Question

What is the reason respiration is considered an exothermic reaction?

Answer 1

Try to feel it by your hand when you breathe out air. The air which is coming out during expiration is a little warmer. Exothermic reaction is a reaction which is likely to produce heat as a by-product. So, basically, it releases heat.

Complete answer:
In order to carry out normal biological functions we need energy and this energy is provided by the food we consume. The food undergoes a variety of processes and reactions before it provides us with the energy we need, such as digestion, absorption etc. Let us go through these processes to understand the concept.
We consume different types of food which have different nutritional values according to the ingredients present in them. Nutritional value indicates the nutritional contents present in the food and also the amount of the ingredient providing the nutrition and its physiological effects on our body. Some foods have more protein content such as egg, pulses etc and some have various types of vitamins like milk, carrots, eggs, etc. Some eatables contain carbohydrates like rice, wheat, etc.
Carbohydrates are macromolecules that have carbon, oxygen and hydrogen as their building blocks. Common examples are glucose and starch. When we consume food that contains carbohydrates, they are converted into simpler forms like glucose by various enzymes in our body. The glucose molecule is to be passed through a combustion process which releases oxygen, water and carbon dioxide.
Breathing is a process where we inspire oxygen and breathe out carbon dioxide, and this is the same carbon dioxide that we got from the combustion of glucose.
This whole process involves a lot of heat generation and that refers to the energy which we get from the food we consume. As explained above, when a reaction generates heat as a by-product, they are known as exothermic reactions.
So, considering that we conclude that respiration is also an exothermic reaction and the heat which is generated is provided throughout the body as an energy source and this is the reason for the fact that when we breathe out, our breath is warm.

Note:
Endothermic reactions, on contrary to exothermic reactions are the ones that utilizes the energy to proceed. One can understand as the heat is being absorbed from the surroundings and this accounts for the reduction in temperature during the process involving endothermic reactions.