When equal masses of X and Y absorb the same amount of energy, their temperatures rise by 5 °C and 10 °C respectively. Which is correct?

A. The specific heat capacity of X is twice that of Y.

B. The specific heat capacity of X is half that of Y.

C. The specific heat capacity of X is one fifth that of Y.

D. The specific heat capacity of X is the same as Y.

A

Answered correctly by 66 %, this question relating specific heat capacity to temperature rise was handled better by higher scoring candidates.

Which statements about bond strength and activation energy are correct for this reaction?

${\mathrm{CH}}_4 \left(\mathrm{g}\right)+2{\mathrm{O}}_2 \left(\mathrm{g}\right)\to {\mathrm{CO}}_2 \left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)$      $∆H^{⦵}=-891 \mathrm{kJ}$

B

More than 60% could select relative bond strength of products and reactants in an exothermic reaction, and correctly apply the magnitude of activation energy between forward and reverse reactions for this.

The enthalpy change for the dissolution of NH₄NO₃ is +26 kJ mol^–1 at 25 °C. Which statement about this reaction is correct?

A. The reaction is exothermic and the solubility decreases at higher temperature.

B. The reaction is exothermic and the solubility increases at higher temperature.

C. The reaction is endothermic and the solubility decreases at higher temperature.

D. The reaction is endothermic and the solubility increases at higher temperature.

D

The combustion of glucose is exothermic and occurs according to the following equation:

C₆H₁₂O₆ (s) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (g)

Which is correct for this reaction?

A

Which equation represents the bond enthalpy for H–Br in hydrogen bromide?

A.  HBr (g) → H⁺ (g) + Br⁻ (g)

B.  HBr (g) → H (g) + Br (g)

C.  HBr (g) → $\frac{1}{2}$H₂ (g) + $\frac{1}{2}$Br₂ (l)

D.  HBr (g) → $\frac{1}{2}$H₂ (g) + $\frac{1}{2}$Br₂ (g)

B

Only a quarter of the candidates identified the equation representing the bond enthalpy of HBr. The most commonly chosen distractor had the elements H₂ and Br₂ as the products.

The potential energy profile of a reaction is shown.

What can be determined about stability and energy change from the potential energy profile shown?

C

What is the enthalpy change of reaction for the following equation?

C₂H₄ (g) + H₂ (g) → C₂H₆ (g)

C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (l)        ΔH = x

C₂H₆ (g) + $\frac{7}{2}$O₂ (g) → 2CO₂ (g) + 3H₂O (l)    ΔH = y

H₂ (g) + $\frac{1}{2}$O₂ (g) → H₂O (l)                           ΔH = z

A. x + y + z

B. −x − y + z

C. x − y − z

D. x − y + z

D

Nearly 89 % of candidates answered this Hess Law question correctly. This topic was obviously well covered.

Which statement is correct?

A.  ${\mathrm{O}}_3$ bond dissociation occurs at a longer wavelength of light than ${\mathrm{O}}_2$ bond dissociation.

B.  ${\mathrm{O}}_3$ bond dissociation occurs at a higher energy than ${\mathrm{O}}_2$ bond dissociation.

C.  ${\mathrm{O}}_3$ bond lengths are shorter than ${\mathrm{O}}_2$ bond lengths.

D.  ${\mathrm{O}}_3$ bond dissociation occurs at a higher frequency of light than ${\mathrm{O}}_2$ bond dissociation.

A

Another well answered question where a good majority of candidates understood how ozone and oxygen bond dissociation energies would be affected by various wavelengths or frequencies of light.

Methane undergoes incomplete combustion.

2CH₄ (g) + 3O₂ (g) → 2CO (g) + 4H₂O (g)

What is the enthalpy change, in kJ, using the bond enthalpy data given below?

A. [2(1077) + 4(463)] − [2(414) + 3(498)]

B. [2(414) + 3(498)] − [2(1077) + 4(463)]

C. [8(414) + 3(498)] − [2(1077) + 8(463)]

D. [2(1077) + 8(463)] − [8(414) + 3(498)]

C

Only 70 % of the candidates chose the correct calculation of the enthalpy change using bond enthalpy data. The most commonly chosen distractor (D) reversed the signs.