

D

The hydrolysis of tertiary bromoalkanes with a warm dilute aqueous sodium hydroxide solution proceeds by a two-step \({{\text{S}}_{\text{N}}}{\text{1}}\) mechanism.

     Step I:     \({\text{R}} - {\text{Br}} \to {{\text{R}}^ + }{\text{B}}{{\text{r}}^ - }\)

     Step II:     \({{\text{R}}^ + } + {\text{O}}{{\text{H}}^ - } \to {\text{R}} - {\text{OH}}\)

Which description of this reaction is consistent with the above information?

B

Which statement about a first-order reaction is correct?

A.     The reactant concentration decreases linearly with time.

B.     The reactant concentration decreases exponentially with time.

C.     The rate of reaction remains constant as the reaction proceeds.

D.     The rate of reaction increases exponentially as the reaction proceeds.

B

Students found this question to be somewhat difficult with 42.15% correct answers. However, almost 40% of the candidates chose A, namely that for a first-order reaction, the reactant concentration decreases linearly with time (rather than exponentially); this suggests the importance of reading the statements more thoroughly.

The rate information below was obtained for the following reaction at a constant temperature.

\[2{\text{N}}{{\text{O}}_2}({\text{g)}} + {{\text{F}}_2}({\text{g)}} \to {\text{2N}}{{\text{O}}_2}{\text{F(g)}}\]

What are the orders of the reaction with respect to \({\text{N}}{{\text{O}}_{\text{2}}}\) and \({{\text{F}}_{\text{2}}}\)?

A.     \({\text{N}}{{\text{O}}_{\text{2}}}\) is first order and \({{\text{F}}_{\text{2}}}\) is second order

B.     \({\text{N}}{{\text{O}}_{\text{2}}}\) is second order and \({{\text{F}}_{\text{2}}}\) is first order

C.     \({\text{N}}{{\text{O}}_{\text{2}}}\) is first order and \({{\text{F}}_{\text{2}}}\) is first order

D.     \({\text{N}}{{\text{O}}_{\text{2}}}\) is second order and \({{\text{F}}_{\text{2}}}\) is second order

C.

Experimental data shows that a reaction in which Y is a reactant is first order with respect to Y. Which graph shows this first-order relationship?

C

The graph in C showed a constant half-life and so represented a 1^st order relationship.

The reaction between NO₂ and F₂ gives the following rate data at a certain temperature.

What is the overall order of reaction?

A.     3

B.     2

C.     1

D.     0

A

Decomposition of hydrogen peroxide in an aqueous solution proceeds as follows.

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

The rate expression for the reaction was found to be: rate = k [H₂O₂].

Which graph is consistent with the given rate expression?

D

D

C