O Level Chemistry 5070 Paper 21, May/June 2025

is prepared using a precipitation reaction

in aqueous solution, reacts with an excess of aqueous ammonia to give a dark blue solution

reacts with warm aqueous sodium hydroxide and aluminium foil to give a gas that turns damp red litmus paper blue

has an aqueous solution that is used to test for an oxidising agent

has an aqueous solution that reacts with copper metal.

Graphite has good electrical conductivity. Explain why graphite has good electrical conductivity. Use ideas about structure and bonding.

State one other property of graphite that makes it suitable for use as an electrode during electrolysis.

Predict the products of the electrolysis of concentrated aqueous copper(II) bromide with graphite electrodes. product at anode ....................................................................................................................... product at cathode ....................................................................................................................

Dilute sulfuric acid is electrolysed using graphite electrodes to form oxygen and hydrogen. Construct the ionic half-equation for the reaction at each electrode. reaction at anode ...................................................................................................................... reaction at cathode ...................................................................................................................

Hydrogen and oxygen are used in a fuel cell to produce electricity. Name the only chemical product formed in a fuel cell.

Describe one disadvantage of using hydrogen–oxygen fuel cells in vehicles compared to gasoline or petrol engines.

State one condition for this reaction.

Explain why this reaction is an example of substitution.

Table 3.1 shows some bond energies. Table 3.1 Show by calculation that the enthalpy change of the reaction between methane and chlorine, $\mathrm{\Delta }H$, is $-119\text{ kJ/mol}$.

Complete the reaction pathway diagram in Fig. 3.2 for the reaction between methane and chlorine. Label the: • reactants • products • enthalpy change of the reaction, $\mathrm{\Delta }H$ • activation energy, $E_{\mathrm{a}}$.

Draw a dot-and-cross diagram to show the electronic configuration in a molecule of methane. Show only the outer shell electrons.

Give the general formula of the homologous series of alcohols.

Members of a homologous series have the same general formula and share similar chemical properties. State two other general characteristics of a homologous series. 1 ................................................................................................................................ 2 ................................................................................................................................

The equation for the reaction between ethanol and sodium is shown. $$2\mathrm{N}\mathrm{a}+2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2+2{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{N}\mathrm{a}$$ A sample of $1.35\text{ g}$ of sodium is added to excess ethanol. Calculate the volume of hydrogen formed measured at room temperature and pressure. Give your answer to two significant figures. volume = .................................................. ${\text{dm}}^3$

Water is added to the reaction mixture to make an aqueous solution. A few drops of litmus are then added. The litmus changes colour to blue. Suggest the name of the ion present in the aqueous solution responsible for the colour change.

State two uses for ethanol. 1 ................................................................................................................................ 2 ................................................................................................................................

Describe the manufacture of ethanol from ethene. Include the other reactant and the conditions for the manufacture.

Petrol is a mixture of hydrocarbons which includes octane, ${\mathrm{C}}_8{\mathrm{H}}_{18}$. Explain why octane is a hydrocarbon.

Two of the air pollutants produced are carbon monoxide and nitrogen monoxide. Explain how carbon monoxide, $\mathrm{C}\mathrm{O}$, is formed in a petrol engine. Include a symbol equation.

Explain how nitrogen monoxide, $\mathrm{N}\mathrm{O}$, is formed in a petrol engine. Include a symbol equation.

A catalytic converter removes most of the $\mathrm{C}\mathrm{O}$ and $\mathrm{N}\mathrm{O}$ formed in a petrol engine. $$2\mathrm{C}\mathrm{O}(\mathrm{g})+2\mathrm{N}\mathrm{O}(\mathrm{g})\to 2{\mathrm{C}\mathrm{O}}_2(\mathrm{g})+{\mathrm{N}}_2(\mathrm{g})$$ Explain why this reaction involves both oxidation and reduction.

The reaction is catalysed using platinum metal. Explain how a catalyst increases the rate of a reaction.

State and explain the effect of increasing the temperature on the rate of this reaction.

State and explain the effect of decreasing the pressure on the rate of this reaction.

The melting point of chlorine is $-101{}^{\circ }\mathrm{C}$ and the boiling point is $-35{}^{\circ }\mathrm{C}$. Explain why chlorine is a liquid at $-50{}^{\circ }\mathrm{C}$.

Describe the arrangement and motion of chlorine molecules at $-50{}^{\circ }\mathrm{C}$.

A sample of chlorine gas contains $1.204\times {10}^{20}$ molecules. One mole of chlorine gas contains $6.02\times {10}^{23}$ molecules. Calculate the mass of this sample of chlorine gas. mass of chlorine = ...................................................... $\text{g}$

The ionic equation for the reaction of chlorine with cold dilute aqueous sodium hydroxide is shown. $${\mathrm{C}\mathrm{l}}_2+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{C}\mathrm{l}\mathrm{O}}^{-}+{\mathrm{C}\mathrm{l}}^{-}+{\mathrm{H}}_2\mathrm{O}$$ State the oxidation number of chlorine in ${\mathrm{C}\mathrm{l}}_2$ and in ${\mathrm{C}\mathrm{l}}^{-}$. ${\mathrm{C}\mathrm{l}}_2$ ............................................. ${\mathrm{C}\mathrm{l}}^{-}$ .............................................

During the reaction chlorine is reduced. Explain why, using ideas about electrons.

Chlorine reacts with cold water to form an equilibrium mixture containing the acids $\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$ and $\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$. The forward reaction releases thermal energy into the surroundings. $${\mathrm{C}\mathrm{l}}_2(\mathrm{a}\mathrm{q})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\rightleftharpoons \mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})+\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$$ The temperature of the equilibrium mixture is increased. State and explain what happens to the acidity of the equilibrium mixture. statement .......................................................................................................................... explanation ........................................................................................................................ ........................................................................................................................................... ...........................................................................................................................................

$\mathrm{H}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$ is a strong acid and $\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$ is a weak acid. Describe the difference between a strong acid and a weak acid.

A chloride of iron contains $34.5\mathrm{\%}$ iron by mass. Calculate the empirical formula of this chloride. Show your working. empirical formula .........................................................

Give one other reason why aluminium is used in the manufacture of aircraft.

Explain why aluminium is resistant to corrosion by water and oxygen.

Aluminium metal reacts with hot dilute sulfuric acid to form hydrogen and aqueous aluminium sulfate as the only products. Construct the symbol equation for this reaction. Include state symbols.

Aluminium oxide reacts with sulfuric acid and with the alkali aqueous sodium hydroxide. State the name of the type of oxide that reacts with both acids and alkalis.

Methylbut-2-enoate is an unsaturated ester. Explain why methylbut-2-enoate is unsaturated.

Describe a chemical test to show that methylbut-2-enoate is unsaturated.

Methylbut-2-enoate is made by the reaction of an alcohol and a carboxylic acid in the presence of a catalyst. State the name of the type of catalyst used in this reaction.

State the name of the alcohol used in this reaction.

Draw the displayed formula of the carboxylic acid used in this reaction.

Methylbut-2-enoate is a monomer used to make an addition polymer. Draw the structure of this addition polymer. Include at least two repeat units.

A sample of $80\text{ g}$ of methylbut-2-enoate is reacted to make the addition polymer. There is a $100\mathrm{\%}$ yield. State the mass of addition polymer made. Explain your answer. mass of addition polymer = ........................... g explanation ........................................................................................................................ ...........................................................................................................................................

Fig. 8.2 shows compound B. Fig. 8.2 Explain why methylbut-2-enoate and compound B are a pair of structural isomers.