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Question
At 25oC, the following heats of formation are given:
Compound SO2(g) H2O(l) ΔHof(kJ/mol) −296.81 −285.83
For the reactions at 25oC
2H2S(g)+Fe(s)⟶FeS2(s)+2H2;ΔHo=−137 kJ mol−1
H2S(g)+32O2(g)+H2O(l)+SO2(g);ΔHo=−562 kJ mol−1
Calculate heats of formation of H2S(g) and FeS2(s)
| Compound | SO2(g) | H2O(l) |
| ΔHof(kJ/mol) | −296.81 | −285.83 |
2H2S(g)+Fe(s)⟶FeS2(s)+2H2;ΔHo=−137 kJ mol−1
H2S(g)+32O2(g)+H2O(l)+SO2(g);ΔHo=−562 kJ mol−1
Calculate heats of formation of H2S(g) and FeS2(s)
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Solution
Heat of formation of H2S=x kJ/mol
Heat of formation of FeS2=y kJ/mol
2H2S(g)+Fe(s)⟶FeS2(s)+2H2(g);ΔHo=−137kJmol−1
−137=y−2x...(i)
2H2S(g)+32O2(g)+H2O(l)+SO2(g);ΔHo=−562kJmol−1
−562=−285.83−296.81−x.....(ii)
From eq. (ii) x=−20.64kJmol−1
From eq. (i), we get
y=2x−137=2−(−20.64)−137=−178.28kJmol−1
ΔHfH2S=−20.64kJ/mol
ΔHfFeS2=−178kJ/mol
Heat of formation of FeS2=y kJ/mol
2H2S(g)+Fe(s)⟶FeS2(s)+2H2(g);ΔHo=−137kJmol−1
−137=y−2x...(i)
2H2S(g)+32O2(g)+H2O(l)+SO2(g);ΔHo=−562kJmol−1
−562=−285.83−296.81−x.....(ii)
From eq. (ii) x=−20.64kJmol−1
From eq. (i), we get
y=2x−137=2−(−20.64)−137=−178.28kJmol−1
ΔHfH2S=−20.64kJ/mol
ΔHfFeS2=−178kJ/mol
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