Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2019 Winter Zone 1

This question is about ions and ionic compounds.
(a) Choose from the following list of ions to answer the questions.
Br⁻ Ca²⁺ Cl⁻ Cr³⁺ Cu²⁺ K⁺ Li⁺ Na⁺ SO₃²⁻ SO₄²⁻
Each ion may be used once, more than once or not at all.
State which ion:
(i) gives a lilac colour in a flame test ...................................................... [1]
(ii) forms a grey-green precipitate with aqueous ammonia .................... [1]
(iii) forms a white precipitate with aqueous sodium hydroxide ............... [1]
(iv) forms a cream precipitate with acidified aqueous silver nitrate ........ [1]
(v) forms a white precipitate with acidified aqueous barium nitrate ...... [1]
(b) Describe how to do a flame test on a sample of a salt.
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(c) Magnesium phosphate contains magnesium ions, Mg²⁺, and phosphate ions, PO₄³⁻.
Deduce the formula of magnesium phosphate.
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(a) Sulfur exists as a number of different isotopes.
What is meant by the term isotopes?
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(b) A sulfide ion has the symbol shown.
$$^{34}_{16}\text{S}^{2-}$$
(i) How many neutrons are contained in this sulfide ion?
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(ii) How is a sulfide ion, $\text{S}^{2-}$, formed from a sulfur atom?
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(iii) Which element forms an ion with a 2+ charge that has the same number of electrons as a $\text{S}^{2-}$ ion?
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(c) The manufacture of sulfuric acid by the Contact process occurs in four stages.

stage 1 Molten sulfur is burned in air to produce sulfur dioxide gas.
stage 2 Sulfur dioxide is reacted with oxygen to form sulfur trioxide.
stage 3 Sulfur trioxide is combined with concentrated sulfuric acid to form oleum, $\text{H}_{2}\text{S}_{2}\text{O}_{7}$.
stage 4 Oleum is added to water to form sulfuric acid.

(i) Complete the chemical equation for stage 1 by adding the appropriate state symbols.
$\text{S}(.....) + \text{O}_{2}(.....) \rightarrow \text{SO}_{2}(.....)$ [1]

(ii) Name the catalyst used in stage 2 and state the temperature used.
catalyst .........................................
temperature .................................. °C [2]

(iii) Write chemical equations for the reactions in stage 3 and stage 4.
stage 3 .........................................................................................................................................
stage 4 ......................................................................................................................................... [2]

(d) Sulfur dioxide is a toxic gas.

(i) State one environmental reason why sulfur dioxide should not be released into the atmosphere.
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(ii) Describe the test for sulfur dioxide.
test .................................................................................................................................
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observations ......................................................................................................................................... [2]

(e) Sulfur dioxide reacts with aqueous sodium sulfite to produce a compound with the following composition by mass: 29.1% Na, 40.5% S and 30.4% O.

Calculate the empirical formula of this compound.
empirical formula = ......................................... [3]

This question is about metals and metal oxides.
(a) Most metals have a high melting point.
State one other physical property that all metals have.
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(b) Iron often rusts.
Name the two substances, other than iron, that must be present for iron to rust.
1 ..............................................................................................................................
2 ..............................................................................................................................

(c) Iron can be obtained by heating iron(III) oxide with zinc powder.
$$ \text{Fe}_2\text{O}_3 + 3\text{Zn} \rightarrow 2\text{Fe} + 3\text{ZnO} $$
(i) What can be deduced about the reactivity of zinc from this reaction?
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(ii) The ionic equation for this reaction is shown.
$$ 2\text{Fe}^{3+} + 3\text{Zn} \rightarrow 2\text{Fe} + 3\text{Zn}^{2+} $$
Identify the oxidising agent in this reaction. Explain your answer in terms of electron transfer.
oxidising agent ..............................................................................................................................
explanation ..............................................................................................................................
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(d) Zinc oxide is amphoteric.
Describe two simple experiments to show that zinc oxide is amphoteric.
Name the reagents you would use and describe the observations you would make.
reagent 1 ..............................................................................................................................
observation ..............................................................................................................................
reagent 2 ..............................................................................................................................
observation ..............................................................................................................................

Insoluble salts can be made by precipitation reactions. A student mixed solutions of some soluble salts. The results the student obtained are shown in the table.
[Table_1: second salt solution table with NaI(aq), Na2CO3(aq), Na2SO4(aq) interactions]
All sodium salts are soluble in water. Use only results from the table to answer the following questions.
(a) Name:
(i) an insoluble cobalt salt ......................................................... [1]
(ii) an insoluble yellow lead salt ......................................................... [1]
(b) Write the chemical equation for the reaction in which silver carbonate is formed. ................................................................. [2]
(c) Write the ionic equation for the reaction in which lead(II) iodide is formed. ................................................................. [2]
(d) Aqueous silver nitrate produces a yellow precipitate with both iodide ions and carbonate ions. When testing an unknown solution for iodide ions, the aqueous silver nitrate is acidified.
Explain why the aqueous silver nitrate is acidified. ................................................................. [1]

(a) Part of the structure of synthetic polymer A is shown.

