Thermodynamics

Question

For the gas phase, reaction,

PCl5 (g) ⇌ PCl3 (g) Cl2 (g)
Which of the following conditions are correct?

  • Δ H = 0 and ΔS > 0

  • ΔH >0 and ΔS > 0

  • Δ H < 0 and ΔS < 0

  • ΔH > 0 and ΔS < 0

Answer

B.

ΔH >0 and ΔS > 0

ΔH = ΔE + ΔnRT

Δn = number of moles of product - number of moles of reactants
ΔG = ΔH - TΔS
For a spontaneous process, ΔG must be negative

PCl5 (g) ⇌ PCl3 (g) Cl2 (g)
 In this  reaction
Δn = 2-1 = 1
Thus, ΔH is positive, ie, >0
If ΔH is positive, then  to maintain the value of ΔG negative, ΔS should be positive , ie, ΔS>0.

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