Question

Question: Calculate $∆$H⁰ for the rate-determining step of the reaction of CH₄ with I₂. Explain why this result illustrates that this reaction is extremely slow.

Short answer

Answer

The value of $∆$H⁰ is +138 kJ/mol. This reaction is slow because its activation energy value is higher than that of other reactions.

Step-by-step solution

Halogenation

This halogenation reaction takes place in the presence of sunlight leading to the formation of alkyl halide product.

Formation of products

The rate-determining step is shown below:

Slow reaction in halogenation

The value of $∆$H⁰ for the above reaction is calculated as follows:

$∆$H⁰=$\sum _{}$BE(bonds broken)+ $\sum _{}$BE(bonds formed)

= [+435+(-297)] kJ/mol

= 138 kJ/mol

Thus, the value of $∆$H⁰ is +138 kJ/mol. The reaction is endothermic, and its activation value is higher than Cl₂ and Br₂ . For this reason, the step is slow in this reaction.