Activation Energy

What you can expect to learn from this page:

Students should be able to identify the activation energy from a graph in both endothermic and exothermic situations.

Background Summary:

It is necessary to know that catalysts lower activation energy and to know what an energy graph looks like.

Lesson:

Once the molecule gets enough energy to get to the top of the hill, or the Activation Complex, the molecule will roll down the other side releasing energy and becoming a product. In exothermic situations, the system is releasing energy into its surroundings, so the energy level of the molecule will be less once it is a product than it was while it was a reactant. If it is an endothermic, the system is absorbing energy from the surroundings so the molecule will have more energy as a product than as a reactant.

Click here to view an energy diagram for an endothermic reaction.

Click here to view an energy diagram for an exothermic reaction.

 

Imagine a golf ball rolling toward a small hill. Without enough momentum, the ball will not make it over the hill to roll down to the other side. Now replace the golf ball with a molecule and the hill with the energy level.

What does the hill represent?

a) the products

b) the reactants

c) the energy level

d) the molecule

answer: c) the energy level. A) would be wrong because the products are not mentioned here. B) would be wrong because the reactants are also not mentioned D) would be wrong because the golf ball represents the molecule.

The "golf ball" starts out as a reactant that must obtain enough energy to get over the hill and to the lower side to become a product. The change in energy that is needed to get the ball over the hill is called the Activation Energy.

What is the change in energy that the molecule undergoes called?

a)

Homework (if any):

1. Once a reactant gets enough energy to get over the "hill", what does it become?

-a product. reactants are what need the energy to achieve a reaction and turn into a product.

2. In which situation, endothermic or exothermic, will the product have less energy than the reactant?

-exothermic because exothermic reactions release energy into the surroundings, leaving less in the system.