Choosing Cp or Cv [ENDORSED]

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Abigail Urbina 1K
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Choosing Cp or Cv

When it is not explicitly stated in the question whether pressure or volume are constant, when do we know which to use?

Chem_Mod
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Re: Choosing Cp or Cv  [ENDORSED]

If it is reasonable to assume the process or reaction is occurring under ambient conditions, using constant pressure heat capacities is appropriate. For instance, a phase change occurring in a lab context without further descriptions of the apparatus can be assumed to take place in constant pressure conditions and you would use Cp.

Constant volume conditions are usually specialized and the wording of the problem will imply it. Read all problems carefully and consider the context before you begin.

Mitch Mologne 1A
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Re: Choosing Cp or Cv

I think usually it can be implied because one of two is usually changing, thus you would use the other.

Anna Goldberg 2I
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Re: Choosing Cp or Cv

Also-in problems that have both volume and temperature changing, for example-use Cv in your delta S=nCln(T2/T1) temperature change calculation, as volume is constant while temperature is changing (even though volume changes in another part of the reaction).
Conversely, in problems that have both delta P and delta T, use Cp in your delta S=nCln(T2/T1) temperature change calculation, as pressure is constant while temperature is changing (even though pressure changes in another part of the reaction).

Context clues in the problem can help as well. For example, "sealed box" means use Cv, as volume will be constant.