3.2 Potential Energy Diagrams Revisited

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In our unit on Thermochemistry the terms endothermic andexothermic were discussed. These are terms you likely learned in Chemistry 20 but it is important to review them:

Endothermic reactions
require a net input of energy.

Exothermic reactions
release energy to the surroundings.

Potential Energy Diagrams

Potential energy diagrams were used to illustrate the input or release of energy:

	Endothermic Reactions - the reactants have less potential energy than do the products. Energy must be input in order to raise the particles up to the higher energy level.
	Exothermic Reactions the reactants have more potential energy than the products have. The extra energy is released to the surroundings.
We now need to modify these diagrams to better fit with our collision model, specifically with what we have learned about activation energy and the activated complex. All reactions, even exothermic reactions, require some initial addition of energy. This energy is required to reach the unstable, high energy state known as the activated complex. Our potential energy graphs become:

Values for potential energy are often included along the y-axis, allowing us to determine values for activation energy and the heat of the reaction.