Question

Why is the reaction between ammonia and oxygen exothermic?

Answer 1

The chemical reactions are often preceded by either absorbing the energy or by releasing the energy into the atmosphere. Depending upon the nature of chemical reactions to evolve or absorb energy they are categorized as endothermic reaction or exothermic reaction.
Exothermic reaction – when any chemical reaction proceeds by releasing energy into their atmosphere to form the final product is termed as endothermic reaction.
In this type of reaction, energy of the product formed is always lesser than the energy of reactant.

Complete answer:
Reaction between ammonia and oxygen is an example of exothermic reaction because when ammonia reacts with oxygen in absence of any catalyst large amounts of energy is released into the atmosphere to form the final product with more stability.
Initially ammonia and oxygen have higher energy and when they undergo chemical reactions they tend to release energy into the atmosphere due to which the final product of the reaction has low energy and has more stability.
Ammonia and oxygen react with each other in the absence of any catalyst to form nitrogen and water. $N{H_3}_{\left( g \right)} + {O_2}_{\left( g \right)} \to {N_{2\left( g \right)}} + {H_2}{O_{\left( g \right)}} - - - - - - - - \left( a \right)$
In order to write the balanced chemical equation multiply$4$ with ammonia and $3$ with oxygen. Now the final equation become-
$4N{H_3}_{\left( g \right)} + 3{O_2}_{\left( g \right)} \to 2{N_{2\left( g \right)}} + 6{H_2}{O_{\left( g \right)}} - - - - - - - - \left( b \right)$
After completing the reaction nitrogen gas and water vapors are formed as the final stable product of reaction.

Note:
Remember that ammonia and oxygen also react in the presence of a catalyst (platinum-rhodium or copper metal) to form nitrous oxide and water.
$4N{H_{3(g)}} + 5{O_{2(g)}}\dfrac{{pt - Rh}}{{Cu}}4N{O_{(g)}} + 6{H_2}{O_{(g)}}$
Always write a balanced chemical equation for any chemical reaction.
In an exothermic reaction the enthalpy of the product will be always less than the enthalpy of reactant so total enthalpy of product is always negative.