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

A mixture of 0.40 mol of CO (g) and 0.40 mol of H₂ (g) was placed in a 1.00 dm³ vessel. The following equilibrium was established.

CO (g) + 2H₂ (g)  CH₃OH (g)

At equilibrium, the mixture contained 0.25 mol of CO (g). How many moles of H₂ (g) and CH₃OH (g) were present at equilibrium?

D

Components X and Y are mixed together and allowed to reach equilibrium. The concentrations of X, Y, W and Z in the equilibrium mixture are 4, 1, 4 and $\text{2 mol}\text{d}{\text{m}}^{-3}$ respectively.

X + 2Y $\rightleftharpoons$ 2W + Z

What is the value of the equilibrium constant, K_c?

A.     $\frac{1}{8}$

B.     $\frac{1}{2}$

C.     2

D.     8

D

Which statement is correct for a spontaneous reaction?

A

Nearly all candidates knew that  is negative for a spontaneous reaction, however some were confused about the effect on the equilibrium constant.

At 700 ºC, the equilibrium constant, K_c, for the reaction is 1.075 × 10⁸.

2H₂ (g) + S₂ (g) $\rightleftharpoons$ 2H₂S (g)

Which relationship is always correct for the equilibrium at this temperature?

A. [H₂S]² < [H₂]² [S₂]

B. [S₂] = 2[H₂S]

C. [H₂S] < [S₂]

D. [H₂S]² > [H₂]²[S₂]

D

1.0 mol of N₂(g), 1.0 mol of H₂(g) and 1.0 mol of NH₃(g) are placed in a 1.0 dm³ sealed flask and left to reach equilibrium. At equilibrium the concentration of N₂(g) is 0.8 mol dm⁻³.

N₂(g) + 3H₂(g) $\rightleftharpoons$ 2NH₃(g)

What are the equilibrium concentration of H₂(g) and NH₃(g) in mol dm⁻³?

B

1.0 mol each of sulfur dioxide, oxygen, and sulfur trioxide are in equilibrium.

$2{\mathrm{SO}}_2 \left(\mathrm{g}\right)+{\mathrm{O}}_2 \left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{SO}}_3 \left(\mathrm{g}\right)$

Which change in the molar ratio of reactants will cause the greatest increase in the amount of sulfur trioxide?

Assume volume and temperature of the reaction mixture remain constant.

D

Which combination describes the system at equilibrium?

B

Which is correct for a reaction with a positive change in Gibbs free energy, ΔG^θ?

A. The formation of reactants is favoured.

B. The formation of products is favoured.

C. The reaction is at equilibrium.

D. The reaction is spontaneous.

A

The higher scoring candidates did better on identifying the direction of spontaneity given a positive ΔG

Which is correct for a redox reaction where the standard electrode potential is negative?

ΔG^Θ = −nFE^Θ and ΔG^Θ = −RT ln K

A.   ΔG^Θ is negative and K is less than 1.

B.   ΔG^Θ is negative and K is greater than 1.

C.   ΔG^Θ is positive and K is less than 1.

D.   ΔG^Θ is positive and K is greater than 1.

C

Iodine and bromine gases were mixed and allowed to reach equilibrium.

What is the value of the equilibrium constant?

A. 0.05

B. 1

C. 4

D. 10

C

This was a challenging question with the highest discrimination index on the paper. 62 % of the candidates were able to deduce the equilibrium concentration of IBr and calculate the equilibrium constant correctly. The most commonly chosen distractor was B where the stoichiometric ratio was not taken into account when calculating the equilibrium concentration of IBr.

Which corresponds to a system at equilibrium?

B