Question

For the following fuels burning in air, write down the stoichiometric reactions, the respective fuel–air ratios (F/A), and the heats of combustion per mole ( ¯qc) and per gm (qc). Assume the product water is in the gaseous state. Fuels: CO, H2, CH4, C7H16, C16H34, C6H6, CH3OH, C7H15OH. What can you say about the qcs for (a) CH4, C7H16, C16H34, and (b) C7H16, CH3OH, C7H15OH?

Answer 1

i) 2CO(g) + O₂(g) $\rightarrow$ 2CO₂(g) (F/A = 1:2)

Heat of combustion ( $\Delta$ Hc) = -294 KJ/mol = -294/28 KJ/g = -10.5 KJ/g

ii) 2H₂(g) + O₂(g) $\rightarrow$ 2H₂O(g) (F/A = 2:1)

Heat of combustion ( $\Delta$ Hc) = -286 KJ/mol = -286/2 KJ/g = -143 KJ/g

iii) CH₄(g) + 2O₂(g) $\rightarrow$ CO₂(g) + 2H₂O(g) (F/A = 1:2)

Heat of combustion ( $\Delta$ Hc) = -890 KJ/mol = -890/16 KJ/g = -55.625 KJ/g

iv) C₇H₁₆(l) + 11O₂(g) $\rightarrow$ 7CO₂(g) + 8H₂O(g) (F/A = 1:11)

Heat of combustion ( $\Delta$ Hc) = -4817 KJ/mol = -4817/100 KJ/g = -48.17 KJ/g

v) 2C₁₆H₃₄(l) + 49O₂(g) $\rightarrow$ 32CO₂(g) + 34H₂O(g) (F/A = 2:49)

Heat of combustion ( $\Delta$ Hc) = -10699 KJ/mol = -10699/226 KJ/g = -47.34 KJ/g

vi) C₆H₆(l) + 9O₂(g) $\rightarrow$ 6CO₂(g) + 6H₂O(g) (F/A = 1:9)

Heat of combustion ( $\Delta$ Hc) = -3271 KJ/mol = -3271/78 KJ/g = -41.94 KJ/g

vii) 2CH₃OH(l) + 3O₂(g) $\rightarrow$ 2CO₂(g) + 4H₂O(g) (F/A = 2:3)

Heat of combustion ( $\Delta$ Hc) = -726 KJ/mol = -726/32 KJ/g = -22.69 KJ/g

viii) 2C₇H₁₅OH(l) + 21O₂(g) $\rightarrow$ 14CO₂(g) + 16H₂O(g) (F/A = 2:41)

Heat of combustion ( $\Delta$ Hc) = 4638 KJ/mol = 4638/116 KJ/g = 39.98 KJ/g

Note: Simple hydrocarbons release more heat than the alcohols in the corresponding combustion reaction. Higher alcohols absorb heat in their combustion.