C1-Higher-Topic 3 Chemical Changes - Electrolytic ...

C1-Higher – Topic 3 – Chemical Changes - Electrolytic Processes Q1. Figure 5 shows the apparatus that can be used to electrolyse sodium chloride solution in the laboratory.

Figure 5

Gases are produced at both electrodes.

(i) State the name of the yellow-green gas X formed at the positive electrode. (1)

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(ii) Describe the test to show that the gas formed at the negative electrode is hydrogen. (2)

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Q2. (i) When copper sulfate solution is electrolysed using inert electrodes, oxygen is formed at the positively charged anode.

Explain how the oxygen is formed from ions in the solution. (2)

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.............................................................................................................................................. (ii) The other product is copper.

1.27 g of copper were produced in an experiment. Calculate the number of moles of copper, Cu, produced in this experiment. (Relative atomic mass: Cu = 63.5)

(1) ..............................................................................................................................................

amount of copper produced = .................................. mol

Q3. The ions in sodium chloride solution are sodium ions, Na+ chloride ions, Cl– hydrogen ions, H+ hydroxide ions, OH– Sodium chloride solution is electrolysed using a direct electric current. (i) Which of these ions will be attracted to the cathode during the electrolysis of sodium chloride solution?

Put a cross ( ) in the box next to your answer. (1)

A H+ ions only B H+ and Na+ ions C Cl– ions only D Cl– and OH– ions

(ii) Chlorine is one of the products of the electrolysis. The half-equation for the production of chlorine is

2Cl– → Cl2 + 2e Explain how the half-equation shows that chloride ions are oxidised.

(2) .............................................................................................................................................. .............................................................................................................................................. (iii) Suggest why the solution remaining at the end of the electrolysis is alkaline.

(1) .............................................................................................................................................. .............................................................................................................................................. (iv) The electrolysis of sodium chloride solution does not produce metallic sodium.

State what change you would make to the electrolyte to obtain metallic sodium. (1)

.............................................................................................................................................. .............................................................................................................................................. Q4. Water, acidified with a small amount of dilute sulfuric acid, can be decomposed by electrolysis using the apparatus shown.

(i) State the form of energy used to carry out the electrolysis. (1)

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(ii) During the electrolysis, hydrogen is formed at one of the electrodes.

Describe a test to show that this gas is hydrogen. (2)

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............................................................................................................................................. Q5. A solution of sodium chloride contains the four ions shown in the box.

(a) The sodium chloride solution is electrolysed. Give the formulae of the two ions that will be attracted to the positively charged electrode.

(1) .............................................................................................................................................

(b) Complete the sentence by putting a cross ( ) in the box next to your answer.

When molten lead bromide is electrolysed the products are (1)

A lead and hydrogen

B hydrogen and bromine

C hydrogen and oxygen

D lead and bromine

(c) During electrolysis, oxidation takes place at the anode and reduction takes place at the cathode.

Explain, in terms of electrons, what is meant by oxidation and reduction. (2)

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(d) Explain why some metal objects are electroplated.

(2) .............................................................................................................................................

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............................................................................................................................................. Q6. Impure copper is purified by electrolysis using the apparatus shown.

(i) Give the name of the electrode which is made of pure copper. (1)

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(ii) Describe how each electrode will have changed at the end of the electrolysis process.

(2) .............................................................................................................................................

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(iii) Complete the sentence by putting a cross ( ) in the box next to your answer.

Cu2+(aq) + 2e− → Cu(s) This half equation shows the process of

(1)

A displacement

B oxidation

C redox

D reduction

(iv) Copper sulfate solution is the electrolyte used in this electrolysis process.

Explain how copper sulfate solution conducts electricity. (2)

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............................................................................................................................................. Q7.

Electrolysis

(a) When molten sodium chloride is electrolysed, the products are sodium and chlorine. (i) Give the formula of sodium chloride.

