Cambridge IGCSE Chemistry - 0620 - Supplement Paper 4 2015 Summer Zone 2

Complete the following table which gives the number of protons, electrons and neutrons in each of the five particles.

[Table_1]
| particle | number of protons | number of electrons | number of neutrons |
|----------|-------------------|--------------------|--------------------|
| ............ | 19 | 19 | 20 |
| $^{56}_{26}\text{Fe}$ | ............ | ............ | ............ |
| ............ | 3 | 2 | 4 |
| $^{70}_{31}\text{Ga}^{3+}$ | ............ | ............ | ............ |
| ............ | 34 | 36 | 45 |

The table shows the melting points, boiling points and electrical properties of five substances, A to E.

[Table_1]

Choose a substance from the table above to match each of the following descriptions. A substance may be used once, more than once or not at all. Justify each choice with evidence from the table.

One has been completed as an example.

This substance is covalent and is a solid at room temperature (25°C). ............... D
evidence Its melting point is above room temperature. It has a low melting point and it does not conduct as a liquid, so it is covalent.

(a) This substance has a giant covalent structure. ...............
evidence .........................................................................................................................................
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(b) This substance is a metal. ...............
evidence .........................................................................................................................................
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(c) This substance is a liquid at room temperature (25°C). ...............
evidence .........................................................................................................................................
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(d) This substance is an ionic solid. ...............
evidence .........................................................................................................................................
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Calcium reacts with nitrogen to form the ionic compound calcium nitride, $\text{Ca}_3\text{N}_2$.
(a) Draw a diagram, based on the correct formula, which shows the charges on the ions and the arrangement of the electrons around the negative ion.
Use o to represent an electron from a calcium atom.
Use x to represent an electron from a nitrogen atom.

[3]
(b) In the lattice of calcium nitride, the ratio of calcium ions to nitride ions is 3:2.
(i) What is meant by the term *lattice*?
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(ii) In terms of ionic charges, explain why the ratio of ions is 3:2.
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(c) The reaction between calcium and nitrogen to form calcium nitride is a redox reaction.
In terms of electron transfer, explain why calcium is the reducing agent.
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Ammonia is made by the Haber process.

N$_2$(g) + 3H$_2$(g) ⇌ 2NH$_3$(g)
The forward reaction is exothermic.
Typical reaction conditions are:
• finely divided iron catalyst,
• temperature 450°C,
• pressure 200 atmospheres.
(a) Explain why the catalyst is used as a very fine powder and larger pieces of iron are not used.
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(b) Using the above conditions, the equilibrium mixture contains about 15% ammonia.
State two changes to the reaction conditions which would increase the percentage of ammonia at equilibrium.
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(c) Suggest why the changes you have described in (b) are not used in practice.
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[Table_1]
(c) decreased \underline{temperature} would reduce rate/reaction \underline{slower}/too slow;
increased \underline{pressure} expensive/uneconomic/safety risks/leaks/explosions/yield or rate good enough at lower pressure/strong pipes needed/thick pipes needed/sturdy pipes needed/requires a lot of energy;

Three common methods of preparing salts are shown below.

method A adding an excess of an insoluble base or carbonate or metal to a dilute acid and removing excess by filtration
method B using a burette and indicator
method C mixing two solutions to obtain the salt by precipitation

For each of the following salt preparations, choose a method, A, B or C. Name any additional reagent which is needed and complete the equation.

(a) the soluble salt, nickel chloride, from the insoluble compound nickel carbonate
method ....................................................................................................................
reagent .....................................................................................................................
word equation .....................................................................................................
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(b) the insoluble salt, lead(II) bromide, from aqueous lead(II) nitrate
method ...................................................................................................................
reagent ...................................................................................................................
ionic equation ........ + ........ → PbBr₂
[3]

(c) the soluble salt, lithium sulfate, from the soluble base lithium hydroxide
method ....................................................................................................................
reagent .....................................................................................................................
equation ................................................................................................................
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The Atacama desert in Chile has deposits of the salt sodium nitrate. Very large amounts of this salt were exported to Europe for use as a fertiliser. After the introduction of the Haber process in 1913, this trade rapidly diminished.

(a) (i) Explain why the introduction of the Haber process reduced the demand for sodium nitrate.
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(ii) Suggest why surface deposits of sodium nitrate only occur in areas with very low rainfall such as desert areas.
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(iii) The desert has smaller surface deposits of potassium nitrate. Suggest why potassium nitrate is a better fertiliser than the sodium salt.
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[1]

(b) All nitrates decompose when heated. The extent to which a nitrate decomposes is determined by the metal in the salt.

(i) Sodium nitrate decomposes to form sodium nitrite, NaNO₂. Write the equation for decomposition of sodium nitrate.
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[2]

(ii) Sodium nitrite is a reducing agent. What would be observed if an excess of sodium nitrite solution was added to a solution of acidified potassium manganate(VII)?
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[2]

(iii) Copper(II) nitrate decomposes to form copper(II) oxide, nitrogen dioxide and oxygen. What is the relationship between the extent of decomposition and the reactivity of the metal in the nitrate?
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[1]

(c) The equation for the decomposition of copper(II) nitrate is given below.

$$2 ext{Cu(NO}_3 ext{)}_2 ightarrow 2 ext{CuO} + 4 ext{NO}_2 + ext{O}_2$$

(i) Predict what you would observe when copper(II) nitrate is heated.
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(ii) Copper(II) nitrate forms a series of hydrates with the formula Cu(NO₃)₂·xH₂O. All these hydrates decompose to form copper(II) oxide. 1 mole of Cu(NO₃)₂·xH₂O forms 1 mole of CuO. What is meant by 1 mole of a substance?
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[2]

(iii) 7.26g of a hydrate, Cu(NO₃)₂·xH₂O, formed 2.4g copper(II) oxide.
number of moles of CuO formed = .......................
number of moles of Cu(NO₃)₂·xH₂O in 7.26g = .......................
mass of 1 mole of Cu(NO₃)₂·xH₂O = ....................... g
mass of 1 mole of Cu(NO₃)₂ is 188g
the value of x in this hydrate = .......................
[4]

(a) Ethanol can be made by the fermentation of glucose.
$$C_6H_{12}O_6(aq) \overset{yeast}{\longrightarrow} 2C_2H_5OH(aq) + 2CO_2(g)$$ exothermic reaction
Yeast are living single-cell fungi which ferment glucose by anaerobic respiration. This reaction is catalysed by enzymes from the yeast.
(i) What is meant by the term respiration?
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(ii) \textit{Anaerobic} means in the absence of oxygen.
Name the products formed from respiration in the \textit{presence} of oxygen.
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(iii) What are enzymes?
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(iv) Suggest a method of measuring the rate of this reaction.
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(b) The following observations were noted.
• When a small amount of yeast was added to the aqueous glucose the reaction started and the solution went slightly cloudy.
• The reaction rate increased and the solution became cloudier and warmer.
• After a while, the reaction rate decreased and eventually stopped, leaving a 14% solution of ethanol in water.
(i) Why did the reaction rate increase?
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(ii) Suggest an explanation for the increase in cloudiness of the solution.
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(iii) Give \textit{two} reasons why the fermentation stopped.
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(c) One use of ethanol is in alcoholic drinks.
Give \textit{two} other uses of ethanol.
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(d) Alcohols can be made from petroleum by the following sequence of reactions.
alkanes from petroleum \longrightarrow alkene \longrightarrow alcohol
Describe the manufacture of ethanol from hexane, $C_6H_{14}$. Include in your description an equation and type of reaction for each step.
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