S.4 Chemistry Summer Assignment 2012-2013
Name: _________________________________ Block: _____
Class and class number: S.4 ___ (
)
Mark:
1. The wastewater generated from all electroplating factory contains dichromate ions. Before the
wastewater is discharged, it is treated in two stages as described below to remove the
chromium-containing substances.
Stage 1: Treat the wastewater with excess sodium sulphite solution in the presence of acid to reduce
the dichromate ions to chromium(III) ions.
Stage 2: Add a suitable chemical to the treated wastewater from to precipitate the chromium(III) ions.
(a) Why is it necessary to remove chromium-containing substances from the wastewater?
(b) In Stage 1, the sulphite ions are oxidised to sulphate ions by the dichromate ions.
(i) Write the half equation for the oxidation of sulphite ions.
(ii) Write the half equation for the reduction of dichromate ions
(iii)Write the overall equation for the redox reaction.
(c) Suggest a suitable chemical for the precipitation of chromium(III) ions in Stage 2.
[1 mark]
[3 marks]
[1 mark]
2. For each of the following experiments, state an expected observation and write a chemical equation for
the reaction involved.
(a) A magnesium ribbon is placed in a Bunsen flame.
(b) Excess iron(II) sulphate solution is added to an acidified potassium permanganate solution.
(c) Chlorine gas is bubbled into a sodium bromide solution.
[6 marks]
3. Discuss the similarities and differences in chemical properties of concentrated sulphuric acid and dilute
sulphuric acid. Illustrate your answer using appropriate examples.
[9 marks]
4. The diagram below shows the set-up used in performing a microscale experiment. A drop of
hydrochloric acid was added to the drop of chlorine bleach in a petri dish and the dish was immediately
covered with its lid. Chlorine gas formed by the reaction between chlorine bleach and hydrochloric acid
eventually filled up the whole set-up.
(a) State the expected observation at position C and position D. In each case, write a relevant ionic
equation if a reaction occurred.
[3 marks]
(b) Suggest one advantage of replacing test tube experiments with microscale experiments.
[1 mark]
5. Consider the conversions between three nitrogen compounds shown in the flow diagram below:
(a) Refer to Reaction 1.
(i) Suggest how NO(g) can be converted to NO2(g).
[1 mark]
(ii) State the expected observation for the reaction involved.
[1 mark]
(iii) Write a chemical equation for the reaction involved.
[1 mark]
(b) Refer to Reaction 2.
(i) Suggest how HNO3(aq) can be converted to NO2(g).
[1 mark]
(ii) State the expected observation for the reaction involved.
[1 mark]
(iii)Write an ionic equation for the reaction involved.
[1 mark]
(c) Refer to Reaction 3.
(i) Suggest how HNO3(aq) can be converted to NO(g).
[1 mark]
(ii) Write an ionic equation for the reaction involved.
[1 mark]
6. A student used an aluminium can, a carbon rod and household bleach to make a chemical cell. The
diagram below shows the set-up of the cell connected to a load and an ammeter.
(a) The materials used by the student to make the cell are readily available at home. Suggest ONE
household item
(i) which contains a carbon rod.
[1 mark]
(ii) which includes an aluminium can.
[1 mark]
(b) When the cell is producing a current, the aluminium can undergoes oxidation to give aluminate ions,
Al(OH)4-(aq), while at the carbon rod the hypochlorite ions undergo reduction in the presence of
water to give chloride ions and hydroxide ions.
Given that household bleach is alkaline, write half equations for
(i) the oxidation of the aluminium metal, and
[1 mark]
(ii) the reduction of the hypochlorite ions.
[1 mark]
7.
The following diagram shows the electrolysis of molten lead(II) bromide using graphite electrodes.
electrode B
electrode A
molten lead(II) bromide
heat
(a) Explain why electrolysis takes place only after lead(II) bromide is heated.
[1 mark]
(b) Write ionic half equations for the reactions taking place at electrode A and electrode B respectively.
[2 marks]
(c) Describe any observable changes at electrode A and electrode B respectively.
[2 marks]
(d) Suggest ONE safety precaution for the experiment and explain.
[2 marks]
8.
The flow diagram below shows the stages involved in the extraction of zinc from zinc blends, ZnS.
(i)
The reaction in Stage I gives, apart from zinc oxide, a gaseous product.
(1) Write the chemical equation for the reaction.
[1 mark]
(2) Give ONE industrial use of the gaseous product. [1 mark]
(ii) Suggest how zinc oxide can be converted to zinc sulphate solution in Stage II. [1 mark]
(iii) The zinc sulphate solution contained ions of other metals. During the electrolysis in Stage III,
zinc metal is liberated at one of the electrodes.
(1) Suggest ONE way to remove ions of metals which are less reactive than zinc from the zinc
sulphate solution before electrolysis.
[1 mark]
(2) Why is it not necessary to remove ions of metals which are more reactive than zinc from the
solution? [1 mark]
(3) Write half equations for the reactions occuring at the anode and cathode during the
electrolysis. [2 marks]
(iv) Give ONE use of zinc in daily life.
[1 mark]
9.
In the above diagram, P and Q are two different metals. When the circuit is closed, a current flows in
the external circuit. After some time, a reddish brown substance is deposited on the carbon electrode
R.
(i) What is the direction of electron flow in the external circuit?
Explain your answer. [2 marks]
(ii) After the circuit has been closed for some time, what would be observed
(1) at the carbon electrode S?
[1 mark]
(2) in the copper(II) sulphate solution?
[1 mark]
(iii) What is the function of set-up X in this experiment?
-End-
[1 mark]