(1)

........................................................... (ii) State why molten sodium is used in some nuclear reactors.

(1) .............................................................................................................................................

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(b) Molten lead bromide is electrolysed.

(i) Complete the sentence describing the type of reaction.

(1) During the electrolysis the lead ions gain electrons at the cathode. The type of reactions in which metal ions gain electrons is called .....................................................

(ii) Complete the sentence by putting a cross ( ) in the box next to your answer.

Molten lead bromide, rather than solid lead bromide, has to be electrolysed because (1)

A ions can only move freely in molten lead bromide B electrolysis is too fast in solid lead bromide C molten lead bromide contains free electrons D lead bromide has a very low melting point

(iii) When molten lead bromide is electrolysed, a silver-coloured liquid is produced at the cathode and a red-brown gas is produced at the anode.

Complete the word equation by showing the products formed when molten lead bromide is electrolysed.

(2)

lead bromide → ..................................................... + .....................................................

(c) Three colourless liquids, A, B and C, are tested.

Tests 1, 2 and 3 are carried out on each of these liquids. Test 1: the liquid is tested to see if it conducts electricity. Test 2: 20 cm3 of the liquid is shaken vigorously with a few drops of soap solution.

Test 3: 20 cm3 of the liquid is boiled and then test 2 is carried out on the boiled liquid. The results are shown in the table.

(i) State which of the liquids are electrolytes.

(1)

........................................................... (ii) Use the results of the tests to explain whether each of the liquids A, B and C is soft, shows permanent hardness, or shows temporary hardness.

(3) .............................................................................................................................................

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Q8. (a) The ions in sodium chloride solution are

sodium ions, Na+ chloride ions, Cl– hydrogen ions, H+ hydroxide ions, OH–

Sodium chloride solution is electrolysed using a direct electric current. (i) Which of these ions will be attracted to the cathode during the electrolysis of sodium chloride solution?

Put a cross ( ) in the box next to your answer. (1)

A H+ ions only B H+ and Na+ ions C Cl– ions only D Cl– and OH– ions

(ii) Chlorine is one of the products of the electrolysis. The half-equation for the production of chlorine is

2Cl– → Cl2 + 2e Explain how the half-equation shows that chloride ions are oxidised.

(2) .............................................................................................................................................. .............................................................................................................................................. (iii) Suggest why the solution remaining at the end of the electrolysis is alkaline.

(1) .............................................................................................................................................. .............................................................................................................................................. (iv) The electrolysis of sodium chloride solution does not produce metallic sodium.

State what change you would make to the electrolyte to obtain metallic sodium. (1)

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(b) (i) When copper sulfate solution is electrolysed using inert electrodes, oxygen is formed at the positively charged anode. Explain how the oxygen is formed from ions in the solution.

(2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (ii) The other product is copper.

1.27 g of copper were produced in an experiment. Calculate the number of moles of copper, Cu, produced in this experiment. (Relative atomic mass: Cu = 63.5)

(1) ..............................................................................................................................................

amount of copper produced = .................................. mol (Total for Question = 8 marks)

Q9. Electrolysis of hydrochloric acid produces chlorine and hydrogen. The apparatus used is shown.

(i) Explain what is meant by electrolysis.

(2) .............................................................................................................................................. .............................................................................................................................................. (ii) Describe the test to show that a gas is chlorine.

(2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (iii) Chlorine is used in the manufacture of a polymer. Part of this polymer molecule is

State the name of the polymer.

(1) .............................................................................................................................................. Q10. Figure 5 shows the apparatus that can be used to electrolyse sodium chloride solution in the laboratory.

Figure 5

Write the half equation for the formation of hydrogen gas from hydrogen ions at a negative electrode. (2)


Q11.

Objects made from transition metals are sometimes coated with a thin layer of another transition metal to improve their appearance and to protect against corrosion.

Figure 10 shows equipment that can be used to electroplate an iron spoon with silver.

