Calculate ΔHrxn for the following reaction: 5C(s)+6H2(g)→C5H12(l)5C(s)+6H2(g)→C 5H12(l)

Use the following reactions and given ΔH values:

C5H12(l)+8O2(g)→ 5CO2(g)+6H2O(g) ΔH= -3505.8 kJ
C(s)+O2(g)→CO2(g) ΔH= -393.5 kJ
2H2(g)+O2(g)→ 2H2O(g) ΔH= -483.5 kJ

ΔH = 87.8kJ

Explanation:

You can determine ΔH of a reaction from the sum of similar reactions (Hess's law). In the problem with the given ΔH:

1. C5H12(l)+8O2(g)→ 5CO2(g)+6H2O(g) ΔH= -3505.8 kJ

2. C(s)+O2(g)→CO2(g) ΔH= -393.5 kJ

3. 2H2(g)+O2(g)→ 2H2O(g) ΔH= -483.5 kJ

5 times 2. + 3 times 3:

5C(s) + 8O2(g) + 6H2(g) → 5CO2(g) + 6H2O(g)  

ΔH= 5*(-393.5 kJ) +  3*(-483.5 kJ) = -3418kJ

Now, this reaction - 1:

5C(s)+6H2(g) → C5H12(l) ΔH= -3418kJ - (-3505.8kJ)