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An important aspect of chemistry is purity and methods of purification.
(a) Give an example of substances used in everyday life which must be pure.
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(b) A list of techniques used to separate mixtures is given below.
chromatography crystallisation diffusion dissolving
evaporation filtration fractional distillation simple distillation
(i) From the list, choose the most suitable technique to separate the following.
water from sea-water ..........................................................................................................................
helium from a mixture of helium and methane ......................................................................
ethanol from a mixture of ethanol and propanol ...............................................................
iron filings from a mixture of iron filings and water ..........................................................
a mixture of two amino acids, glycine and alanine ........................................................... [5]
(ii) Describe how you would obtain a pure sample of copper(II) sulfate-5-water crystals from a mixture of copper(II) sulfate-5-water with copper(II) oxide using some of the techniques listed above.
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597
510
526
Aluminium is obtained by the reduction of aluminium ions to aluminium atoms.
(a) Write an ionic equation for the reduction of an aluminium ion to an aluminium atom.
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(b) The original method of extracting aluminium involved the reduction of aluminium chloride using the reactive metal sodium. Aluminium obtained by this method was very expensive due to the high cost of extracting sodium from sodium chloride.
(i) Complete the equation for this reduction.
$$\text{AlCl}_3 + \text{.......Na} \rightarrow \text{.......... + ..........}$$
(ii) How can sodium metal be obtained from sodium chloride?
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(c) In the modern method, aluminium is obtained by the electrolysis of aluminium oxide (alumina) dissolved in molten cryolite, $$\text{Na}_3\text{AlF}_6$$.
(i) The major ore of aluminium is impure aluminium oxide. What is the name of this ore?
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(ii) This ore is a mixture of aluminium oxide, which is amphoteric, and iron(III) oxide which is basic. Explain how these two oxides can be separated by the addition of aqueous sodium hydroxide.
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(iii) Give two reasons why the electrolyte contains cryolite.
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(iv) The mixture of gases evolved at the positive electrode includes:
carbon dioxide
carbon monoxide
fluorine
oxygen
Explain the presence of these gases in the gaseous mixture formed at the positive electrode. Include at least one equation in your explanation.
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(d) A major use of aluminium is the manufacture of pots and pans. One reason for this is its resistance to corrosion.
(i) Explain why aluminium, a reactive metal, is resistant to corrosion.
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(ii) Suggest two other reasons why aluminium is suitable for making pots and pans.
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504
544
502
(a) A hydrocarbon has the following structural formula.
(i) State the molecular formula and the empirical formula of this hydrocarbon.
molecular formula ........................................................
empirical formula ........................................................
[2]
(ii) Draw the structural formula of an isomer of the above hydrocarbon.
[1]
(iii) Explain why these two hydrocarbons are isomers.
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[2]
(iv) Are these two hydrocarbons members of the same homologous series?
Give a reason for your choice.
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[1]
(b) Alkenes can be made from alkanes by cracking.
(i) Explain the term cracking.
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[2]
(ii) One mole of an alkane, when cracked, produced one mole of hexane, $C_6H_{14}$, and two moles of ethene.
What is the molecular formula of the original alkane?
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[1]
(c) Alkenes are used in polymerisation reactions and addition reactions.
(i) Draw the structural formula of the product formed by the addition polymerisation of but-2-ene. Its formula is given below.
[3]
(ii) Give the name and structural formula of the addition product formed from ethene and bromine.
name ........................................................
structural formula ........................................................
[2]
552
520
599
(a) Name an alloy which contains zinc. What is the other metal in this alloy?
name of alloy ...............................................................
other metal in alloy ...............................................................
(b) The main ore of zinc is zinc blende, ZnS.
(i) The ore is heated in the presence of air to form zinc oxide and sulfur dioxide. Write the equation for this reaction.
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(ii) Give a major use of sulfur dioxide.
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(c) Zinc can be obtained from zinc oxide in a two step process. Aqueous zinc sulfate is made from zinc oxide and then this solution is electrolysed with inert electrodes. The electrolysis is similar to that of copper(II) sulfate with inert electrodes.