Figure 10

(i) Which row of the table correctly shows the charge on the silver rod electrode and the type of reaction occurring at this electrode?

(1)

(ii) Silver metal is deposited on the spoon.

Which half-equation represents this reaction? (1)

A Ag + e → Ag+

B Ag → Ag+ + e−

C Ag+ + e → Ag

D Ag+ → Ag + e− (Total for question = 2 marks)

Q12. During electrolysis, reduction takes place at the cathode. Explain, in terms of electrons, what is meant by reduction.

(2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. Q13. Figure 5 shows the apparatus that can be used to electrolyse sodium chloride solution in the laboratory.

Figure 5

Some of the solution remaining after the electrolysis was tested with litmus paper. The paper turned blue.

Explain why the litmus paper turned blue. (2)

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Q14. Figure 5 shows the apparatus that can be used to electrolyse sodium chloride solution in the laboratory.

Figure 5

Explain why sodium chloride solution can conduct electricity. (2)

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Q15.

(a) Electrolysis of hydrochloric acid produces chlorine and hydrogen. The apparatus used is shown.

(i) Explain what is meant by electrolysis.

(2) .............................................................................................................................................. .............................................................................................................................................. (ii) Describe the test to show that a gas is chlorine.

(2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (iii) Chlorine is used in the manufacture of a polymer. Part of this polymer molecule is

State the name of the polymer.

(1) ..............................................................................................................................................

(b) Dilute hydrochloric acid reacts with silver nitrate solution to form silver chloride and nitric acid. (i) Complete the sentence by putting a cross ( ) in the box next to your answer. The reaction produces silver chloride as a precipitate. In an equation this would be shown as

(1) A AgCl(aq) B AgCl(g) C AgCl(l) D AgCl(s)

(ii) This apparatus is used to investigate the mass of the reactants and the mass of products in this reaction.

The total mass of this apparatus was measured. The flask was shaken to allow the silver nitrate solution and dilute hydrochloric acid to react. After the reaction the total mass of the apparatus was measured again. State how the total mass of the apparatus after the reaction will compare with the total mass of the apparatus before the reaction.

(1) .............................................................................................................................................. (iii) Write the balanced equation for the reaction of silver nitrate solution, AgNO3, with dilute hydrochloric acid to form silver chloride, AgCl, and nitric acid.

(2) ..............................................................................................................................................

(Total for Question = 9 marks) Q16. Sodium sulfate solution is electrolysed in the apparatus shown.

Sodium sulfate solution contains sodium ions, Na+, sulfate ions, SO42−, hydrogen ions, H+, and hydroxide ions, OH−.

Hydrogen is produced at one electrode and oxygen is produced at the other electrode.

Explain how these products are formed from the ions in the electrolysis process, indicating how you would identify the products.

You may give ionic equations in your answer. (6)

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Q17.

(a) A solution contains a mixture of ions with the formulae shown.

Give the formulae of all the ions that will be attracted to the negatively charged cathode during electrolysis.

(1) ..............................................................................................................................................

(b) During electrolysis, reduction takes place at the cathode. Explain, in terms of electrons, what is meant by reduction.

(2) .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. ..............................................................................................................................................

(c) Solid lead bromide cannot be electrolysed. Molten lead bromide can be electrolysed.

Explain how, when solid lead bromide is melted and electrolysed, bromine is formed at the anode. (3)

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*(d) Impure copper can be purified using electrolysis. The impure copper is used as the anode. A pure copper cathode is used. The electrodes are placed in copper sulfate solution. A direct electric current is passed through the solution.

Describe and explain what is seen when this apparatus is used to purify a piece of impure copper.

(6)

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(Total for Question = 12 marks) Q18.

Electrolysis

(a) Some metal objects are electroplated to improve their appearance. Give another reason why some metal objects are electroplated.

(1) .............................................................................................................................................

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(b) Copper sulfate solution was electrolysed using copper electrodes. The mass of each electrode was determined before it was placed in the solution.

