# Exothermic vs. endothermic reactions

An exothermic reaction is a chemical reaction that releases energy by light or heat.

Does this mean that if I mix two chemicals in a test tube when the reaction between them is endothermic then the gas evolved is hot but the test tube becomes cooler than before or does it become hotter because the gas evolved is hot?

• Would Chemistry be a better home for this question? – Qmechanic Oct 26 '15 at 22:19

You're mixing up several concepts here.

Exothermic is defined as when system loses heat to the surroundings.

This is not necessarily true. Often for the purpose of measurements of Reaction Enthalpy reactions are conducted in a adiabatic enclosure, i.e. a container from which no heat can escape (or enter) and which doesn't perform any work (constant volume).

In that case the heat released by the reaction is absorbed by the reaction products, any solvent (like water) present and to some extent by the container. This then leads inevitably to an increase in temperature acc.:

$$\large{\Delta Q = - m\int_{T_1}^{T_2}C_VdT}.$$

With $m$ the total mass, $C_V$ the heat capacity of the mass, $T$ the temperature and $\Delta Q (<0)$ (for exothermic reactions) the reaction heat.

In the simple case where $C_V$ is a true constant, this simplifies to:

$$\large{\Delta Q = - mC_V\Delta T}.$$

The situation is similar for an endothermic reaction but there $\Delta Q > 0$. Whether or not gas evolves doesn't change anything.

Here's an example of the lab measurement of an endothermic reaction.