Chloralkali Questions

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March 2012

10 The simplified diagram of a cell used in the chlor-alkali industry is shown below.



10.1 Write down the CHEMICAL FORMULA of brine. (1)
10.1   NaCl     (Na+(aq) and Cl-(aq))

10.2 At which electrode, X or Y, is chlorine gas formed? (1)
10.2   Y

10.3 Write down a half-reaction that explains the formation of hydrogen gas at one of the electrodes. (2)
10.3

10.4 The purity of the sodium hydroxide produced in the chlor-alkali industry depends on the extent to which it is separated from the chlorine gas produced by this cell. Briefly describe how chlorine gas and sodium hydroxide are prevented from mixing in this cell. (2)
10.4 The membrane:
• prevents chloride ions from moving to the cathode
• only allows positive ions.

10.5 Apart from advantages and disadvantages of products produced, write down for this process:

10.5.1 ONE positive impact on humans (2)
10.5.1 Job creation resulting in more people having a better life.

10.5.2 ONE negative impact on humans (2)
10.5.2 Use huge amounts of electricity resulting in load shedding

OR

Chemical plant uses a lot of space that could have been used for housing/gardens, etc.


Nov 2011

10 The simplified diagram of a membrane cell used in the chlor-alkali industry is shown below. Gas A, gas B and compound C are the three major products formed during this process. X and Y represent the two electrodes.



10.1 Write down the function of the membrane in this cell. (1)
10.1 Allows only positive ions (cations / Na+ ions) to migrate to cathode half-cell.

OR

Prevents chloride ions Cl- ions from migrating to the cathode half-cell.

10.2 Which electrode, X or Y, is connected to the positive terminal of the power supply? Briefly explain how you arrived at the answer. (2)
10.2   Y Chloride ions are oxidised at Y.

OR

Chloride ions are negative and must be attracted to Y.

10.3 Write down the NAME or FORMULA of:

10.3.1 Gas A (1)
10.31 Hydrogen (H2)

10.3.2 Gas B (1)
10.3.2 Chlorine (Cl2)

10.3.3 Compound C (1)
10.3.3 Sodium hydroxide (NaOH)

10.4 Write down the balanced net (overall) cell reaction taking place in this cell. (3)
10.4


OR


10.5 The chlor-alkali industry is sometimes blamed for contributing to the greenhouse effect. Briefly explain how the above cell contributes to the greenhouse effect. (2)
10.5 Uses huge amounts of electricity (energy).
Combustion of coal during generation of electricity releases huge amounts of carbon dioxide into atmosphere.


March 2010

11 The diagram below is a simplified version of a membrane cell, one of the electrolytic cells used in the chlor-alkali industry. The letters P and Q represent the two gases formed during this process.



11.1 Write down the letters P and Q in your answer book. Next to each, write down the half-reaction that shows how gas P and gas Q are respectively formed.
11.1



11.2 Water (H2O(l)) and sodium ions (Na+(aq)) are both present in the cathode side of the membrane cell. Explain why hydrogen gas, and not sodium metal, is formed in the membrane cell. Refer to the relative strengths of oxidising agents to explain your answer.
11.2 H2O is a stronger oxidising agent (than Na+) and is more readily reduced than the Na+.

11.3 State ONE function of the membrane. (1)
11.3 Allows only the cation (Na+) to move across to the cathode compartment.

OR

To separate the Cl- ions from the OH-.

11.4 State TWO uses of chlorine. (2)
11.4 Any two:
As chemical reactant in the production of:
• Medicines to cure diseases
• Polymers
    • PVC to make plastic products e.g. pipes, insulation, handbags
    • Nylon for carpeting, clothing, etc.
• Household products e.g. toiletries, cosmetics, CD's etc
• Hydrochloric acid used in building industry and swimming pools
• Bromine used in photography
• Solvents e.g. "tippex"
• Solvents used for dry cleaning
• Titanium dioxide used as white pigment in paint
• Dyes used in textile industry
• Pesticides used to protect crops
• Compounds that can be used to sterilize medical equipment e.g. kidney dialysis machines, wounds and work surfaces in medical labs
• Extraction of titanium used in aircrafts

As disinfectant to:
• Purify/sterilize drinking water

As bleaching agent in the:
• Textile industry
• Paper industry


Nov 2009 Unused

11 Industrially, chlorine gas is produced by the electrolysis of brine. In addition to chlorine gas, hydrogen gas and sodium hydroxide are also produced.



