State whether the following is true or false.
Values from a steam table and from a heat capacity correlation may be mixed freely in one calculation.
Answer: C
They use different reference states, so combining them introduces an arbitrary constant into the answer. Either source may be used, but consistently for the whole species. Mixing two references leaves an arbitrary offset behind.
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All of the following are needed to compute the enthalpy change of a stream that is heated and vaporised EXCEPT
Answer: D
Correct answer: the heat of reaction of the substance
Heating and vaporising involve no chemical change, so no heat of reaction enters. The three quantities required are the liquid heat capacity to reach the boiling point, the latent heat to vaporise there, and the vapour heat capacity for any further heating - which is the standard three-segment path for a stream that changes phase.
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Arrange the steps of a hypothetical path from a cold liquid to a superheated vapour in order.
1. heat the vapour to the final temperature
2. heat the liquid to its boiling point
3. sum the three enthalpy changes
4. vaporise it at the boiling point
Answer: A
The path is chosen so that each step matches a kind of tabulated data. Any other path between the same end states gives the same total, since enthalpy is a state function. The real process need not follow this path at all.
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A liquid of heat capacity 2 kilojoules per kilogram kelvin is heated from 20 to 80 degrees Celsius and then vaporised with a latent heat of 300 kilojoules per kilogram. What is the total enthalpy change per kilogram?
Answer: B
Correct answer: 420 kilojoules
Sensible heat first, then latent: \[ 2 \times (80-20) + 300 = 120 + 300 = 420\ \mathrm{kJ/kg}. \] The two contributions must be added, and the latent heat is the larger by a factor of two and a half - which is the usual pattern and the reason a phase change dominates any balance containing one. Omitting either term gives the 300 or the 120 offered.
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Assertion (A): Latent heat falls as the temperature rises towards the critical point.
Reason (R): The liquid and vapour become more alike as the critical point is approached.
Answer: C
Both are true and the reason gives the cause: the two phases become identical at the critical point, so no energy is needed to convert one into the other and the latent heat goes to zero. The saturation curve terminates at that same point.
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The Watson correlation is used to convert a latent heat
Answer: D
It scales the known value by a power of the ratio of one minus the reduced temperatures. It is a convenient way of using a single tabulated value at the normal boiling point over a wide range. Its exponent is usually taken as about two fifths.
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Trouton's rule states that the entropy of vaporisation at the normal boiling point is
Answer: A
A value near eighty-eight joules per mole kelvin holds for many non-associating liquids, which allows a latent heat to be estimated from the boiling point alone. Hydrogen bonded liquids such as water depart noticeably from it.
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Three of the following require a phase change. Which is the odd one out?
Answer: B
Sensible heat is the energy of a temperature change within a single phase, whereas the other three all accompany a change of state at constant temperature. The distinction sets which term belongs in each step of a path.
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Consider the following statements about an enthalpy-concentration diagram.
1. It plots enthalpy against composition at several temperatures.
2. It accounts for the heat of mixing directly.
3. It is needed for every binary mixture.
Answer: C
The first two are correct and are why such diagrams exist for sulphuric acid and for caustic soda. The third overstates it, since most mixtures have negligible heats of mixing and need no such chart. Sulphuric acid and water are the classic exception.
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A heat of solution must be included in an energy balance when
Answer: D
Dissolving sulphuric acid in water releases enough heat to boil the mixture if it is done carelessly, so it cannot be ignored. Most organic mixtures release far less and the term is dropped safely. Adding acid to water rather than the reverse follows from it.
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