Question

For the formation of Silica: Si(s) + O₂ --> SiO₂(s)

Find the enthalpy of formation for SiO₂ at 500 K,1 atm (in symbols and in numbers)

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Find the equilibrium constant for ther above reaction at 500K (in symbols & in numbers).

Answer 1

Solution :-

Lets first calculate the Delta H and Delta S of the reaction

Si(s) + O2(g) ----- > SiO2(s)

Delta H= sum of delta Hf product – sum of delta Hf reactant

             = [SiO2 * 1]

Si and O2 are not involved because they are in elemental state so the delta Hf is 0 kj

             = -910.9 kJ * 1

            = -910.9 kJ

Now lets calculate the Delta S reaction

Delta S = sum of delta S product – sum of delta S reactant

             = [SiO2*1] –[(Si*1)+(O2*1)]

             = [41.5 * 1] – [ (18.0*1)+(205 *1)]

             = -178.5 J per mol

Now lets calculate the Delta G at 500 K

Delta G = Delta H – T* delta S

            = (-910.9 kJ per mol * 1000 J / 1 kJ) – [500 K * (-178.5 J per mol )]

          = -821650 J

Now using the Delta G we can calculate the equilibrium constant

Delta G = - RT ln K

-821650 J per mol = - 8.314 J per mol K * 500 K * ln K

-821650 J per mol / - 8.314 J per mol K * 500 K = ln K

197.7 = ln K

Antiln 197.7 = k

6.9*10^85 = K