The electrolysis was carried out for a period of time. The electrodes were removed, washed, dried and their masses redetermined. The table shows the masses of the electrodes before and after electrolysis.

Explain these results.

(3) .............................................................................................................................................

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(c) In an electrolysis experiment, oxide ions, O2−, form oxygen gas, O2.

Write the balanced half equation for the reaction. (2)

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*(d) Sodium chloride is an ionic compound. It contains sodium ions, Na+, and chloride ions, Cl−.

When molten sodium chloride is electrolysed, sodium metal and chlorine gas are formed. Describe how the sodium ions and chloride ions in solid sodium chloride are converted into sodium and chlorine by electrolysis.

(6) .............................................................................................................................................

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Mark Scheme Q1.

Q2.

Answer Acceptable answers

Mark

(i) An explanation linking: Marking point 1 • hydroxide/OH- ions (from water) (1) Marking point 2 • (ions) lose electrons /are oxidised (1)

half equation, even unbalanced, showing hydroxide ions losing electrons (2) do not allow marking point 1 if only oxygen/sulfate ions mentioned

(2) (ii)

1.27 / 63.5 (1) (= 0.02)

0.02 with no working (1) correct working with incorrect answer (1) (1)

Q3.


Mark

(i) B H+ and Na+ ions (1) (ii) An explanation

linking • electron(s) (1) • (have been) lost/removed (1) conditional on electrons

ignore reference to number of electrons do not allow negative charge chlorine gains electrons (0) allow chlorine loses electrons (1) (2)

(iii) Any one from • it contains (excess) hydroxide/OH¯ ions (1) • sodium/Na+ ions and hydroxide/ OH¯ ions remain (1) • it is sodium hydroxide/NaOH (1) • hydrogen/H+ ions have been removed (at the cathode) (1)

ignore solution has pH greater than 7 allow no hydrogen ions left/acidic ions removed

(1) (iv) use molten/liquid

sodium chloride ignore just liquid/liquid sodium (1)

/electrolyte / melt it/sodium chloride/electrolyte

Q4.

Q5.

Q8.


Mark

(a)(i) B H+ and Na+ ions (1) (a)(ii) An explanation

linking • electron(s) (1)

ignore reference to number of electrons do not allow negative (2)

• (have been) lost/removed (1) conditional on electrons

charge chlorine gains electrons (0) allow chlorine loses electrons (1)

(a)(iii) Any one from • it contains (excess) hydroxide/OH¯ ions (1) • sodium/Na+ ions and hydroxide/ OH¯ ions remain (1) • it is sodium hydroxide/NaOH (1) • hydrogen/H+ ions have been removed (at the cathode) (1)

ignore solution has pH greater than 7 allow no hydrogen ions left/acidic ions removed

(1) (a)(iv) use molten/liquid

sodium chloride /electrolyte / melt it/sodium chloride/electrolyte

ignore just liquid/liquid sodium

(1) (b)(i) An explanation

linking: Marking point 1 • hydroxide/OH- ions (from water) (1) Marking point 2 • (ions) lose electrons /are oxidised (1)

half equation, even unbalanced, showing hydroxide ions losing electrons (2) do not allow marking point 1 if only oxygen/sulfate ions mentioned

(2) (b)(ii)

1.27 / 63.5 (1) (= 0.02)

0.02 with no working (1) correct working with incorrect answer (1) (1)

Q9.


Mark

an explanation linking the following • decomposition (of compound/substance /electrolyte)(1) • using (direct) current (1)

splitting up/breaking down/breaking up ignore separating reject splitting of atoms/elements/molecules for M1 using electrical energy /electricity reject alternating current/ac

(2)

• (damp blue) litmus (paper) (1) • (turns red then) bleaches / goes

allow bleaches indicator for 1 mark ignore indicator goes lighter ignore smells of

(2)

white (1) chlorine/swimming pools ignore any incorrect middle colour mentioned use of suitable named indicator with correct result e.g. (damp) universal indicator paper (1) (turns red then) bleaches (1) starch-iodide paper(1) turns blue-black(1)

poly(chloroethene) PVC/polyvinylchloride/ polychloroethene/poly(chlorethene) (1)

Q10.

