I am having difficulty differentiating endothermic and exothermic reactions when using the enthalpy and not using it.
Can you help me? Thank you
Explanation from Alloprof
This Explanation was submitted by a member of the Alloprof team.
Thank you for your question!
First, let's get back to the basics:
Hproducts − Hreactants = + ∆H
Concretely, endothermic reactions lower the temperature of their environment.
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!