Why is it that endothermic reactions decrease in temperature?


If endothermic reactions gain heat, shouldn’t their temperature increase?

In: Chemistry

Those reactions are, more or less, “eating” heat energy and turning it into mass in the form of bond energy. So the surrounding area gets colder because the energy to create those bonds has been transformed into something other than heat.

Exothermic/endothermic depends on whether or not the temperature of the *environment* increases or decreases. If a reaction increases the temperature of the environment, it is exothermic. If the reaction reduces the temperature of the environment, it is endothermic.

It depends on what you’re measuring. Usually you’re not measuring the temperature of the reaction itself, but rather the environment the reaction is happening in.

For example, let’s say you have an endothermic reaction happening in water. Your thermometer is sitting in the water, not on the individual molecules involved in the reaction, so as the reaction progresses the water loses heat (which is transferred to the reactants) and the measured temperature (of the water) decreases.

Visualized another way, there’s typically a lot more solvent (the water) than reactants. Imagine a (perfectly insulated) bathtub full of boiling water, in which you drop a single frozen ball bearing. The ball bearing is warming up (endothermic from the point of view of the bearing) but because it’s small and rolling around the bathtub, you can’t keep the thermometer on it. So instead, you have to measure the temperature of the water, which decreases as it transfers energy to the bearing.

To put it short, they gain “energy” from the surrounding to break the bonds between atoms or ions, thus their enthalpy is increasing, and so the surrounding temperature decreases