Calculate ΔHrxn for the following reaction: 7C(s)+8H2(g)→C7H16(l) Use the following reactions and given ΔH values: C7H16(l)+11O2(g)→7CO2(g)+8H2O(g),ΔHC(s)+O2(g)→CO2(g),ΔH2H2(g)+O2(g)→2H2O(g),ΔH===−4464.3kJ−393.5kJ−483.5kJ

ΔH = -224.2kJ

Explanation:

You can obtain ΔH of any reaction from the sum of another similar reactions (Hess's law).

With the reactions:

(1) C₇H₁₆(l) + 11O₂(g) → 7CO₂(g) + 8H₂O(g) , ΔH = -4464.3kJ

(2) C(s) + O₂(g) → CO₂(g), ΔH = -393.5kJ

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

7 times (2) - (1):

7(2) 7C(s) + 7O₂(g) → 7CO₂(g), ΔH = 7*-393.5kJ = -2754.5kJ

-(1) 7CO₂(g) + 8H₂O(g) → C₇H₁₆(l) + 11O₂(g) ΔH = +4464.3kJ

7C(s) + 8H₂O(g) → C₇H₁₆(l) + 4O₂(g) ΔH = -2754.5kJ +4464.3kJ = 1709.8kJ

This reaction + 4 times (3):

4(3) 8H₂(g) + 4O₂(g) → 8H₂O(g), ΔH = 4*-483.5kJ = -1934kJ

7C(s) + 8H₂(g) → C₇H₁₆(l) ΔH = 1709.8kJ - 1934kJ