HSC Chemistry sample questions

Question 1

Which sample contains the greatest number of atoms?

Accepted Answer

C. 1.0 mol of CH₄

Question 2

A 0.500 g sample of magnesium is added to excess hydrochloric acid.

a. Write a balanced chemical equation for the reaction, including states.
b. Calculate the volume of hydrogen gas produced at 25 °C and 100 kPa. (M(Mg) = 24.31 g/mol, molar volume at these conditions = 24.79 L/mol)

Accepted Answer

a. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

b. n(Mg) = 0.500 / 24.31 = 0.02057 mol
The ratio Mg : H₂ is 1 : 1, so n(H₂) = 0.02057 mol
V = n × Vₘ = 0.02057 × 24.79 = 0.510 L (510 mL)

Question 3

Using bond energies, explain why the combustion of methane is exothermic.

Accepted Answer

Breaking bonds requires energy, while forming bonds releases it. In the combustion of methane, the C–H and O=O bonds in the reactants are broken and the C=O and O–H bonds in the products are formed. The total energy released in forming the bonds in carbon dioxide and water is greater than the total energy required to break the bonds in methane and oxygen, so there is a net release of energy to the surroundings and ΔH is negative.

Question 4

A reaction has ΔH = −92 kJ/mol and ΔS = −198 J/K/mol. Determine whether the reaction is spontaneous at 25 °C.

Accepted Answer

ΔG = ΔH − TΔS
T = 298 K, ΔS = −0.198 kJ/K/mol
ΔG = −92 − (298 × −0.198) = −92 + 59.0 = −33 kJ/mol
ΔG is negative, so the reaction is spontaneous at 25 °C.

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