The chemical reaction that causes magnesium to corrode in air is given by in whi

Question

The chemical reaction that causes magnesium to corrode in air is given by

in which

A)What is the standard Gibbs free energy for this reaction? Assume the commonly used standard reference temperature of 298 K.

Explanation / Answer

For the reaction, MgO + O2 ------> 2MgO Given : Delta H^0(rxn)=-1204kJ, Delta S^0(rxn) =-217.1 J/K A) For temperature, T=298K The standard free energy change of the forward reaction is given by : Delta G^0(rxn) = Delta H^0(rxn) - T*{ Delta S^0(rxn)} =-1204 - (298*-217/1000) = -1139.334 kJ B) Assuming the value of standard free enthalpy and standard free entropy of reaction to be same at temp.= 5958 K The standard free energy change of the forward reaction is given by : Delta G^0(rxn) = Delta H^0(rxn) - T*{ Delta S^0(rxn)} =-1204 - (*-217/1000) = +88.886 kJ C) At equilibrium, The standard free energy change of the forward reaction = 0. Thus, Delta H^0(rxn) - T*{ Delta S^0(rxn)} = 0 => T = Delta H^0(rxn) / Delta S^0(rxn) =-1204*1000/-217 = 5548.387 K. This is the forward and reverse corrosion reactions occur in equilibrium.

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