How do exothermic reactions occur?

1 Answer
Apr 22, 2018

Answer:

#|Enthalphy_(BondF)|#>#|Enthalphy_(BondB)|#

Explanation:

https://www.chemguide.co.uk/physical/basicrates/energyprofiles.html

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)|#