Endothermic and Exothermic Reactions

Last class we learnt about endothermic and exothermic reactions and how all chemistry reactions involve changes in energy. Here are some of the notes we took last class.

An exothermic reaction is when energy is released to its surroundings.
An endothermic reaction is when energy is absorbed.

Molecules are held together by chemistry bonds

• When energy is added, bonds are broken
• When energy is given off, bonds are created

- If it takes more energy to break a bond than it does to form one, then it's an endothermic reaction.
- If it takes less energy to break a bond than it does to form one, then it's an exothermic reaction.

Energy Diagrams
- We can chart potential energy of the chemicals as they change from reactants to products.
- Reactants start with a certain amount of energy, energy is added to start the reaction and then energy is released as the reaction proceeds.
- Relative amounts of energy determines if a reaction is endothermic or exothermic.

*Enthalpy, H, is the heat contained in the system.

Here are some vocabulary you should know before you can understand an energy diagram.
Energy of Reactants: Total potential energy of all reactants in the reaction
Energy of Products: Total potential energy of all products in the reaction
Activation Energy: The energy that must be added to get the reaction to progress
Delta H: The change in potential energy during the reaction. (Energy of products minus the energy of reactants)

*In this diagram substrate = energy of reactants

Energy in the Equation

- Energy absorption/release can be placed directly in the equation

- Exothermic reactions have the energy term on the right hand side of the equation and a negative delta H

- Endothermic reactions have the energy term on the left hand side of the equation and have a positive delta H

- Energy is represented as KJ in equations (kilojoules)

- Example: CH4 + 2O2 -> CO2 + 2H2O + 812KJ