11.1 Write down the equation for the half-reaction that takes place at the cathode. (2)
11.1

11.2 Write the balanced overall (net) cell reaction, omitting spectator ions, for this cell. (3)
11.2

11.3 State TWO functions of the membrane in the above cell. (2)
11.3 • Allows the migration of positive ions from anode to cathode
        • Prevents mixing of products

11.4 Use the relative strengths of oxidising agents present in a brine solution to explain why sodium metal is NOT one of the products in this process. (2)
11.4 H2O is a stronger oxidising agent than Na+ and will be reduced.

OR

Na+ is a weaker oxidising agent than H2O and will not be reduced.

11.5 Chlorine is used in many useful products such as plastics, drugs and disinfectants. Environmentalists are protesting against the large-scale production of chlorine. They base their argument on the negative impact of chlorine on humans.

Name ONE negative impact of chlorine on humans. (1)
11.5 Any one:
• Chlorine gas is poisonous - causes health problems / breathing complications
• Chlorine gas is used to make drugs that can be dangerous when overdosing
• Chlorine used as nerve gas.


Nov 2009

11 The chloralkali industry is the second largest consumer of electricity among electrolytic industries. It makes use of brine as electrolyte to produce chlorine gas, hydrogen gas and sodium hydroxide. The overall reaction can be represented by the following equation:



11.1 Define the term electrolyte. (2)
11.1 A liquid / solution that conducts electricity.

11.2 Give a reason why brine conducts electricity. (1)
11.2 Contains (positive / Na+ and negative / Cl-) ions that are free to move.

11.3 Write down the NAME of the reducing agent in the above reaction. Give a reason for your choice. (2)
11.3 Chloride ions / Sodium chloride
Chloride ions are oxidised / lose electrons to form Cl2.

OR

Chloride ions reduce water / causes reduction of water.

11.4 Write down a half-reaction to explain how hydroxide ions are formed during this reaction. (2)
11.4

11.5 At which electrode (anode or cathode) is chlorine gas formed? Give a reason for your answer. (2)
11.5 Anode
Cl-(aq) is oxidised to Cl2(g) - oxidation at the anode

OR


11.6 The chloride ions present in the brine solution can contaminate the sodium hydroxide. Briefly describe how this contamination is prevented in the membrane cell. (2)
• The (selectively permeable) membrane separates the chloride ions from the cathode compartment by allowing only positive ions / cations (Na+) to move from the anode compartment to the cathode.
• By preventing negative ions (anions) to pass through.

11.7 Give ONE reason why it is not advisable to build a chloralkali plant close to a residential area. (1)
11.7 Any ONE:
• Chlorine is poisonous / dangerous / harmful to humans and the environment.
• Leaching of sodium hydroxide into groundwater / water carriers is a health risk to humans and the environment.
• Hydrogen gas can result in an explosion.


Exemplar 2008

13The chloralkali (also called 'chlorine-caustic') industry is one of the largest electrochemical technologies in the world. Chlorine is produced using three types of electrolytic cells. The simplified diagram below shows a membrane cell.



13.1 Give TWO reasons why the membrane cell is the preferred cell for the preparation of chlorine. (2)
13.1 Any two:
• Less pollution
 • Cheaper
 • Product more pure

13.2 Why do you think it is advisable to use inert electrodes in this process? (2)
13.2 Do not react and can be used more than once 

SInce these electrodes can be used more than once, this saves on costs.

13.3 Write down the equation for the half-reaction taking place at electrode M. (2)
13.3

13.4 Which gas is chlorine gas? Write down only Gas A or Gas B. (2)
13.4 Gas A

13.5 Briefly explain how sodium hydroxide forms in this cell. (3)
13.5
Na+ ions migrate through the membrane to the cathode 
H+ ions (from the H2O in solution) are reduced to H2 that escapes
Na+ ions combine with remaining OH- ions (from the H2O in solution) to form NaOH


Additional Exemplar 2008

12 The diagram below shows a type of membrane cell used in the chloroalkali industry.



12.1 Name the gases A and B. (2)
12.1
A: Chlorine
B: Hydrogen

12.2 Why is the membrane called a cationic membrane? (1)
12.2 Allows only the cations (positive ions) to pass through it.

12.3 Write down the half-reaction that takes place at electrode N. (2)
12.3

12.4 Apart from its use in household products, name ONE industrial use of chlorine. (1)
12.4 Any one
• Manufacture of PVC, paper, drugs etc.
• Disinfectant for water

12.5 Explain why this electrolytic process cannot be done in one large container without a membrane. (2)
12.5 In a single pot the chlorine will react with water to form chlorine water

OR

the chlorine will react with the OH- ions to form bleach. 

OR

Products formed will be contaminated






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