The chlor alkali industry is built on the electrolysis of
Answer: A
Aqueous brine gives chlorine at the anode and hydrogen with caustic soda at the cathode. Molten salt electrolysis gives sodium metal instead and belongs to a different industry. The aqueous route also yields hydrogen at the cathode.
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State whether the following is true or false.
Chlorine, caustic soda and hydrogen are produced together in fixed proportions.
Answer: B
The stoichiometry of the cell fixes the ratio, so the market for one product constrains the economics of the others. This coupling is the central commercial fact of the industry. Neither product can be made alone by this route.
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Assertion (A): A chlor alkali plant cannot adjust its caustic to chlorine ratio to suit the market.
Reason (R): Both are produced by the same electrolysis in a ratio set by the stoichiometry.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the cause. The electrolysis produces one mole of chlorine for every two of caustic soda, and no operating change alters that ratio - it is fixed by the chemistry. The industry therefore lives with a structural problem: demand for the two products moves independently, so one is often in surplus while the other is short, and the plant can only respond by changing its total rate.
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Match each cell type with its distinguishing feature.
List I
A. Mercury cell
B. Diaphragm cell
C. Membrane cell
D. Solvay process
List II
1. an ion selective membrane passing only sodium ions
2. a flowing mercury cathode forming an amalgam
3. no electrolysis at all, using ammonia and limestone
4. an asbestos or polymer separator with liquid flow through it
Answer: D
The three cells differ in how they keep the chlorine and the caustic apart, and each gives a different product purity and energy consumption. The Solvay process is included because it makes soda ash by an entirely different route.
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The membrane cell has displaced the others chiefly because it
Answer: A
Correct answer: gives pure caustic at lower energy use
The ion-exchange membrane passes sodium ions and blocks chloride, so the caustic produced is essentially free of salt and needs no evaporation to purify it. Combined with a lower cell voltage than the diaphragm and mercury routes, that gives the lowest energy consumption of the three - and it avoids both mercury and asbestos, which settled the matter.
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Consider the following statements about the mercury cell.
1. It produces caustic of very high purity.
2. It has been phased out on environmental grounds.
3. It uses less electrical energy than a membrane cell.
Answer: B
The first two are correct. The third is wrong, because the mercury cell is the most energy intensive of the three, which together with mercury losses sealed its fate. Membrane conversion has been under way for decades.
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Brine purification before a membrane cell removes
Answer: C
Correct answer: calcium and magnesium to a very low level
Hardness ions must be brought down to parts per billion before brine enters a membrane cell, which takes precipitation with soda ash and caustic followed by ion-exchange polishing. It is the most demanding purification in the plant, and the reason is given directly in the next item.
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Assertion (A): Membrane cells require far purer brine than diaphragm cells.
Reason (R): Hardness ions deposit within the membrane and raise its electrical resistance irreversibly.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Calcium and magnesium hydroxides precipitate inside the membrane where the pH rises, blocking the ion-conducting channels and raising the cell voltage permanently - the damage cannot be washed out. A diaphragm is far more tolerant because it is a porous barrier rather than a selective one, which is why membrane plants carry ion-exchange polishing that diaphragm plants do not need.
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At the anode of a chlor alkali cell the reaction produces
Answer: A
Chloride is oxidised to chlorine, while hydrogen and hydroxide appear at the cathode. A little oxygen is also formed at the anode as an unwanted side reaction that lowers the current efficiency. Anode coatings are chosen to suppress it.
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How many moles of electrons are needed to produce one mole of chlorine from brine?
Answer: B
Correct answer: 2
The anode reaction is \[ 2\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2e^-, \] so two moles of electrons are needed per mole of chlorine - that is, two faradays, about 193,000 coulombs. The same two electrons produce two moles of caustic soda at the cathode, which is exactly the fixed ratio the first item in this topic describes.
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