Question

The value of ΔG∘ at 221.0∘C for the formation of phosphorous trichloride from its constituent elements,

P2(g)+3Cl2(g)→2PCl3(g)

is ________ kJ/mol. At 25.0∘C for this reaction, ΔH∘ is −720.5kJ/mol, ΔG∘ is −642.9kJ/mol, and ΔS∘ is −263.7J/K.

Answer 1

From Gibbs free energy relationship, we have

ΔG^∘ = ΔH^∘ - TΔS^∘

given T= 221.0^oC = 221 + 273.15 = 494.15 K

ΔG^∘ = ΔH^∘ - TΔS^∘ = −720.5 kJ/mol - [ 494.15 K x −263.7 J/K],

but ΔS^∘ value is in J/K convert it into kJ/K as 1 kJ = 100J

assume ΔH^∘ and ΔS^∘ values are independent over the temperature change.

ΔG^∘ = ΔH^∘ - TΔS^∘ = −720.5 kJ/mol - [ 494.15 K x −0.2637 kJ/K]

ΔG^∘ = -590.192645 kJ/mol

ΔG^∘ = -590.2 kJ/ mol   -------------- Your Answer with 4 sig fig (Please note that negative sign before value)

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