State whether the following is true or false.
The caustic soda from a diaphragm cell contains significant sodium chloride.
Answer: C
Brine flows through the diaphragm, so the cell liquor carries salt that must be removed by evaporation and crystallisation. This extra step is a large part of the energy cost of the diaphragm route. Membrane caustic needs far less evaporation.
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All of the following are products or co-products of a chlor alkali plant EXCEPT
Answer: D
Correct answer: sulphuric acid
A chlor alkali plant makes chlorine, caustic soda and hydrogen - the three products of electrolysing brine. Sulphuric acid is not among them; it may well be *bought in* for drying the chlorine, but it is a raw material to the plant rather than a co-product of it.
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The hydrogen from a chlor alkali plant is most often
Answer: A
Correct answer: burned as fuel or used to make acid
The hydrogen is clean and comes at a useful pressure, so it is burned for steam or process heat, or combined with part of the chlorine to make hydrogen chloride and hence hydrochloric acid. Venting it wastes a saleable stream and is now unusual, and it cannot be turned back into chlorine.
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The Solvay process manufactures
Answer: B
Ammonia is recovered and recycled, so the net raw materials are salt and limestone. Calcium chloride is the by-product and its disposal is the main environmental difficulty of the process. Salt and limestone are the net raw materials.
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Assertion (A): Ammonia is recycled in the Solvay process rather than consumed.
Reason (R): It is recovered from the filtrate by treatment with lime and returned to the ammoniation stage.
Answer: C
Both are true and the reason gives the mechanism: the ammonia acts as a carrier for carbon dioxide rather than as a reactant, which is what makes the economics of the process work. Lime from the same limestone recovers it.
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Arrange the main steps of the Solvay process in order.
1. carbonate it to precipitate sodium bicarbonate
2. purify and saturate the brine
3. calcine the bicarbonate to soda ash
4. ammoniate the brine
Answer: D
Ammonia must be present before carbon dioxide is introduced, since it is what makes the solution absorb the gas. The calcination releases carbon dioxide which is returned to the carbonating tower. The order of the middle two steps cannot be reversed.
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Three of the following are raw materials for the Solvay process. Which is the odd one out?
Answer: A
Correct answer: sulphur dioxide gas
The Solvay process uses brine, limestone and ammonia. The limestone is calcined to give carbon dioxide and lime, the carbon dioxide and ammonia react with the brine to precipitate sodium bicarbonate, and the lime recovers the ammonia from the ammonium chloride so that it circulates rather than being consumed. Sulphur dioxide has no part in it.
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Consider the following statements about soda ash.
1. It can be made by the Solvay process or mined as trona.
2. It is used in large quantities by the glass industry.
3. It is chemically the same as caustic soda.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: soda ash is made synthetically by the Solvay process and also mined as natural trona, and glass manufacture is its largest single use. Statement 3 confuses two different compounds - soda ash is sodium carbonate and caustic soda is sodium hydroxide, made by quite different routes and not interchangeable.
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Chlorine is normally shipped and stored
Answer: C
Correct answer: as a liquid under pressure
Chlorine liquefies at moderate pressure at ambient temperature, and shipping it as a liquid reduces the volume by a factor of several hundred. It is carried in pressure vessels, road tankers and rail cars, and because a release would flash to a dense toxic gas, the storage and transport of chlorine are among the most heavily regulated in the chemical industry.
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Assertion (A): Dry chlorine can be handled in ordinary steel while wet chlorine cannot.
Reason (R): Moisture allows hydrochloric and hypochlorous acids to form, which attack the metal.
Answer: D
Both are true and the reason gives the chemistry: drying the gas with sulphuric acid before liquefaction is therefore a standard step, and titanium is used where wet chlorine is unavoidable. Sulphuric acid drying towers precede liquefaction.
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