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Find the heat of formation of ethyl alco...

Find the heat of formation of ethyl alcohol for following data
`C(s) +O_(2)(g) rarr CO_(2)(g) DeltaH =- 94 kcal`
`H_(2)(g) +1//2O_(2)(g) rarr H_(2)O(l), DeltaH =- 68 kcal`
`C_(2)H_(5)OH(l)+3O_(2)(g) rarr 2CO_(2)(g) +3H_(2)O(l) DeltaH =- 327 kcal`

Text Solution

Verified by Experts

Always write the balanced thermodynamic equation for which `Delta_(r )H` is to be calculated.
`2C(s)+3H_(2)(g)+(1)/(2)O_(2)(g)rarr C_(2)H_(5)OH(l) , Delta_(f)H=?`
Given : I. `C(s)+O_(2)(g)rarr CO_(2)(g) " " , Delta_(r )H_(1)=-94 kcal//mol -=(Delta_(f)H)_(CO_(2))-=(Delta_(c )H)_(C )`
II. `H_(2)(g)+(1)/(2)O_(2)(g)rarr H_(2)O(l) " " , Delta_(f)H_(2)=-68 kcal//mol -=(Delta_(f)H)_(H_(2)O)-=(Delta_(c )H)_(H_(2))`
III. `C_(2)H_(5)OH(l)+3O_(2)(g)rarr 2CO_(2)(g)+3H_(2)O(l) , Delta_(f)H_(3)=-327 kcal -= (Delta_(c )H)_(C_(2)H_(5)OH)`
Now adding appropriately to get the required equation (Using Hess's Law) :
Operate : `2(I)+3(II)-(III)` to get :
`2C(s)+3H_(2)(g)+(1)/(2)O_(2)(g)rarr C_(2)H_(5)OH(l)`
From Hess's Law : `Delta_(f)H=2(Delta_(f)H_(1))+3(Delta_(r )H_(2))-Delta_(r )H_(3)=2(-94)+3(-68)-(-327)=-65` kcal/mol.
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