Chemistry 122
Cape Breton University
Dr. D. Keefe
Problem Set 5

  1. How does the entropy of a system change for each of the following processes?
    1. A solid melts. Entropy increases
    2. A liquid freezes. Entropy decreases
    3. A liquid boils. Entropy increases
    4. A vapor is converted to a solid. Entropy decreases
    5. A vapor condenses to a liquid. Entropy decreases
    6. A solid sublimes. Entropy increases
  2. Using thermodynamic tables, calculate the standard entropy change for the following reactions at 25°C:
    1. H2(g) + CuO(s) ® Cu(s) + H2O(g)
    2. 2Al(s) + 2ZnO(s) ® Al2O3(s) + 2Zn(s)
    3. CH4(g) + 2O2(g) ® CO2(g) + 2H2O(l)
  3. Without consulting a table of thermodynamic data, predict whether the entropy change is positive or negative for the following reactions. Explain your predictions.
    1. 2KClO4(s) ® 2KClO3(s) + O2(g)
      moles of gas increases, therefore more disorder, change in entropy is positive
    2. H2O(g) ® H2O(l)
      moles of gas decreases, therefore more order, change in entropy is negative
    3. S(s) + O2(g) ® SO2(g)
      moles of gas stays same, but number of molecules is reduced. Change in entropy is probably small and negative.
    4. 2Na(s) + 2H2O(l) ® 2NaOH(aq) + H2(g)
      moles of gas increases, therefore more disorder, change in entropy is positive
    5. CH4(g) + 2O2(g) ® CO2(g) + 2H2O(l)
      moles of gas decreases, therefore more order, change in entropy is negative
    6. N2(g) ® 2N(g)
      moles of gas increases, therefore more disorder, change in entropy is positive
    7. 2LiOH(aq) + CO2(g) ® Li2CO3(aq) + H2O(l)
      moles of gas decreases, therefore more order, change in entropy is negative
  4. For the reaction
    H2(g) + I2(g) º 2HI(g)
    D G° = 2.60 kJ. Calculate KP for the reaction at 25°C.
  5. Consider the following facts: Water freezes spontaneously at -5°C and 1 atm, and ice has a more ordered structure than liquid water. Explain how a spontaneous change can lead to a decrease in entropy.
    The change in the entropy of the system doe not determine whether or not a process is spontaneous. It is the change in entropy of the universe or the change in the free energy of the system that determines whether a process is spontaneous or not. The change in the free energy of the system combines both an enthalpy and entropy change. In this case the change in enthalpy is negative enough to out weigh the decrease in entropy associated with the process, thus making the free energy decrease and the process spontaneous.
  6. Use the following data to determine the normal boiling point of mercury. What assumptions must you make in order to do the calculation?
    Hg(l): S° = 77.4 J K-1 mol-1
    Hg(g): S° = 174.7 J K-1 mol-1.



    At the normal boiling point, the liquid and gas will be in equilibrium, thus

    It is necessary to assume that both the enthalpy and entropy of vaporization do not change with temperature. (In this case the approximations are quite good, the experimental boiling point of Hg is 357°C)
  7. Use thermodynamic tables to estimate a value of KP at 100°C for the reaction
    2NO(g) + O2(g) º 2NO2(g).