QUESTION 1 One Difference in the Electrode Reactions of an Electrolytic Cell Compared to a Galvanic Cell Is

QUESTION 1 One difference in the electrode reactions of an electrolytic cell compared to a galvanic cell is:

A Oxidation occurs at the cathode and reduction at the anode B Oxidation occurs at the positive electrode and reduction at the negative electrode C Oxidation is a spontaneous reaction but reduction is not D Reduction is a spontaneous reaction but oxidation is not

QUESTION 2 In an electrolytic cell the redox reactions:

A Are spontaneous B Convert chemical energy into electrical energy C Are driven by electrical energy D Are driven by chemical energy

QUESTION 3 If pure water were to be used as the only electrolyte in an electrolytic cell, the reaction products would be:

A Oxygen gas at the anode and hydrogen gas at the cathode B Hydrogen gas at the anode and oxygen gas at the cathode C Both hydrogen and oxygen gas at each electrode D No products would form as no reaction would occur

QUESTION 4 The reason elemental sodium is extracted from molten sodium chloride in the Downs cell, instead of a sodium chloride solution is:

A Sodium will immediately react with water as soon as it is formed B Too high a current is needed to drive the reaction C Chlorine is a better reductant than sodium D Water is a better oxidant than sodium ions

QUESTION 5 In the diaphragm cell used to make chlorine gas and sodium hydroxide, it is important to have the asbestos diaphragm because:

A Two different electrolytes are used in each chamber and they are not allowed to mix

B The sodium and chloride ions have to be prevented from reacting

C The diaphragm acts as a filter

D Hydrogen gas and chlorine gas have to be prevented from mixing, otherwise they react to form hydrochloric acid.

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 1 QUESTION 6 The Hall-Heroult cell, using molten alumina mixed with cryolite as the electrolyte, produces aluminium electrolytically.

(a) Is it possible to extract aluminium by chemical reduction? Give a reason for your answer.

(b) Explain the function of the cryolite.

Cathode: Al 3 (l)  3e  Al(l)

Overall: 2Al2O3 (l)  3C(s)  4Al(l)  3CO2 (g)

(d) Why do anodes have to be periodically replaced?

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 2

QUESTION 7 Sketch an electrolytic cell for the electrolysis of a 5.0M solution of sodium chloride, NaCl including all possible electrode reactions and actual products of the electrolysis at each electrode. Comment on whether different results would be obtained if a 0.001M solution were to be electrolysed.

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 3 QUESTION 8 Sodium is obtained from operating the Downs cell, which electrolyses molten sodium chloride.

(a) Why can’t sodium be extracted from an aqueous solution? Use the electrochemical series to explain your answer.

(b) Is sodium obtained from the positive or negative electrode? Give a reason for your answer.

(c) What other useful product is obtained from the Downs cell? Write an equation to show the formation of this product and name the electrode where the reaction occurs.

QUESTION 9 Sketch a diagram of the diaphragm cell, showing electrodes, electrolyte and electrode reactions as well as where products are collected from. Mention an occupational hazard associated with the use of asbestos in this cell.

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 4

QUESTION 1 Answer is B

QUESTION 2 Answer is C

QUESTION 3 Answer is A

QUESTION 4 Answer is D

QUESTION 5 Answer is D

QUESTION 6

(a) Theoretically is possible to extract aluminium by chemical reduction as a compound such as aluminium chloride could be reacted with stronger reductants (I.e.: lower in the electrochemical series) such as sodium and potassium. Because of its high reactivity, the reductants required to extract aluminium have to be more reactive metals, which are expensive.

(b) Cryolite is able to dissolve molten alumina and is used to lower the melting temperature of the electrolyte and hence maintain a lower operating temperature. The operating temperature is halved from around 20000C for pure alumina to 10000C for the solution with cryolite.

Cathode: Al 3 (l)  3e  Al(l)

Overall: 2Al2O3 (l)  3C(s)  4Al(l)  3CO2 (g)

2  Anode: C(s)  2O (l)  CO2 (g)  4e

(d) Anodes need to be replaced because they react with oxide ions, as shown in the anode half-reaction above.

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 5 QUESTION 7

Anode Cathode

Inert Electrodes

5.0M NaCl

Substances able to react

• At anode: Water, chloride ions

Possible reactions:   2H 2O(l)  O2 (g)  4H (aq)  4e   2Cl (aq)  Cl2 (g)  2e

The two equations on the electrochemical series are very close, with the water reaction being directly below the chloride reaction. Water is a slightly better reductant and should be oxidised first. In reality, because of the medium concentration of the solution it is very likely that both the above reactions would occur at the anode thus producing both oxygen and chlorine gas.

• At cathode: Water, sodium ions

Possible reactions:   2H 2O(l)  2e  H 2 (g)  2OH (aq) Na  (aq)  e  Na(s)

The equation with water is higher than the equation with sodium ions, making water a better oxidant than sodium ions. Even at a concentration of 5.0M, only water would react at the cathode producing hydrogen gas.

If a very dilute solution of 0.001M were to be used as electrolyte the likely products would be oxygen gas at the anode and hydrogen gas at the cathode.

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 6 QUESTION 8

(a) The equation for the reduction of sodium ions is lower in the electrochemical series table compared to the reduction of water, making water a stronger oxidant. Even at high concentrations of sodium ions, water will be preferentially reduced.

(b) Sodium is produced at the negative electrode as in electrolytic cells the cathode (where reduction occurs) is connected to the negative terminal of the power supply.

(c) The other product obtained from the Downs cell is chlorine gas. It is produced at the positive electrode (anode) according to the following equation:

  2Cl (aq)  Cl2 (g)  2e

QUESTION 9 The cell diagram should include details as shown below:

Graphite Anode Steel mesh Cathode

Hydrogen Chlorine gas Gas Brine

Asbestos diaphragm; to prevent mixing of hydrogen and chlorine. NaOH/NaCl Brine is free to flow to cathode.

The main hazard associated with asbestos is that fibres can reach the lungs and cause a condition commonly known as asbestosis, which often results in premature death of the patient.

 The School For Excellence 2018 Unit 3 & 4 Chemistry – Topic Test Page 7