The bond dissociation energy depends upon the nature of the bond and nature of the molecule. If any molecule more than 1 bonds of similar nature are present then the bond energy reported is the average bond energy.
Determine `C-C and C-H` bond enthalpy (in kJ/mol). Given: `Delta_(f)H^(0) (C_(2)H_(6),g)= -85kJ//mol, Delta_(f) H^(0) (C_(3)H_(8), g)= -104kJ//mole, Delta_("sub")H^(0) (C,s)= 718kJ//mol`, B.E. (H-H)= 436 kJ/mol,

Options

A. 414345
B. 345414
C. 287405.5
D. none of these

Answer

B

Explanation

The given reactions are `2C_(("graphite")) + 3H_(2(g)) rarr C_(2)H_(6(g)), Delta H = -85 kJ`/mole …(1)
`3C_(("graphite")) + 4H_(2(g)) rarr C_(3)H_(8(g)), Delta H = - 104` kJ/mole …(2)
`C_(("graphite")) rarr C_((g)), Delta H = 718` kJ/mole …(3)
`H_(2) rarr 2H, Delta H = 436` kJ/mole ...(4)
To get expression for heat of reaction in terms of Bond energy we have to use the expression with all the components are in gaseous state. So, from Eq. (1) and (3)
`2C_((g)) + 3H_(2(g)) rarr C_(2)H_(6(g)), Delta H = - 1521 kJ` ...(5)
From Eq.(2) and (3)
`3C_((g)) + 4H_(2(g)) rarr C_(3)H_(8(g)) , Delta H = - 2258kJ` ...(6)
We know that heat of reaction `Sigma` B.E of reactants `= Sigma` B.E of products. So,
`-1521= 3 (H- H) - [6 C-H + C-C]` ...(7)
`-2258 = 4(H-H) - [8C- H + 2C -C]` ..(8)
On solving the above two equations. C- H bond energy = 414
C- C bond energy = 345