Q11.

Q12.


Mark

An explanation linking (reduction is) gain of electrons(2)

if cathode gains electrons – 1 mark loss of oxygen – 1 mark

(2)

Q13.

Q14.

Q15.


Mark

(a)(i) an explanation linking the following • decomposition (of compound/substance /electrolyte)(1) • using (direct) current (1)

splitting up/breaking down/breaking up ignore separating reject splitting of atoms/elements/molecules for M1 using electrical energy /electricity reject alternating current/ac

(2)

(a)(ii) allow bleaches (2)

• (damp blue) litmus (paper) (1) • (turns red then) bleaches / goes white (1)

indicator for 1 mark ignore indicator goes lighter ignore smells of chlorine/swimming pools ignore any incorrect middle colour mentioned use of suitable named indicator with correct result e.g. (damp) universal indicator paper (1) (turns red then) bleaches (1) starch-iodide paper(1) turns blue-black(1)

(a) (iii) poly(chloroethene) PVC/polyvinylchloride/ polychloroethene/poly(chlorethene) (1)

(b)(i) D AgCl(s) (1) (b)(ii) same/no change (1) (b)(iii) HCl + AgNO3 →

AgCl + HNO3 • reactant formulae (1) • product formulae (1)

Ag+ + Cl- → AgCl max 1 if any incorrect attempt to balance reject incorrect use of cases and non-subscripts

(2)

Q16.


Mark

(a) Na+and Mg2+ reject if any other ions are given allow names

(1)

(b) An explanation linking (reduction is) gain of electrons(2)

if cathode gains electrons – 1 mark loss of oxygen – 1 mark

(2)

(c) An explanation linking three of the following ions / particles cannot move in solid (1) ions / particles move in molten liquid / lattice structure is broken on melting (1) bromide (ions) / negatively charged (ions /bromine) move / attracted to anode (1) bromide (ions) lose electrons / (bromide ions) are oxidised(1) to form bromine atoms (1) bromine molecules / Br2 formed (1)

Half equation, even unbalanced, showing bromide ions losing electrons (2)

(3)

Indicative Content Mark

QWC *(d) A description / explanation including some of the following points anode • the anode decreases in mass / size • copper atoms form ions by losing electrons • copper ions pass into electrolyte • oxidation takes place at the anode • (anode) sludge forms as • impurities are left behind / fall to the bottom (under electrode) cathode • the cathode increases in mass / size • copper ions gain electrons to form (6)

copper • pure copper deposits on the cathode • reduction takes place at the cathode electrolyte • copper sulfate solution does not change in colour • no change in concentration of copper ions • copper ions in the electrolyte move toward the cathode

Level 0 No rewardable content 1 1 - 2

• a limited description e.g. copper leaves the anode and deposits on the cathode. • the answer communicates ideas using simple language and uses limited scientific terminology • spelling, punctuation and grammar are used with limited accuracy

2 3 - 4 • a simple description e.g. copper leaves the anode and deposits on the cathode and impurities fall to the bottom of the beaker. • the answer communicates ideas showing some evidence of clarity and organisation and uses scientific terminology appropriately • spelling, punctuation and grammar are used with some accuracy

3 5 - 6 • a detailed description and explanation e.g. copper atoms from the anode lose electrons to form copper ions; copper ions are attracted to the cathode where they gain electrons to form pure copper. • the answer communicates ideas clearly and coherently uses a range of scientific terminology accurately • spelling, punctuation and grammar are used with few errors

Q18.

C1-Higher-Topic 3 Chemical Changes - Electrolytic ...

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