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Compute the heat of formation of liquie ...

Compute the heat of formation of liquie methyl alcohol is kilojoule per mol using the following data. Heat of vaporisation of liquid methyl alcohol `=38kJ//mol`. Heat of formation of gaseous atoms from the elements in their standard states `:H=218kJ // mol, C=715kJ //mol, O=249kJ //mol.`
Average bond energies`:`
`C-H415kJ//mol, C-O 356kJ//mol, O-H 463kJ //mol.`

Text Solution

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We aim at `: C(s) +2H_(2)(g) + (1)/(2)O_(2) rarr CH_(3)OH(l), DeltaH =?`
We are given `: (i) CH_(3)OH(l) rarr CH_(3) OH (g), DeltaH = 38 kJ mol^(-1)` (iii) `C(s)rarr C(g), DeltaH =715 kJ mol^(-1)`
(iv) `(1)/(2)O_(2)(g) rarrO(g) , DeltaaH -249 kJ mol^(-1)`
Also from the given bond energies, we have
(v) `CH_(3)OH(g)(H- underset(H) underset(|) overset(H)overset(|) (C) -O-H) rarr C(g) + 4H(g)+O(g), DeltaH = 3 xx 415 +356 +463 = 2064kJ mol^(-1)`
Eqn. (iii) `+4 ` Eqn. (ii) `+` Eqn. (iv)- Eqn.(i) - Eqn. (v) gives the requried result
i.e., `DeltaH = 715 + 4( 218) + 249-38 -2064 = -266 kJ mol^(-1)`
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