CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Homework, Week 5


Homework, week 5
Answer key

For this problem set, you will need to download the handout of thermodynamic data from the link on the bottom of the course homepage.  Keep this handy throughout the semester.

1. For each of the following processes determine whether deltaS(sys) is greater than zero, less than zero, or equal to zero.  Explain your reasoning.

            a) An irreversible adiabatic process

            b) A reversible adiabatic process

           c) An irreversible isothermal expansion of an ideal gas

            d) A reversible isothermal expansion of an ideal gas

2.  Consider Figure 20.3 in McQuarrie & Simon.  Determine qrev and deltaS for a reversible cooling of 1.0 mole of an ideal gas at a constant volume V1 from P1, V1, T1 to P2, V1, T4 followed by a reversible expansion at constant pressure P2 from P2, V1, T4 to P2, V2, T1.  Compare your result to qrev and deltaS determined from pathway B + C in Figure 20.3 (assume both paths are reversible) and comment on any differences.

3.  Vaporization at the normal boiling point of a substance (the boiling point at 1 atm, Tvap) is a reversible process.  If deltaHvap of water is 40.65 kJ mol-1, determine deltaSvap when 2.0 moles of water are vaporized at 100˚C.  Comment on the sign of deltaSvap.

4.  (Please download the pdf file to look at these molecular structures.)  For each of the following pairs of molecules, predict which has the greater molar entropy, assuming all are gaseous species under the same conditions.  Briefly discuss your reasoning.

5.  Without referring to reference tables, arrange the following reactions according to increasing values of deltaSrxn, and briefly discuss your reasoning.

            a.  S(s) + O2(g) --> SO2(g)

            b.  H2(g) + O2(g) -->  H2O2(l)

            c.  CO(g) + 3 H2(g) --> CH4(g) + H2O(l)

            d.  C(s) + H2O(g) -->  CO(g) + H2(g)

6.  Consider the mixing of two ideal gases, 1 and 2.  Plot deltaSmix against the mole fraction of species 1, n1.  Determine the value of n1 that gives maximum deltaSmix and provide a physical interpretation for this result. 

7.  A 10 g block of gold at 900 K is brought in contact with a 10 g block of silver at 300 K until thermal equilibrium is reached.  If there is no heat exchange between the blocks and the surroundings, calculate the final temperature of the system, deltaSsys, deltaSsurr, and deltaSTot. 

Assume Cp(Au) = 25.41 J K-1 mol-1 and Cp(Ag) = 25.38 J K-1 mol-1, and remain constant over this temperature range.