Question

Calculate the standard entropy change for the following reactions at 25 degrees celcius.

4NH3(g)+7O2(g)--->4NO2(g)+6H20(g)

2H2O2(l)--->2H20(l)+O2(g)

Answer 1

(1) 4NH₃(g)+7O₂(g)   ----> 4NO₂(g)+6H₂O(g)   ; ΔS = ?

ΔS = ΔS⁰_fproducts – ΔS⁰_freactants

ΔS = [(4x ΔS⁰_fNO2(g) )+ (6 x Δ⁰_{fH2O (g)} )]- [( 4x ΔS⁰_fNH3(g)) + (7 x ΔS⁰_{fO2 (g)} )]

     = [(4x239.95 )+ (6 x188.72 )]- [( 4x 192.34) + (7 x205.03 )]

    = -112.45 J/(mol-K)

(2) 2H₂O₂(l) --->   2H₂O(l)+O₂(g)   ;ΔS = ?

ΔS = ΔS⁰_fproducts – ΔS⁰_freactants

ΔS = [(2 x Δ⁰_{fH2O (l)} )+ (1 x ΔS⁰_{fO2 (g)} )]- [( 2x ΔS⁰_fH2O2(l)) ]

     = [(2x 69.91) + (1x205.03) ] - (2x232.88 )

    = -120.91 J/(mol-K)