Question
This question is about chemical reactions and electricity.
(a) The diagram shows the apparatus used in the production of electrical energy in a simple cell.
The zinc electrode dissolves in the electrolyte forming Zn2+(aq) ions.
(i) Draw an arrow on the diagram to show the direction of electron flow. [1]
(ii) Write the ionic half-equation for the reaction that occurs when the zinc electrode dissolves.[2]
(b) The reading on the voltmeter can be increased if either zinc or iron is replaced by another metal.
(i) Name a metal that can replace zinc and increase the reading on the voltmeter.[1]
(ii) Name a metal that can replace iron and increase the reading on the voltmeter.[1]
(c) Fuel cells are used to generate electricity.
(i) Name the reactants in a fuel cell.[1]
(ii) Name the waste product of a fuel cell.[1]
(d) Electricity can be used to break down aqueous or molten ionic compounds.
(i) Name the process which uses electricity to break down aqueous or molten ionic compounds.[1]
(ii) Explain why the ionic compound needs to be aqueous or molten.[1]
(e) Brine is concentrated aqueous sodium chloride.
(i) Name three substances which are manufactured by passing electricity through brine. [3]
1
2
3
(ii) Name a different substance formed when molten sodium chloride is used instead of concentrated aqueous sodium chloride.[1] [Total: 13]
Answer/Explanation
Ans:
4(a)(i) arrow going from Zn to Fe
4(a)(ii) Zn → Zn2+ + 2e–
Zn as only reactant and Zn2+ as only product
correct equation
4(b)(i) any metal above zinc in reactivity series
4(b)(ii) any metal below iron in reactivity series
4(c)(i) hydrogen and oxygen
4(c)(ii) water
4(d)(i) electrolysis
4(d)(ii) mobile ions
4(e)(i) hydrogen
chlorine
sodium hydroxide
4(e)(ii) sodium
Question
Potassium reacts with chlorine to form potassium chloride, KCl.
(a) Write a chemical equation for this reaction.
(b) Potassium chloride is an ionic compound.
Complete the diagram to show the electron arrangement in the outer shells of the ions present
in potassium chloride.
Give the charges on both ions.
(c) Molten potassium chloride undergoes electrolysis.
(i) State what is meant by the term electrolysis.
(ii) Name the products formed at the positive electrode (anode) and negative electrode
(cathode) when molten potassium chloride undergoes electrolysis.
anode ………………………………………………………………………………………………………………….
cathode ……………………………………………………………………………………………………………….
(d) Concentrated aqueous potassium chloride undergoes electrolysis.
(i) Write an ionic half-equation for the reaction at the negative electrode (cathode).
(ii) Name the product formed at the positive electrode (anode).
(iii) Name the potassium compound that remains in the solution after electrolysis.
(e) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of
chlorine, \(Cl_2\).
Show the outer electrons only.
(f) The melting points and boiling points of chlorine and potassium chloride are shown.
(i) Deduce the physical state of chlorine at –75°C. Use the data in the table to explain your
answer.
physical state ……………………………………………………………………………………………………….
explanation ………………………………………………………………………………………………………….
(ii) Explain, in terms of structure and bonding, why potassium chloride has a much higher melting point than chlorine.
Your answer should refer to the:
● types of particle held together by the forces of attraction
● types of forces of attraction between particles
● relative strength of the forces of attraction.
Answer/Explanation
Answer:
(a) \(2K + Cl_2 → 2KCl\)
\(Cl_2\) on left hand side (1)
equation fully correct (1)
(b) K outer shell with 8 crosses (1)
Cl outer shell with 7 dots and 1 cross (1)
\(^+\) and – (1)
(c) (i) breakdown by (the passage of) electricity (1)
of an ionic compound in molten or aqueous (state) (1)
(ii) (anode) chlorine
(cathode)potassium
(d) (i) \(2H^+ + 2e(^–) → H_2\)
\(H^+\) and \(e(^–)\) on left hand side (1)
equation fully correct (1)
(ii) chlorine
(iii) potassium hydroxide (1)
(e) one shared pair of electrons and 6 non-bonding electrons on each chlorine atom
(f) (i) liquid (1)
BOTH melting point is below –75 oC AND boiling point is above –75 oC
OR
BOTH –75 oC is higher than –101 oC / melting point AND lower than –35 oC / boiling point
OR
–75 oC is between melting point or –101 oC and boiling point or –35 oC
(ii) ionic bonds in KCl (1)
attraction between molecules in \(Cl_2\) (1)
weaker attraction (between particles) in \(Cl_2\) ORA (1)
Question
This question is about electrolysis.
Concentrated hydrochloric acid is electrolysed using the apparatus shown.
(a) Chloride ions are discharged at the anode.
(i) Complete the ionic half-equation for this reaction.
……….Cl– (aq) → ……….(g) + ……….e–
(ii) State whether oxidation or reduction takes place. Explain your answer.
(b) Describe what is seen at the cathode.
(c) Write the ionic half-equation for the reaction at the cathode.
(d) The pH of the electrolyte is measured throughout the experiment.
(i) Suggest the pH of the electrolyte at the beginning of the experiment.
(ii) State how the pH changes, if at all, during the experiment.
Explain your answer.
(e) The electrolysis is repeated using molten lead(II) bromide.
Describe what is seen at the:
● cathode ……………………………………………………………………………………………………………….
● anode. …………………………………………………………………………………………………………………
(f) State two properties of graphite (carbon) which make it suitable for use as an electrode.
1 ……………………………………………………………………………………………………………………………….
2 ……………………………………………………………………………………………………………………………….
Answer/Explanation
Answer:
(a) (i) \(2Cl^– → Cl_2 + 2e^–\)
\(Cl_2\) (1)
rest of equation (1)
(ii) Oxidation AND lose electrons
(b) effervescence (of colourless gas)
(c) \(2H^+ + 2e^– → H_2\)
\(H^+ + e\) as only species on LHS (1)
rest of equation fully correct (1)
(d) (i) 1
(ii) M1 increase (1)
M2 H+ ions being removed (1)
(e) M1 cathode: silver / grey solid (1)
M2 anode: bubbles of orange / brown gas (1)
(f) M1 inert (1)
M2 conducts electricity (1)