Which energy profile diagram represents an exothermic S_N1 reaction?

C

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 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 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

Which graph represents the relationship between the rate constant, $k$, and temperature, $T$, in kelvin?

C

60% of candidates selected the appropriate graph showing how rate constant, k, varies with temperature. Incorrect answers were equally split between a linear and inverse relationship.

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

What are the units of the rate constant, $k$, if the rate equation is $\mathrm{Rate}=k\left[\mathrm{A}\right]{\left[\mathrm{B}\right]}^2$?

A.  $\mathrm{mol} {\mathrm{dm}}^{-3} {\mathrm{s}}^{-1}$

B.  ${\mathrm{dm}}^3 {\mathrm{mol}}^{-1 }{\mathrm{s}}^{-1}$

C.  ${\mathrm{dm}}^6 {\mathrm{mol}}^{-2 }{\mathrm{s}}^{-1}$

D.  ${\mathrm{dm}}^9 {\mathrm{mol}}^{-3 }{\mathrm{s}}^{-1}$

C

Nearly three quarters of candidates could identify the units of a rate constant for a 3rd order reaction.

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

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

Compounds X and Y were mixed and the time taken for a colour to appear was recorded at various reactant concentrations.

What are the orders of reaction with respect to X and Y?

A

Which graph is obtained from a first order reaction?

B

More than 78 % of candidates correctly identified a first order reaction graph.

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

The rate expression for the reaction is: rate = k [NO]²[O₂].

2NO (g) + O₂ (g) → 2NO₂ (g)

Which mechanism is not consistent with this rate expression?

C

Which is correct for the reaction mechanism shown?

D

Another well answered question on rate mechanisms.

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 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

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

D

Which graph represents a second order reaction with respect to X?

X → Y

D

A reaction proceeds by the following mechanism:

step 1: $\mathrm{A}+\mathrm{A}\to \mathrm{B}$
step 2: $\mathrm{B}+\mathrm{C}\to \mathrm{D}$

Which rate equation is consistent with this mechanism?

A.  Rate = k[B]²[C]

B.  Rate = k[A]²[B][C]

C.  Rate = k[A]²

D.  Rate = k[A][C]

C

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

What is the activation energy according to the following plot of the linear form of the Arrhenius equation?

Arrhenius equation: $k=A{e}^{\frac{\mathit{-}Ea}{RT}}$.

A.  $E_a=\frac{2\times 8.31}{0.06}$

B.  $E_a=\frac{-2\times 8.31}{0.06}$

C.  $E_a=e^{\frac{2\times 8.31}{0.06}}$

D.  $E_a=e^{\frac{-2\times 8.31}{0.06}}$

A

A discriminating question in which high scoring candidates had much greater success in transforming the Arrhenius equation into the units used in the graph by taking its natural log and using the dimensions of the slope to arrive at the correct answer.

Which is correct?

B

What is the order with respect to each reactant?

2NO (g) + Cl₂ (g) → 2NOCl (g)

C

88 % of the candidates deduced the order of the reaction with respect to each reactant based on the experimental data of initial rate and concentrations of reactants.

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

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

What is the intercept on the y-axis when a graph of lnk is plotted against $\frac{1}{T}$ on the x-axis?

$\ln k=-\frac{E_a}{RT}+\ln A$

A.  lnA

B.  $-\frac{E_{\mathrm{a}}}{R}$

C.  $-\frac{R}{E_{\mathrm{a}}}$

D.   $E_{\mathrm{a}}$

A

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

The rate equation for a reaction is:

rate = k[A][B]

Which mechanism is consistent with this rate equation?

A.  2A $\rightleftharpoons$ I       Fast
     I + B → P    Slow

B.  A + B $\rightleftharpoons$ I   Fast
     I + A → P    Slow

C.  A → I          Slow
     I + B → P    Fast

D.  B $\rightleftharpoons$ I         Fast
     I + A → P    Slow

D

Which graph shows a first order reaction?

A

Which graphs show a first order reaction?

A.  V and X

B.  V and Y

C.  W and X

D.  W and Y

B

Which statement is correct about a catalyst?

A. It decreases the activation energy of the forward reaction but not the reverse.

B. It increases the proportion of products to reactants in an equilibrium.

C. It decreases the enthalpy change of the reaction.

D. It changes the mechanism of the reaction.

D

This was surprisingly one of the most challenging questions on the paper and discriminated very well between high scoring and low scoring candidates. It tested understanding of the role of the catalyst. 52 % of candidates chose the correct answer (a catalyst changes the mechanism of the reaction). The most commonly chosen distractor was A (a catalyst decreases the activation energy of the forward reaction but not the reverse).

The table shows data for the hydrolysis of a halogenoalkane, RCl.

Which statements are correct?

I.   The reaction is first order with respect to RCl.
II.  The reaction is second order overall.
III. The reaction proceeds by an S_N2 mechanism.

A.  I and II only

B.  I and III only

C.  II and III only

D.  I, II and III

D

Average performance on a high discriminatory question on rate of reaction and mechanism question, with no clear misconception based on the other choices.