Chapter 5: Q. 5.21 (page 163)
Is heat capacity (C) extensive or intensive? What about specific heat (c) ? Explain briefly.
Short Answer
Heat capacity is an extensive property
Specific heat is an intensive property.
Chapter 5: Q. 5.21 (page 163)
Is heat capacity (C) extensive or intensive? What about specific heat (c) ? Explain briefly.
Heat capacity is an extensive property
Specific heat is an intensive property.
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Get started for freeSuppose that a hydrogen fuel cell, as described in the text, is to be operated at and atmospheric pressure. We wish to estimate the maximum electrical work done by the cell, using only the room temperature data at the back of this book. It is convenient to first establish a zero-point for each of the three substances, . Let us take for both to be zero at , so that G for a mole of is at .
(a) Using these conventions, estimate the Gibbs free energy of a mole of at . Repeat for .
(b) Using the results of part (a), calculate the maximum electrical work done by the cell at , for one mole of hydrogen fuel. Compare to the ideal performance of the cell at.
Functions encountered in physics are generally well enough behaved that their mixed partial derivatives do not depend on which derivative is taken first. Therefore, for instance,
where each is taken with fixed, each is taken with fixed, and is always held fixed. From the thermodynamic identity (for) you can evaluate the partial derivatives in parentheses to obtain
a nontrivial identity called a Maxwell relation. Go through the derivation of this relation step by step. Then derive an analogous Maxwell relation from each of the other three thermodynamic identities discussed in the text (for ). Hold fixed in all the partial derivatives; other Maxwell relations can be derived by considering partial derivatives with respect to , but after you've done four of them the novelty begins to wear off. For applications of these Maxwell relations, see the next four problems.
Use a Maxwell relation from the previous problem and the third law of thermodynamics to prove that the thermal expansion coefficient (defined in Problem 1.7) must be zero at T=0.
Problem 5.35. The Clausius-Clapeyron relation 5.47 is a differential equation that can, in principle, be solved to find the shape of the entire phase-boundary curve. To solve it, however, you have to know how both L and V depend on temperature and pressure. Often, over a reasonably small section of the curve, you can take L to be constant. Moreover, if one of the phases is a gas, you can usually neglect the volume of the condensed phase and just take V to be the volume of the gas, expressed in terms of temperature and pressure using the ideal gas law. Making all these assumptions, solve the differential equation explicitly to obtain the following formula for the phase boundary curve:
P= (constant) x e-L/RT
This result is called the vapour pressure equation. Caution: Be sure to use this formula only when all the assumptions just listed are valid.
How can diamond ever be more stable than graphite, when it has
less entropy? Explain how at high pressures the conversion of graphite to diamond
can increase the total entropy of the carbon plus its environment.
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