Page 181 - Physical chemistry understanding our chemical world
P. 181

148    REACTION SPONTANEITY AND THE DIRECTION OF THERMODYNAMIC CHANGE

                      the negative value of  H overcomes the unfavourable positive − S term. In
                      fact, although the reaction is thermodynamically feasible, the rate of reaction (see
                      Chapter 8) is so small that we need to heat the reaction vessel strongly to about 550 K
                      to generate significant quantities of product to make the reaction viable.

                                      SAQ 4.5 Consider       the   reaction   2H 2 S (g) + SO 2(g) →
              Some zoologists be-     2H 2 O (g) + 3S (s) , where all species are gaseous except the
              lieve this reaction                           O
              inspired the myth of    sulphur. Calculate  G r  for this reaction at 298 K with the
              fire-breathing dragons:  thermodynamic data below:
              some large tropical
              lizards have glands
              beside their mouths to                          H 2 S    SO 2     H 2 O    S
              produce both H 2 Sand        H /kJ mol  −1     −22.2   −296.6   −285.8     0
                                             O
              SO 2 . Under advanta-          f          −1
                                             O
              geous conditions, the        S /JK  −1  mol    205.6    247.9      70.1   31.9
                                             f
              resultant sulphur reacts
              so vigorously with the
                                      [Hint: it is generally easier first to determine values of  H r
              air that flames form as  and  S r by constructing separate Hess’s-law-type cycles.]
              a self-defence mecha-
              nism.
                                      SAQ 4.6 The      thermodynamic     quantities  of  charge-
                                      transfer complex formation for the reaction
                         methyl viologen + hydroquinone −−−→ charge-transfer complex      (4.26)

                      are  H r =−22.6kJ mol  −1  and  S r =−62.1J K −1  mol −1  at 298 K. Such val-
                      ues have been described as ‘typical of weak charge-transfer complex
                      interactions’. Calculate the value of  G r .


              4.4     The effect of pressure on thermodynamic
                      variables



                       How much energy is needed?

                      The Gibbs–Duhem equation

                      ‘How much energy is needed?’ is a pointless question. It is too imprecise to be useful
                      to anyone. The amount of energy needed will depend on how much material we wish
                      to investigate. It also depends on whether we wish to perform a chemical reaction
                      or a physical change, such as compression. We cannot answer the question until we
                      redefine it.
                        The total amount we need to pay when purchasing goods at a shop depends both
                      on the identity of the items we buy and how many of each. When buying sweets and
   176   177   178   179   180   181   182   183   184   185   186