State whether the following is true or false.
Kirchhoff's relation corrects a heat of reaction from one temperature to another using heat capacities.
Answer: A
The change in the heat of reaction with temperature is the difference between the product and reactant heat capacities. Integrating that difference is the standard way of moving a tabulated value to the reaction temperature.
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A reaction has a standard heat of reaction of minus 100 kilojoules per mole at 298 kelvin, and the product heat capacities exceed the reactant ones by 20 joules per mole kelvin. What is the heat of reaction at 398 kelvin?
Answer: B
The correction is twenty joules per kelvin over a hundred kelvin, that is two kilojoules, added to the value because the difference is positive. The reaction remains exothermic but slightly less so. The sign of the capacity difference decides the direction.
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The specific heat of most liquids compared with that of the same substance as a gas is
Answer: C
Liquids retain more modes of motion available for storing energy at a given temperature, so their capacity is generally the larger. Water is an extreme case at about twice its own value as steam. Rotational and vibrational modes account for the difference.
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Consider the following statements about using heat capacity correlations.
1. The temperature must be in the units the correlation was fitted for.
2. Extrapolating far outside the fitted range is unreliable.
3. The correlation may be used for any substance of similar molar mass.
Answer: D
The first two are correct and are basic care with correlations. The third is wrong, since heat capacity depends on molecular structure rather than on molar mass alone. Two isomers of one formula can differ appreciably.
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Assertion (A): Heat capacity correlations state the temperature units they require.
Reason (R): A polynomial fitted in kelvin gives a wrong answer if degrees Celsius are substituted.
Answer: A
Both are true and the reason gives the hazard: the coefficients absorb the scale, so the same numbers cannot serve both scales and the error grows with each higher order term. A quick check at one known temperature catches the slip.
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Enthalpy of a stream containing several species is computed as
Answer: B
Each species carries its own enthalpy relative to its own reference, and the stream total is the sum. Any heat of mixing is added separately when it is large enough to matter. Most mixtures need no such correction at all.
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