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Show that free energy (Delta G) as a criterion for spontaneous and non-spontaneous processes. |
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Answer» Solution :Effect of temperature on the spontaneity of chemical reaction using Gibb's-Helmholtz equation : `Delta G = Delta H - T.Delta S` Case I : For an exothermic process, `Delta H` is negative. (a) If `Delta S` is positive, `Delta G` is always negative. The process is always spontaneous. (b) If `Delta S` is negative, (i) If the temperature is LOW such that `T Delta S lt Delta H`, then `Delta G` is negative. The process is spontaneous. (ii) If the temperature is high such that `T Delta S gt Delta H`, then `Delta G` is positive. The process is not spontaneous Thus, an exothermic process which is not spontaneous at a GIVEN temperature may become spontaneous at low TEMPERATURES. Case II : For an endothermic process, `Delta H` is positive. (a) If `Delta S` is negative, `Delta G` is always positive. `:.` The process is not spontaneous (b) If `Delta S` is positive, (i) If the temperature is low such that `TDelta S lt Delta H`, then `Delta G` is positive. The process is not spontaneous. (ii) If the temperature is high such that `T Delta S gt Delta H`, the `Delta G` is negative. The process is spontaenous. Thus, an endothermic process which is not spontaneous at a given temperature may become spontaneous at high temperatures. Case III :When any process attains equilibrium, `T Delta S gt Delta H, Delta G = 0` i.e., `Delta G = Delta H - T Delta S = 0`or `Delta H = T Delta S` at equilibrium For a chemical reaction, `A + B rarr C + D`, Gibb's ENERGY change is given by, `Delta G = SigmaG_(2) - Sigma G_(1)`. It is the difference between the total free energies of all the PRODUCTS and the total free energies of all the reactants. |
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