Answer: ΔH = 145 kJ/mol
Explanation:
1) Equations given:
C (s) + O₂ (g) → CO₂ (g), ΔH = 395 kJ/mol
H₂ (g) + 1/2O₂ (g) → H₂O (l), ΔH = 285 kJ/mol
C₅H₁₂ (g) + 8O₂ → 5CO₂ (g) + 6H₂O (l), ΔH = 3.54 × 10³ kJ/mol
2) Target equation:
5C(s) +6 H₂(g) → C₅H₁₂(g)
3) Hess law
As per Hess law you can handle the chemical equations (add, subtract, reverse, multiply by a coefficient) to find the target equation, and then the net sum of the heat changes of the individual reactions equals the heat change of the target reaction.
4) Now, you must figure out how to transform the individual reactions to end up with the final equation:
Here is how:
i) Multiply the first one by 5:
5C (s) + 5O₂ (g) → 5CO₂ (g), ΔH = 5×395 kJ/mol
ii) Multiply the second by 6:
6H₂ (g) + 3O₂ (g) → 6H₂O (l), ΔH = 6×285 kJ/mol
iii) Reverse the third:
5CO₂ (g) + 6H₂O (l) → C₅H₁₂ (g) + 8O₂ , ΔH = - 3.54 × 10³ kJ/mol
iv) Add them up:
5C (s) + 5O₂ (g) → 5CO₂ (g), ΔH = 5×395 kJ/mol
6H₂ (g) + 3O₂ (g) → 6H₂O (l), ΔH = 6×285 kJ/mol
5CO₂ (g) + 6H₂O (l) → C₅H₁₂ (g) + 8O₂ , ΔH = - 3.54 × 10³ kJ/mol
----------------------------------------------------------------------------------------------
5C (s) + 6H₂ → C₅H₁₂ (g),
ΔH = 5×395 kJ/mol + 6×285 kJ/mol - 3.54 × 10³ kJ/mol
⇒ ΔH = 145 kJ/mol
