Question

How do exothermic reactions occur?

Answer 1

`|Enthalphy_(BondF)|`>`|Enthalphy_(BondB)|`

Explanation:

In any reaction, there is two stages. The Bond Breaking and Bond Forming stage which occurs after bond breaking.

Lets take `N_2 + 3H_2 = NH_3` as reaction for example. This reaction ,common in the Haber Process, is exothermic. Why? Simply , energy is first used to break the bonds of `N≡N` in `N_2` and `H=H` in `H_2`. Let this energy 'absorbed' be `|Enthalphy_(BondB)|` .

After that, free `N` atoms and free `H` atoms collide to form `N-H` single bonds in newly formed `NH_3`. Let this energy 'released' be `|Enthalphy_(BondF)|` .

Since in this reaction, the energy evolved(released) is larger than the energy absorbed to break the bonds, there is a net release of energy which results in the products having a lower energy content than the reactants as shown in the graph.

aka `|Enthalphy_(BondF)|` > `|Enthalphy_(BondB)|`