Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2010 Winter Zone 2

The following table gives information about six substances.

[Table_1: Contains columns for substances A to F with melting points, boiling points, and electrical conductivity as solid and liquid.]

(a) Which substance could have a macromolecular structure, similar to that of silicon(IV) oxide?
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(b) Which substances are solids at room temperature?
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(c) Which substance could be a metal?
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(d) Which substance could be aqueous sodium chloride?
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(e) Which substance is an ionic compound?
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(f) Which substances are liquids at room temperature?
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An ore of the important metal zinc is zinc blende, ZnS. This is changed into zinc oxide which is reduced to the impure metal by carbon reduction.
(a) (i) How is zinc oxide obtained from zinc sulfide?
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(ii) Write a balanced equation for the reduction of zinc oxide by carbon.
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(iii) The major impurity in the zinc is cadmium. The boiling point of zinc is 907°C and that of cadmium is 767°C. Name a technique which could be used to separate these two metals.
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(b) In common with most metals, zinc is a good conductor of electricity. It is used as an electrode in cells.
(i) Give two other uses of zinc.
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(ii) Describe the metallic bonding in zinc and then explain why it is a good conductor of electricity.
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The decomposition of hydrogen peroxide is catalysed by manganese(IV) oxide.

$\text{2H}_2\text{O}_2(\text{aq}) \rightarrow \text{2H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$

To 50 cm$^3$ of aqueous hydrogen peroxide, 0.50 g of manganese(IV) oxide was added. The volume of oxygen formed was measured every 20 seconds. The average reaction rate was calculated for each 20 second interval.

[Table_1]

(a) Explain how the average reaction rate, 2.4 cm$^3$/s, was calculated for the first 20 seconds.

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(b) Complete the table. [1]

(c) Explain why the average reaction rate decreases with time.

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(d) The experiment was repeated but 1.0 g of manganese(IV) oxide was added.
What effect, if any, would this have on the reaction rate and on the final volume of oxygen?
Give a reason for each answer.

effect on rate ................................................................................................
reason ..........................................................................................................
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effect on final volume of oxygen ...............................................................
reason ..........................................................................................................
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Chromium is a transition element.
(a) (i) Predict two differences in the physical properties of chromium and sodium.
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(ii) Predict two differences in the chemical properties of chromium and sodium.
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(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.
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(ii) The formula of the chromium(III) ion is $\text{Cr}^{3+}$ and of the sulfate ion is $\text{SO}_4^{2-}$. Give the formula of chromium(III) sulfate.
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(iii) Write the equation for the reaction at the negative electrode (cathode).
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(iv) A colourless gas, which relights a glowing splint, is formed at the positive electrode (anode). Name this gas.
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(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.
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In the absence of oxygen, certain bacteria decompose carbohydrates to biogas. This is a mixture of gases mainly methane and carbon dioxide. Biogas is becoming an increasingly important fuel around the world.
A diagram of a simple biogas generator is given below. Typically, it contains biomass - animal manure, plant material etc.
(a) (i) What is meant by the term carbohydrate? ............................................................................................................................. ............................................................................................................................. [2]
(ii) The reaction in the generator is an example of anaerobic respiration. Anaerobic means in the absence of oxygen. What does respiration mean? ............................................................................................................................. ............................................................................................................................. [2]
(iii) The generator must produce some carbon dioxide. Why is it impossible for it to produce only a hydrocarbon such as methane? ............................................................................................................................. [1]
(iv) Suggest a use for the nitrogen-rich solid removed from the generator. ............................................................................................................................. [1]

(b) (i) In an experiment, a 60 cm³ sample of biogas required 80 cm³ of oxygen for the complete combustion of the methane in the sample. Calculate the percentage of methane in the sample of biogas. Assume that biogas contains only methane and carbon dioxide.
$${\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}}$$ ............................................................................................................................. ............................................................................................................................. [2]
(ii) Carbon dioxide is acidic and methane is neutral. Suggest another way of measuring the volume of methane in the sample. ............................................................................................................................. [2]

The alcohols form an homologous series.
(a) Give three characteristics of an homologous series.
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(b) The following two alcohols are members of the series and they are isomers.
$$\text{CH}_3\text{—CH}_2\text{—CH}_2\text{—CH}_2\text{—OH} \quad \text{and} \quad (\text{CH}_3)_2\text{CH—CH}_2\text{OH}$$
(i) Explain why they are isomers.
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(ii) Give the structural formula of another alcohol which is also an isomer of these alcohols. [1]

(c) Copper(II) oxide can oxidise butan-1-ol to liquid X whose pH is 4.

(i) Name another reagent which can oxidise butan-1-ol.
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(ii) What type of compound is liquid X and what is its formula?
type of compound ................................................................................................ [1]
formula of liquid X

(d) The alcohol ethanol can be made by fermentation. Yeast is added to aqueous glucose.
$$\text{C}_6\text{H}_{12}\text{O}_6(\text{aq}) \rightarrow 2\text{C}_2\text{H}_5\text{OH}(\text{aq}) + 2\text{CO}_2(\text{g})$$
Carbon dioxide is given off and the mixture becomes warm as the reaction is exothermic.
The graph shows how the rate of reaction varies over several days.

(i) Suggest a method of measuring the rate of this reaction.
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(ii) Why does the rate increase initially?
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(iii) Suggest two reasons why the rate eventually decreases.
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(iv) Why is fermentation carried out in the absence of air?
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The major use of sulfur dioxide is to manufacture sulfuric acid.
(a) (i) Another use of sulfur dioxide is as the food additive E220. How does it preserve food? .......................................................................................................................................
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(ii) Why is sulfur dioxide used in the manufacture of wood pulp? ....................................................................................................................................... [1]
(iii) How is sulfur dioxide manufactured? ....................................................................................................................................... [1]

(b) Complete the following description of the manufacture of sulfuric acid.
Sulfur dioxide reacts with ............................. to form sulfur trioxide.
The above reaction is catalysed by ............................. .
The optimum temperature for this reaction is ............................. °C.
Sulfur trioxide needs to react with ............................. to form sulfuric acid. [4]

(c) (i) Define the term acid. ....................................................................................................................................... [1]
(ii) Sulfuric acid is a strong acid. Ethanedioic acid is a weak acid. Given solutions of both acids, how could you show that sulfuric acid is a strong acid and ethanedioic acid is a weak acid?
method .......................................................................................................................................
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result for each acid .......................................................................................................................................
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(d) 20.0 cm³ of sulfuric acid, concentration 0.30 mol/dm³, was added to 40 cm³ of sodium hydroxide, concentration 0.20 mol/dm³.

$$2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$$

(i) How many moles of H₂SO₄ were added? ............................. [1]
(ii) How many moles of NaOH were used? ............................. [1]
(iii) Which reagent is in excess? Give a reason for your choice.
reagent in excess .............................
reason ............................. ....................................................................................................................................... [1]
(iv) Is the pH of the final mixture less than 7, equal to 7 or more than 7?
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