Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2015 Winter Zone 3

(a) Describe a chemical test which shows the presence of water.
test ..........................................................
colour change if water is present ..........................................................
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(b) How could you show that a sample of water is pure?
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(c) Describe how water is treated before it is supplied to homes and industry.
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(d) State \textit{two} industrial uses of water.
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Choose from the following list of gases. A gas may be chosen once, more than once or not at all.

sulfur dioxide hydrogen methane carbon monoxide
argon ethene butane

(a) It is used to bleach wood pulp. ............................................................. [1]
(b) When burned in oxygen, the only product is water. ......................................................... [1]
(c) It can polymerise. ............................................................. [1]
(d) It is used to provide an inert atmosphere for welding. ..................................... [1]
(e) When reacted with oxygen, the only product is carbon dioxide. ......................................... [1]
(f) It is produced by the decay of vegetation in the absence of oxygen. ....................................... [1]

Lithium bromide is an ionic compound. It can be electrolysed when it is molten or in aqueous solution. It cannot be electrolysed as a solid.
(a) Solid lithium bromide is a poor conductor of electricity. The ions cannot move to the electrodes, they are held in an ionic lattice by strong forces.
(i) Describe the motion of the ions in the solid state.
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(ii) Define the term ionic bonding.
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(iii) What is meant by the term ionic lattice?
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(b) The diagram shows the electrolysis of molten lithium bromide.

(i) Mark on the diagram the direction of the electron flow. [1]
(ii) Write an ionic equation for the reaction at the negative electrode (cathode).
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(iii) Write an ionic equation for the reaction at the positive electrode (anode).
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(iv) Which ion is oxidised? Explain your answer.
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(c) When aqueous lithium bromide is electrolysed, a colourless gas is formed at the negative electrode and the solution becomes alkaline.
Explain these observations and include an equation in your explanation.
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Two homologous series of hydrocarbons are the alkanes and the alkenes.
(a) (i) One general characteristic of a homologous series is that the physical properties vary in a predictable way.
State three other general characteristics of a homologous series.
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(ii) How can the molecular formula of a hydrocarbon show whether it is an alkane or an alkene?
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(iii) How do alkanes and alkenes differ in their molecular structures?
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(b) Cracking is the thermal decomposition of alkanes into smaller hydrocarbons and possibly hydrogen.
(i) State two conditions required for the cracking of an alkane.
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(ii) One type of cracking produces an alkane and an alkene.
Complete an equation for the cracking of heptane into an alkane and an alkene.
$$C_7H_{16} \rightarrow \text{.................} + \text{.................}$$ [1]

(iii) Complete an equation for the cracking of heptane into hydrogen and two other products.
$$C_7H_{16} \rightarrow \text{.................} + \text{.................} + \text{ } H_2$$ [1]

(iv) Suggest one reason why cracking is important.
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(c) Hydrocarbons burn in excess oxygen to form carbon dioxide and water. 20 cm$^3$ of a gaseous hydrocarbon burned in an excess of oxygen, 200 cm$^3$. After cooling, the volume of the residual gas at r.t.p. was 150 cm$^3$, 50 cm$^3$ of which was oxygen.
(i) Determine the volume of the oxygen used.
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(ii) Determine the volume of the carbon dioxide formed.
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(iii) The hydrocarbon was an alkane.
Determine the formula of the hydrocarbon.
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Sulfuric acid is a strong acid. In aqueous solution, it ionises as shown below.

$$\text{H}_2\text{SO}_4 \to 2\text{H}^+ + \text{SO}_4^{2-}$$

(a) (i) What is meant by the term \textit{acid}?
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(ii) Sulfurous acid, $\text{H}_2\text{SO}_3$, is a weak acid.
State the difference between a weak acid and a strong acid.
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(b) Sulfurous acid forms salts called sulfites, which contain the ion $\text{SO}_3^{2-}$.

When barium nitrate solution is added to aqueous sulfurous acid, a white precipitate, $A$, forms.
Bromine water changes from brown to colourless when added to aqueous sulfurous acid.
Bromine oxidises sulfurous acid. When this solution is tested with acidified barium nitrate solution, a different white precipitate, $B$, is formed.

(i) Identify the white precipitate, $A$.
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(ii) Identify the white precipitate, $B$.
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(iii) Write an ionic equation for the reduction of the bromine molecule.
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(iv) Name the product formed by the oxidation of sulfurous acid.
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(c) Complete the following word equations.

(i) magnesium hydroxide + dilute sulfuric acid
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(ii) zinc + dilute sulfuric acid
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(iii) copper carbonate + dilute sulfuric acid
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(d) Write equations for the reaction of dilute sulfuric acid with each of the following.

(i) ammonia
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(ii) sodium hydroxide
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(iii) iron
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(a) Which two metals will react most vigorously with cold water?
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(b) Which two metals will not react with dilute hydrochloric acid?
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(c) Deduce the formula of iron(III) sulfate.
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(d) What is the formula of a magnesium ion?
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(e) Describe a test-tube experiment which will show that manganese is more reactive than copper.
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(f) Manganese is a typical transition metal.
Predict three physical and two chemical properties of this metal.
physical properties
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chemical properties
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Two salts can be made from potassium hydroxide and sulfuric acid. They are potassium sulfate, $\text{K}_2\text{SO}_4$, and the acid salt potassium hydrogen sulfate, $\text{KHSO}_4$. They are both made by titration.

(a) 25.0 $\text{cm}^3$ of potassium hydroxide, concentration 2.53 mol/dm$^3$, was neutralised by 28.2 $\text{cm}^3$ of dilute sulfuric acid.
$$2\text{KOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{K}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$$
Calculate the concentration of the sulfuric acid.
number of moles of KOH used = .............................
number of moles of $\text{H}_2\text{SO}_4$ needed to neutralise the KOH = .............................
concentration of dilute sulfuric acid = ............................. mol/dm$^3$ [3]

(b) In the conical flask there is a neutral solution of potassium sulfate which still contains the indicator used in the titration.
(i) Describe how you could obtain a solution of potassium sulfate without the indicator.
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(ii) Potassium hydrogen sulfate can be made by the following reaction.
$$\text{KOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{KHSO}_4\text{(aq)} + \text{H}_2\text{O(l)}$$
Suggest how you could make a solution of potassium hydrogen sulfate without using an indicator.
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(c) Describe a test which would distinguish between aqueous solutions of potassium sulfate and sulfuric acid.
test ..................................................................................................................
result ................................................................................................................ [2]