Question

What is the value for ∆Soreaction for the following reaction, given the standard entropy values? Fe2O3(s) + 3H2(g) 2 Fe(s) + 3H2O(g) +138 J/K -138 J/K +5 J/K -5 J/K 189 J/K

Answer 1

Answer: 138 J/K

Explanation:

1) ΔS reaction = ΔS products - ΔS reactants

2) ΔS products = 2ΔS Fe(s) + 3 ΔS H₂O (g)


3) ΔS reactants = ΔS Fe₂O₃ (s) + 3 ΔS H₂ (g)


4) Now you have to search the standard entropy values for each product and each reactant in a table.

I found these values at 25° and 1 bar.


ΔS H₂O = 188.8 J/Kₓmol
ΔS Fe(s) = 27.3 J/Kmol

ΔS Fe₂O₃ (s) = 87.4 J/Kmol
ΔS H₂ (g) = 130.7 J/Kmol

5) Replace those values into above equations:

      ΔS products = 2ΔS Fe(s) + 3ΔS H₂O (g) = 
      2 mol × 27.3 J/K×mol + 3 mol ×188.8 J/K×mol = 621.0 J/K


      ΔS reactants = ΔS Fe₂O₃ (s) + 3 ΔS H₂ (g) =
      1mol × 87.4 J/K×mol + 3×130.7 J/K×mol = 479.5 J/K

      ΔS reaction = 621.0 J/K - 479.5 J/K = 141.5 J/K


Taking into account the differences in the values from different sources (specially due to the temperature), you can consider that the value 141.5 J/K is pretty much close to 138 J/K, and take that answer.