Structured Questions

2009 Oct Nov Paper 41 Q3c

(i) Give an explanation for the following in terms of atoms and ions.

observation at zinc electrode – the electrode becomes smaller. [1]

zinc atoms change into zinc ions

observation at copper electrode – the electrode becomes bigger. [1]

copper(II) ions change into copper atoms

(ii) When a current flows, charged particles move around the circuit.

What type of particle moves through the electrolytes? [1]

ions

Which particle moves through the wires and the voltmeter? [1]

electrons

2010 Oct Nov Paper 42 Q4b

(b) Chromium is used to electroplate steel objects. The diagram shows how this could be done.

(i) Give two reasons why steel objects are plated with chromium. [2]

To improve appearance (the surface become shiny)
To resist corrosion (rusting)

(ii) The formula of the chromium(III) ion is \(\mathrm{Cr}^{3+}\) and of the sulfate ion is \(\mathrm{SO}_{4}{ }^{2-}\). Give the formula of chromium(III) sulfate. [1]

\[\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3}\]

(iii) Write the equation for the reaction at the negative electrode (cathode). [2]

\[
\mathrm{Cr}^{3+}+3 e \rightarrow \mathrm{Cr}
\]

(iv) A colourless gas, which relights a glowing splint, is formed at the positive electrode (anode). Name this gas. [1]

oxygen

(v) During electrolysis, it is necessary to add more chromium(III) sulfate but during copper-plating using a copper anode, it is not necessary to add more copper(II) sulfate. Explain. [2]

In chromium-plating, chromium sulfate will used up, hence need to be refilled.
In copper-plating, copper ions in the electrolyte will also used up, but can be replaced from copper anode.

2011 May June Paper 42 Q2 (b) & (c)

(b) Aqueous tin(II) sulfate is electrolysed using carbon electrodes. This electrolysis is similar to that of aqueous copper(II) sulfate using carbon electrodes.
(i) What is the product at the negative electrode (cathode)? [1]

tin

(ii) Write the equation for the reaction at the positive electrode (anode). [2]

\[
4 \mathrm{OH}^{-} \rightarrow \mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O}+4 \mathrm{e}^{-}
\]

(iii) Name the acid formed in this electrolysis. [1]

sulfuric acid

(c) Steel articles can be plated with tin or zinc to prevent rusting. When the zinc layer is damaged exposing the underlying steel, it does not rust, but when the tin layer is broken the steel rusts. Explain. [4]

Zinc is more reactive than iron [1]. When zinc corrodes, it will provides sacrificial protection to iron. [1]

Tin is less reactive than iron [1]. When both iron and tin exposed to oxygen, iron will corrodes in preference to tin [1].