Several reactions of calcium carbonate with dilute hydrochloric acid are carried out at the same temperature.

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

Which reaction has the greatest rate?

A

74 % of the candidates chose the correct combination to give the greatest rate of reaction. The most commonly chosen distractor was D where “smaller surface area of same mass of CaCO₃(s)” was chosen. It seems these candidates confused “surface area” with “particle size”.

Which change does not increase the rate of this reaction?

${\mathrm{CuCO}}_3 \left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4 \left(\mathrm{aq}\right)\to {\mathrm{CuSO}}_4 \left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)+{\mathrm{CO}}_2 \left(\mathrm{g}\right)$

A.  Increasing the particle size of the ${\mathrm{CuCO}}_3$

B.  Increasing the temperature

C.  Increasing the concentration of ${\mathrm{H}}_2{\mathrm{SO}}_4 \left(\mathrm{aq}\right)$

D.  Stirring the reaction mixture

A

Perhaps asking which factor does not affect a rate threw some candidates. 80% did get this correct but it was not a very discriminating question with higher and lower scorers performing equally well/poor.

Which explains increasing rate of reaction with increasing temperature?

B

Which statements are correct about the action of a catalyst in a chemical reaction?

I.   It increases the energy of each collision.
II.  It alters the mechanism of the reaction.
III. It remains unchanged at the end of the reaction.

A.  I and II only

B.  I and III only

C.  II and III only

D.  I, II and III

C

Which properties can be monitored to determine the rate of the reaction?

Fe (s) + CuSO₄ (aq) → Cu (s) + FeSO₄ (aq)

    I. change in volume
    II. change in temperature
    III. change in colour

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

C

One G2 form queried if visible spectroscopy is on the core syllabus and therefore should candidates be aware of monitoring a reaction via colour change. However, 6.1 clearly states that following change in colour is one method of following reactions.

Which conditions are required for the reaction between two molecules?

    I. a collision
    II. E ≥ E_a
    III. proper orientation

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

D

This question on collision theory was one of the best answered in the exam.

What is the activation energy of the reverse reaction?

C

81 % of the candidates identified the activation energy of the reverse reaction. The most commonly chosen distractor was B, the activation energy of the forward reaction.

The same amount of two gases, X and Y, are in two identical containers at the same temperature. What is the difference between the gases?

A. X has the higher molar mass.

B. Y has the higher molar mass.

C. X has the higher average kinetic energy.

D. Y has the higher average kinetic energy.

A

This question generated debate among teachers. It extended the concept that equal amounts of gases at the same temperature have the same distribution of kinetic energy curve, to the molecular speed distribution among particles. Candidates had to know that kinetic energy is calculated based on speed and mass of the molecule to deduce the answer.

Some teachers welcomed the question as a “good challenge to students’ thinking”, others thought it was difficult and a couple felt it was outside of the syllabus.

It was by far the most challenging question on the paper with only 21 % of candidates obtaining the correct answer. The majority of candidates chose distractor D which did not take note of the fact that the gases were at the same temperature and hence had the same average kinetic energy.

The dotted line represents the formation of oxygen, O₂ (g), from the uncatalysed complete decomposition of hydrogen peroxide, H₂O₂ (aq).

Which curve represents a catalysed reaction under the same conditions?

B

Graphical representation of catalysis was also well answered.