InterviewSolution
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InterviewSolution
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Using the data ( al values are in kilocalories per moleat `25^(@)C `) given below, calculate the bond energy of `C-C` and `C-H` bonds. `DeltaH^(@) ` combustion( ethane) `= - 372.0` `DeltaH^(@) ` combustion ( propane) `= - 530.0``DeltaH^(@) ` forC ( graphite` rarr C (g) = 172.0` Bond energy of `H-H= 104.0` `Delta_(f)H^(@)` of`H_(2)O(l)= - 68.0, Delta_(f)H^(@)` for`CO_(2) = - 94.0` |
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Answer» We are giving (i)`C_(2)H_(6)(g)+(7)/(2) O_(2)(g) rarr 2CO_(2)(g)+H_(2)O(l),DeltaH^(@) =- 372.0 kcal` (ii) `C_(3)H_(8) +5O_(2)(g) rarr 3CO_(2)(g)+4H_(2)O(l),DeltaH^(@) = - 530.0 kcal` (iii) `C(s) rarr C(g), DeltaH^(@) =172.0kcal` (iv) `H_(2)(g) rarr 2H(g) , DeltaH^(@) = 104.0kcal` (vi)` C(g)+ O_(2)(g) rarr CO_(2)(g) , DeltaH^(@) = - 94.0 kcal` Suppose the bond energy of`C-C` bond `=x kcal ` mol`e^(-1)` and thatof`C-H` bond `=y` kcal` mol e^(-1)`. Then for `C_(2)H_(6)(g), i.e.,H- underset(H) underset(|) overset(H) overset(|) (C) - underset( H ) underset(|) overset(H) overset(|) (C)-H rarr C(g) + H(g),DeltaH = x+6y` ....(vii) andfor `C_(3)H_(8) (g)`, i.e., `H-underset(H) underset(|) overset(H) overset(|)(C) - underset(H) underset(|) overset(H) overset(|) (C) - underset(H) underset(|) overset(H) overset(|) (C) - H rarr 3C(g) +8 H(g), DeltaH = 2x + 8 y ` .....(viii) To get Eqn. (vii) , operate Eqn. (i) `+ 2 xx `Eqn. (iii) ` + 3 xx ` Eqn. (iv) `- 3 xx` Eqn. (v) `- 2 xx` Eqn. (vi). This gives`DetlaH = 676 kcal` To get Eqn. (viii) , operateEqn. `+` Eqn. (iii) `+`Eqn. (iii) `+ 4 xx` Eqn. (vi)` - 4 xx` Eqn. (v) `- 3 xx `Eqn. (vi). This gives `DeltaH = 956kcal` Thus, `x + 6y= 676 ` `2x + 8y=956` On solving these equations, we get `x= 82,y = 99` Hence, `C-C` bond energy `= 83 kcal mol^(-1)` and `C-H` bond energy `= 99` kcal `mol^(-1)` |
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