The Gibbs energy of reaction (Delta_r G) is a thermodynamic quantity that determ

Question

The Gibbs energy of reaction (Delta_r G) is a thermodynamic quantity that determines whether a process will occur spontaneously at constant temperature and pressure. For a reaction carried out under standard conditions, we can write an equation relating the changes in standard Gibbs energy, enthalpy and entropy as follows, Delta_r G degree = Delta_r H degree - T Delta_r S degree, For a phase change, such as vapourization, we can write Delta_pc G degree = Delta_pc H degree - T_pc Delta_pc S degree where PC' indicates phase change. When all reactants and products are in their standard states and at the temperature of the phase change Delta pc G degree = 0 kJ mol^-1 which allows us to write Delta_pc G degree H degree = T_pc Delta_pc S degree or Delta_pc S degree = Delta_pc H degree/T_pc In the activity, select CH_3CI. Click on the Run button, and observe the graph of standard molar entropy versus temperature. Observe the equilibrium position, in which the solid and liquid phases exist together. Notice that the entropy increases from 79 J mol^-1 K^-1 to 116 J mol^-1 K^-1 as CH_3CI melts. Calculate the standard enthalpy of fusion (Delta_fus H degree) for this equilibrium. Express the change in enthalpy (Delta_fus H degree) to two significant and include the appropriate units.

Explanation / Answer

Part C) Melting point of CH3Cl from literature was found to be Tm = -97.4 oC = 273 - 97.4 = 175.6 K

Enthalpy change dHofus = TdSo

                                        = [175.6 x (116 - 79)]/1000

                                        = 6.50 kJ/mol

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