Endothermic and Exothermic Reactions

What are endothermic and exothermic reactions?

All chemical reactions have something to do with energy. It just depends on whether energy is released or absorbed. If energy is released, it is exothermic. If energy is absorbed, it is endothermic.

For example, if you hold on to an ice pack, after a few minutes, the ice will start to melt. You can realize that this ice has absorbed energy (heat) and is an endothermic reaction.



In bombs and explosions, heat is released as things explode into pieces, making it an exothermic reaction.



In any kind of reaction, molecules stay together by chemical bonds, and we can add energy to break the bonds or release energy to make them join together.

Now, we can get a more detailed definition of endo and exothermic reactions.

If a reaction needs more energy to break bonds than it does to give off energy in order to form bonds, it is endothermic.

In contrary, if a reaction takes less energy to break bonds than it does to give off energy to from bonds, it is exothermic.

We can express this energy as enthalpy (H), which is the heat contained in the system.

Now, for us to tell whether a reaction is exothermic or endothermic, we can use energy diagrams.
The purpose of the energy diagram is to show potential energy of chemicals as they change from reactants to products vs. time.

Reactants start with a certain amount of energy, and as energy is added to start the reaction, the amount of energy at the end can be lesser or greater, depending on the type of reaction.



In the diagram above, the substrate is the reactant. We can call its starting point the energy of reactants. We can also call the end point of this graph the energy of products.

The energy of activated complex is the highest point of the entire graph, and this is usually located in the middle of the graph.

The activation energy is the amount of energy that must be added to start up the reaction. This can be calculated by Energy of Activated Complex - Energy of Reactants.

Finally, the change in energy between the substrate and the product is ΔH or change in enthalpy. This can be calculated by Energy of Products - Energy of Reactants.

Using the information, we can tell if a reaction is exothermic or endothermic. How?
Well, all we have to do is check if the change in enthalpy is positive or negative.

If it is negative, it means that the Energy of Products is lesser than the Energy of Reactants, or in other words, energy was released, so it is exothermic.



If it is positive, it means that the Energy of Products is greater than the Energy of Reactants, so energy was added or absorbed, and it is endothermic.



Now, after determining whether it was exothermic or endothermic, we can incorporate the amount of energy gained or lost into the chemical equation. If it is exothermic, you would add the energy onto the right hand side of the equation.

For example, 2 C₂H₆ + 7 O₂ --> 4 CO₂ + 6 H₂O + 800 kJ (this 800 is a made-up number, just to give an example. It can be any number, depending on the amount of energy.)

If you see the energy on the right hand side, it is exothermic. Contrarily, if the number in kJ is on the left hand side, it is endothermic.

Here is a video explaining energy diagrams in more detail:




Here is a worksheet for practice:
http://colgurchemistry.com/Chem12/pdfs/Microsoft%20Word%20-%20Ch%2012%20Worksheet%201-2_doc.pdf
http://gohs1.tvusd.k12.ca.us/TeacherWebs/science/JMacLean/IB-AP%20Chemistry/Chapter%2012/Potential%20Energy%20Diagram%20ws.pdf