Chapter 5: Q 5.61 (page 194)
Suppose you need a tank of oxygen that is 95% pure. Describe a process by which you could obtain such a gas, starting with air.
Short Answer
The process is explained.
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Chapter 5: Q 5.61 (page 194)
Suppose you need a tank of oxygen that is 95% pure. Describe a process by which you could obtain such a gas, starting with air.
The process is explained.
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In a hydrogen fuel cell, the steps of the chemical reaction are
Calculate the voltage of the cell. What is the minimum voltage required for electrolysis of water? Explain briefly.
Below 0.3 K the slope of the °He solid-liquid phase boundary is negative (see Figure 5.13).
(a) Which phase, solid or liquid, is more dense? Which phase has more entropy (per mole)? Explain your reasoning carefully.
(b) Use the third law of thermodynamics to argue that the slope of the phase boundary must go to zero at T = 0. (Note that the *He solid-liquid phase boundary is essentially horizontal below 1 K.)
(c) Suppose that you compress liquid *He adiabatically until it becomes a solid. If the temperature just before the phase change is 0.1 K, will the temperature after the phase change be higher or lower? Explain your reasoning carefully.
Suppose you have a box of atomic hydrogen, initially at room temperature and atmospheric pressure. You then raise the temperature, keeping the volume fixed.
(a) Find an expression for the fraction of the hydrogen that is ionised as a function of temperature. (You'll have to solve a quadratic equation.) Check that your expression has the expected behaviour at very low and very high temperatures.
(b) At what temperature is exactly half of the hydrogen ionised?
(c) Would raising the initial pressure cause the temperature you found in part (b) to increase or decrease? Explain.
(d) Plot the expression you found in part (a) as a function of the dimension- less variable t = kT/I. Choose the range of t values to clearly show the interesting part of the graph.
If expression 5.68 is correct, it must be extensive: Increasing both NA and NB by a common factor while holding all intensive variables fixed should increase G by the same factor. Show that expression 5.68 has this property. Show that it would not have this property had we not added the term proportional to In NA!.
The partial-derivative relations derived in Problems 1.46,3.33, and 5.12, plus a bit more partial-derivative trickery, can be used to derive a completely general relation between and.
(a) With the heat capacity expressions from Problem 3.33 in mind, first considerto be a function of andExpand in terms of the partial derivatives and . Note that one of these derivatives is related to
(b) To bring in , considerlocalid="1648430264419" to be a function ofand P and expand dV in terms of partial derivatives in a similar way. Plug this expression for dV into the result of part (a), then set and note that you have derived a nontrivial expression for . This derivative is related to , so you now have a formula for the difference
(c) Write the remaining partial derivatives in terms of measurable quantities using a Maxwell relation and the result of Problem 1.46. Your final result should be
(d) Check that this formula gives the correct value of for an ideal gas.
(e) Use this formula to argue that cannot be less than .
(f) Use the data in Problem 1.46 to evaluatefor water and for mercury at room temperature. By what percentage do the two heat capacities differ?
(g) Figure 1.14 shows measured values of for three elemental solids, compared to predicted values of . It turns out that a graph of vs.T for a solid has same general appearance as a graph of heat capacity. Use this fact to explain why and agree at low temperatures but diverge in the way they do at higher temperatures.
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