Question

Using the bond energies provided below, calculate ?H° for the reaction

Answer 1

Answer:

ΔHºrxn =  - 440 kJ

Explanation:

What we need to remember in this question is that given the bond energies for a reaction, the enthalpy change , ΔHº , is given by the expression:

ΔHºrxn = ∑ energies of the bonds broken   - ∑ energies of the bonds formed.

where s  ∑ is  summatory.

The reaction is CH₄(g) + 4 Cl₂(g) ⇒ CCl₄(g) + 4 HCl(g)

So lets make the inventory of the bond broken and bonds formed

Bonds Broken:

4 C-H = 4 mol (413 kJ/mol ) = 1652 kJ

4 Cl-Cl = 4 mol ( 243 kJ/mol = 972 kJ/mol

Bond Formed:

4 C-Cl = 4 mol (339 kJ/mol ) = 1356 kJ

4 H-Cl = 4 mol (427 kJ/mol ) = 1708 kJ

Now we can calculate  ΔHºrxn :

ΔHºrxn = ∑ energies of the bonds broken   - ∑ energies of the bonds formed

ΔHºrxn = ( 1652 + 972 ) kJ   - ( 1356 + 1708 ) kJ

            = 2624 kj  - 3064 kJ

            = - 440 kJ