Chemistry 9701/21 — October/November 2021

Cambridge AS Level · AS Level Structured Questions · worked solutions for every part, with the mark scheme

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Topics Chemical Bonding · Atoms, Molecules and Stoichiometry · Halogen Compounds · States of Matter · Chemical Energetics · Carbonyl Compounds · +11 more

Q1States of MatterChemical BondingChemical EnergeticsEquilibriaElectrochemistryAtoms, Molecules and StoichiometryNitrogen and SulfurFree sample

Sulfides are compounds that contain sulfur but not oxygen.

(a)

Carbon disulfide, $\text{CS}_2$ , is a volatile liquid at room temperature and pressure.

(i)

State the meaning of volatile.

(ii)

Draw a ‘dot-and-cross’ diagram of the $\text{CS}_2$ molecule.

(iii)

Suggest the bond angle in a molecule of $\text{CS}_2$ .

(iv)

$\text{CS}_2$ is a liquid under room conditions, while $\text{CO}_2$ is a gas.

Explain what causes the difference in the physical properties between $\text{CS}_2$ and $\text{CO}_2$ .

(b)

The enthalpy change of combustion of $\text{CS}_2(\text{l})$ is represented by the following equation.

\text{CS}_2(\text{l}) + 3\text{O}_2(\text{g}) \xrightarrow{\Delta H_c} \text{CO}_2(\text{g}) + 2\text{SO}_2(\text{g})

(i)

Define enthalpy change of combustion.

(ii)

The table shows the enthalpy changes of formation of $\text{CS}_2(\text{l})$ , $\text{CO}_2(\text{g})$ and $\text{SO}_2(\text{g})$ .

compound	enthalpy change of formation, $\Delta H_f / \text{ kJ mol}^{-1}$
$\text{CS}_2(\text{l})$	+89.7
$\text{CO}_2(\text{g})$	-394
$\text{SO}_2(\text{g})$	-297

Use the data in the table to calculate the enthalpy change of combustion, $\Delta H_c$ , of $\text{CS}_2(\text{l})$ , in $\text{kJ mol}^{-1}$ .

Show your working.

(c)

Hydrogen sulfide gas, $\text{H}_2\text{S(g)}$ , is slightly soluble in water. It acts as a weak acid in aqueous solution.

(i)

State the meaning of weak acid.

(ii)

Give the formula of the conjugate base of $\text{H}_2\text{S}$ .

(iii)

$\text{H}_2\text{S(aq)}$ reacts slowly with oxygen dissolved in water. The reaction is represented by the following equation.

\text{H}_2\text{S(aq)} + \frac{1}{2}\text{O}_2(\text{aq}) \rightarrow \text{H}_2\text{O(l)} + \text{S(s)}

Explain, with reference to oxidation numbers, why this reaction is a redox reaction.

(d)

The compound $\text{As}_2\text{S}_3$ is a common mineral.

When $\text{As}_2\text{S}_3$ is heated strongly in air, it forms a mixture of products, as shown.

2\text{As}_2\text{S}_3(\text{s}) + 9\text{O}_2(\text{g}) \rightarrow \text{As}_4\text{O}_6(\text{s}) + 6\text{SO}_2(\text{g})

(i)

A sample containing 0.198 g $\text{As}_2\text{S}_3$ is placed in $0.100 \text{ dm}^3$ of pure oxygen, an excess, in a reaction chamber connected to a gas syringe at room temperature.

The reactants are heated until no further change is observed. The products are then allowed to cool to room temperature.

Calculate the volume, in $\text{dm}^3$ , of gas present at the end of the experiment.

The molar volume of gas is $24.0 \text{ dm}^3 \text{ mol}^{-1}$ under these conditions. Assume that the pressure is constant throughout the experiment.

Show your working.

(ii)

State the environmental consequences of releasing $\text{SO}_2(\text{g})$ into the atmosphere.

(iii)

$\text{SO}_2(\text{g})$ can be removed from the air by reacting it with $\text{NaOH(aq)}$ .

Construct an equation for the reaction of $\text{SO}_2(\text{g})$ with $\text{NaOH(aq)}$ . Include state symbols.

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3 more questions

Q2Group 2 · Chemical Bonding · Atoms, Molecules and Stoichiometry · Halogen Compounds · Carbonyl Compounds13M
Q3States of Matter · Chemical Energetics · Chemical Periodicity · Introduction to Organic Chemistry · Hydroxy Compounds · Halogen Compounds · Hydrocarbons14M
Q4Hydroxy Compounds · Atoms, Molecules and Stoichiometry · Introduction to Organic Chemistry · Halogen Compounds · Reaction Kinetics · Analytical Techniques · Carboxylic Acids and Derivatives · Carbonyl Compounds · Chemical Bonding12M

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