The graph shows values of ΔG for a reaction at different temperatures.

Which statement is correct?

A.     The standard entropy change of the reaction is negative.

B.     The standard enthalpy change of the reaction is positive.

C.     At higher temperatures, the reaction becomes less spontaneous.

D.     The standard enthalpy change of the reaction is negative.

B

The graph shows Gibbs free energy of a mixture of N₂O₄(g) and NO₂(g) in different proportions.

N₂O₄(g) $\rightleftharpoons$ 2NO₂(g)

Which point shows the system at equilibrium?

B

Which energy profile diagram represents an exothermic S_N1 reaction?

C

Which technique is used to determine the bond lengths and bond angles of a molecule?

A.     X-ray crystallography

B.     Infrared (IR) spectroscopy

C.     Mass spectroscopy

D.     ¹H NMR spectroscopy

A

Which shows the first ionization energies of successive elements across period 2, from left to right?

C

Which technique involves breaking covalent bonds when carried out on an organic compound?

A. infrared spectroscopy

B. nuclear magnetic resonance spectroscopy

C. X-ray crystallography

D. mass spectrometry

D

This question challenged candidates to think about analytical techniques more deeply. 67 % of the candidates recognized that mass spectrometry involves breaking covalent bonds. The most commonly chosen distractor was X-ray crystallography (option C).

Given equimolar concentrations, which substance would produce the strongest signal in a ¹H NMR spectrum?

A.  (CH₃)₃CH

B.  C₆H₆

C.  C₈H₁₈

D.  Si(CH₃)₄

D

47% of the candidates identified TMS as the substance producing the strongest signal in a ¹H NMR spectrum. The wording of this question was not ideal and several teachers commented on this. We do not usually refer to the strength but the area under the signal.

What can be deduced from the infrared (IR) spectrum of a compound?

A. Number of hydrogens

B. Number of hydrogen environments

C. Bonds present

D. Molar mass

C

82 % of the candidates identified the bonds present as the information that can be deduced from an infrared spectrum. The most commonly chosen distractor was B (the number of hydrogen environments).

The following data were recorded for determining the density of three samples of silicon, Si.

Which average density value, in g cm⁻³, has been calculated to the correct number of significant figures?

A. 2

B. 2.3

C. 2.27

D. 2.273

B

Candidates found this question relatively challenging and only 67 % chose the answer with two significant figures. The most commonly chosen distractor was C which expressed the answer to three significant figures.

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.

The molar mass of a gas, determined experimentally, is 32 g mol⁻¹. Its literature molar mass is 40 g mol⁻¹.

What is the percentage error?

A.     80%

B.     25%

C.     20%

D.     8%

C

How should a measurement of 5.00 g from a balance be recorded?

A. 5.00 ± 0.1 g

B. 5.00 ± 0.01 g

C. 5.00 ± 1 g

D. 5.00 ± 0.001 g

B

More than 85 % of candidates were able to identify a correct uncertainty, however not a good discriminating question.