# Help Zone

### Student Question

Secondary IV • 1yr.

Hi

I am having difficulty differentiating endothermic and exothermic reactions when using the enthalpy and not using it.

Can you help me? Thank you

Science

## Explanations (1)

• Explanation from Alloprof

Explanation from Alloprof

This Explanation was submitted by a member of the Alloprof team.

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Team Alloprof • 1yr.

First, let's get back to the basics:

• An endothermic reaction is defined as a reaction that absorbs energy. Therefore, the enthalpy difference between the products and the reactants is positive (because the products have more stored energy than the reactants):

Hproducts − Hreactants = + ∆H

Concretely, endothermic reactions lower the temperature of their environment.

• An exothermic reaction is defined as a reaction which releases energy. More precisely, the enthalpy difference between the products and the reactants is negative (because the products do not have as much energy stored energy as the reactants):

Hproducts − Hreactants = −∆H

Concretely, we often observe that the exothermic reactions increase the temperature of their environment or give off light.

So as soon as a reaction releases energy, you can be sure that it is an exothermic reaction. It is the same for any chemical equation whose ∆H is negative or whose energy is on the same side of the equation as the products. For example, cellular respiration, light sticks, and the combustion of propane are all exothermic reactions.

Warning: even if it may seem counterintuitive, phase changes from gas to liquid and from liquid to solid are exothermic: they release energy in their environment.

Then, as soon as a reaction absorbs energy, you can be sure that it is an endothermic reaction. It is the same for any chemical equation whose ∆H is positive or whose energy is on the reactants' side of the equation. For example, melting ice, photosynthesis, and the electrolysis of water are all endothermic reactions.

Please note: even though it may seem counterintuitive, some processes that release heat (ex: cooking food) are endothermic reactions. It is explainable because the chemical reaction does not produce energy. It requires a significant amount of heat to be absorbed in its medium to take place.

Do not hesitate if you have other questions!