Question

I need to understand how to use bond enthalpy values to calculate the enthalpy change for a reaction. The reaction is:CH4 (g) + Br2 (g) --> CH3Br (g) + HBr (g)Any idea how to do this?

Answer 1

To calculate the enthalpy of the reaction using the bond enthalpy values, just use the expression below:

ΔH = Σr - Σp

Where:

ΔH = enthalpy change.

Σr = sum of the energies needed to break each bond in the reactant.

Σp = sum of energies required to form each bond in the product.

Using the values of binding energies from the table, we can determine the value of the ΔH of the reaction.

But first, we need to determine each bond breaking and each bond making.

Step 1: Demonstrate all bondings by setting up the structural formula for each of the participants:

Step 2: Calculate the sum of bond energies in the reactants (Σr).

4 C-H bonds - 413 kJ/mol each

1 Br-Br bond - 193 kJ/mol

So the calculation will be:

Σr = (4x413) + (1x193) = 1,845 kJ/mol

Step 3: Calculate the sum of bond energies in the products (Σp).

In the products, we have the following bonds:

3 C-H bonds - 413 kJ/mol each

1 C-Br bond - 276 kJ/mol

1 H-Br bond - 363 kJ/mol

So the calculation will be:

Σp = (3x413) + (1x276) + (1x363) = 1,878 kJ/mol

Step 4: Calculate the ΔH.

ΔH = Σr - Σp

ΔH = 1,845 - 1878 = -33 kJ/mol

Answer: The enthalpy change is -33 kJ/mol. Since the enthalpy is negative, the reaction is exothermic.