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(i) What type of synthetic polymer is A?
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(ii) Deduce the empirical formula of polymer A.
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(iii) Draw the structure of the monomer from which polymer A is made.
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(b) The formula $C_4H_{10}$ represents two different structural isomers.

(i) What is meant by the term structural isomers?
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(ii) Draw the structures of two structural isomers with the formula $C_4H_{10}$. Show all of the atoms and all of the bonds.
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(iii) All structural isomers of $C_4H_{10}$ are flammable.
Write a chemical equation for the incomplete combustion of $C_4H_{10}$.
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Dilute hydrochloric acid, HCl(aq), reacts with aqueous sodium carbonate, Na_2CO_3(aq).
The chemical equation for the reaction is shown.
$2HCl + Na_2CO_3 \rightarrow 2NaCl + CO_2 + H_2O$
(a) A $25.0 \text{cm}^3$ portion of $Na_2CO_3(aq)$ was placed in a conical flask with a few drops of a suitable indicator. It was titrated against $HCl(aq)$ of concentration $0.180\text{ mol/dm}^3$.
$20.0 \text{cm}^3$ of $HCl(aq)$ was required to reach the end-point.
Calculate the concentration of the $Na_2CO_3(aq)$, in $\text{mol/dm}^3$, using the following steps.
• Calculate the number of moles of $HCl$ used in the titration.
................................. mol
• Calculate the number of moles of $Na_2CO_3$ contained in the $25.0 \text{cm}^3$ portion of $Na_2CO_3(aq)$.
................................. mol
• Calculate the concentration of the $Na_2CO_3(aq)$ in $\text{mol/dm}^3$.
................................. $\text{mol/dm}^3$ [3]
(b) In another experiment, the volume of carbon dioxide, $CO_2$, produced was $48.0\text{cm}^3$, measured at room temperature and pressure.
How many moles of $CO_2$ is this?
moles of $CO_2 =$ .............................. mol [1]

(c) A sample of concentrated hydrobromic acid, HBr(aq), was electrolysed using platinum electrodes.
The concentration of the hydrobromic acid was $8.89 \text{mol/dm}^3$.
(i) Calculate the concentration of the HBr(aq) in $g/dm^3$.
concentration of HBr(aq) = .......................... $g/dm^3$ [1]
(ii) Explain why concentrated HBr(aq) can conduct electricity.
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(iii) Magnesium is \textbf{not} a suitable material from which to make the electrodes.
Explain why.
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(iv) Predict the product formed at the anode when concentrated HBr(aq) is electrolysed.
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[1]
(v) Write the ionic half-equation for the reaction occurring at the cathode.
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[2]

This question is about ethanol.

(a) Ethanol that is suitable for use as a fuel can be manufactured from sugars such as glucose, $C_6H_{12}O_6$, by a two-step process.
Describe how this can be done. In your answer, include:
• an equation for the reaction in which ethanol is formed
• the essential conditions for the reaction in which ethanol is formed
• the name of the process used to obtain ethanol that is pure enough to use as a fuel from the reaction mixture.
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(b) The equation for the complete combustion of ethanol is shown:
       H                                                                         O         O
       |                                                                         ||
       H—C—C  +   3O=     →  2C=O   +  3H—O—H
       |                                                                                                                                            H H H (each letters belong to respective molecules)
Use the bond energies in the table to calculate the energy change, in kJ/mol, for the complete combustion of ethanol.

[Table_1]
bond | bond energy in kJ/mol
C—C | 347
C—H | 413
C—O | 358
C=O | 805
O—H | 464
O=O | 498

• Energy needed to break bonds. ................................. kJ
• Energy released when bonds are formed. ............................. kJ
• Energy change for the complete combustion of ethanol.
energy change = ............................. kJ/mol [3]

(c) Ethanol can be oxidised by hydrogen peroxide to form ethanal, $CH_3CHO$. A catalyst for this reaction is $Fe^{3+}$.
(i) What is meant by the term catalyst?
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(ii) The structure of ethanal is shown.

Complete the dot-and-cross diagram to show the electron arrangement in a molecule of ethanal. Show outer shell electrons only.
[3]

(iii) The table gives the boiling points of ethanal and ethanol.
[Table_2]
substance | boiling point/°C
ethanal | 20
ethanol | 78
In terms of attractive forces between particles, suggest why ethanal has a lower boiling point than ethanol.
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(d) Ethene gas reacts with steam to form gaseous ethanol.

$$C_2H_4(g) + H_2O(g) ightleftharpoons CH_3CH_2OH(g)$$
The reaction can reach a position of equilibrium. The forward reaction is exothermic.

(i) State and explain the effect of increasing the pressure on the position of equilibrium. All other conditions are unchanged.
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(ii) Increasing the pressure of a gas increases its concentration.

State and explain the effect of increasing the pressure on the rate of the reaction. All other conditions are unchanged.
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(iii) State and explain the effect of increasing the temperature on the position of equilibrium. All other conditions are unchanged.
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