(i) Name the reagent which will react with zinc oxide to form zinc sulfate.
................................................................................. [1]
(ii) Complete the following for the electrolysis of aqueous zinc sulfate.
Write the equation for the reaction at the negative electrode.
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Name the product at the positive electrode.
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The electrolyte changes from zinc sulfate to ......................................... . [3]
(d) A dry cell (battery) has a central rod, usually made of graphite. This is the positive electrode which is surrounded by the electrolyte, typically a paste of ammonium chloride and manganese(IV) oxide, all of which are in a zinc container which is the negative electrode.
(i) Draw an arrow on the diagram to indicate the direction of electron flow. [1]
(ii) Suggest why the electrolyte is a paste.
................................................................................. [1]
(iii) The following changes occur in a dry cell. For each change, decide if it is oxidation or reduction and give a reason for your choice.
Zn to $\text{Zn}^{2+}$
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manganese(IV) oxide to manganese(III) oxide
................................................................................. [2]
513
555
598
(a) Glucose, sucrose and starch are all carbohydrates. Their formulae are:
glucose, $C_6H_{12}O_6$,
sucrose, $C_{12}H_{22}O_{11}$,
starch, $(C_6H_{10}O_5)_n$.
(i) Identify two common features in the formulae of these carbohydrates.
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(ii) Draw the structure of a complex carbohydrate, such as starch. The formula of glucose, can be represented by
Include three glucose units in the structure. [2]
(b) Starch hydrolyses to glucose in the presence of the enzyme, amylase.
What is meant by the term enzyme?
................................................................................................................................................................. [2]
(c) The effect of temperature on this reaction can be studied by the experiment shown below.
Starch and iodine form a blue-black colour. Glucose and iodine do not form a blue-black colour.
The experiment is set up as in the diagram and the time measured for the mixture to change from blue-black to colourless. The experiment is repeated at different temperatures. Typical results of this experiment are given in the table below.
[Table_1]
(i) Put the experiments in order of reaction rate - slowest first and fastest last.
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(ii) Explain why the reaction rates in experiments A and B are different.
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(iii) Suggest why the colour remains blue-black in experiment C.
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592
561
550
(a) Give a major use of sulfuric acid.
(b) A group of naturally occurring minerals have the formula of the type FeSO_4.xH_2O where x is 1, 4, 5, 6 or 7. The most common of these minerals is iron(II) sulfate-7-water.
(i) When this mineral is heated gently it dehydrates.
$$\text{FeSO}_4\cdot 7\text{H}_2\text{O} \rightleftharpoons \text{FeSO}_4 + 7\text{H}_2\text{O}$$
green pale yellow
Describe how you could show that this reaction is reversible.
[2]
(ii) When the iron(II) sulfate is heated strongly, further decomposition occurs.
$$2\text{FeSO}_4\text{(s)} \rightarrow \text{Fe}_2\text{O}_3\text{(s)} + \text{SO}_2\text{(g)} + \text{SO}_3\text{(g)}$$
The gases formed in this reaction react with water and oxygen to form sulfuric acid. Explain how the sulfuric acid is formed.
[2]
(iii) A mineral of the type FeSO_4.xH_2O contains 37.2% of water. Complete the calculation to determine x.
mass of one mole of H_2O = 18 g
mass of water in 100 g of FeSO_4.xH_2O = 37.2 g
number of moles of H_2O in 100 g of FeSO_4.xH_2O = ...............
mass of FeSO_4 in 100 g of FeSO_4.xH_2O = ................ g
mass of one mole of FeSO_4 = 152 g
number of moles of FeSO_4 in 100 g of FeSO_4.xH_2O = ...............
x = ................ [4]
(c) When a mixture of sulfur and potassium nitrate is burned and the products are dissolved in water, sulfuric acid is formed.
(i) The sulfuric acid formed by this method is not pure. It contains another acid. Deduce the identity of this acid.
[1]
(ii) The heat causes some of the potassium nitrate to decompose. Write the equation for the action of heat on potassium nitrate.
[2]
532
562
532
