Activation Energy and Catalysts

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  • 0:05 Activation Energy
  • 1:45 Activated Complex
  • 3:07 Catalyst
  • 4:40 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn how to define activation energy and how it relates to a reaction's energy. Learn what an activated complex is and where it fits into an activation energy diagram. Discover how a catalyst works to change the activation energy of a reaction and what this means to metabolism in the body.

Activation Energy

Have you ever played miniature golf? Nearly all courses have a hole with a hill in the middle of the fairway. You have to hit the ball hard enough to get it up and over the hill. The ball slows down as it climbs the hill, then picks up speed on the far side of it. If you don't hit the ball hard enough, it won't get over the hill.

The force you must hit the ball with is analogous to activation energy in chemistry. You know that for a reaction to occur, two molecules must collide with enough energy for bonds to break and new ones to form. Without enough energy, the reaction won't occur. Activation energy is the minimum amount of energy it takes to start a chemical reaction.

Look at the reaction of hydrogen and oxygen forming water: 2H2 + O2 --> 2H2O. This reaction is exothermic, meaning that it gives off energy when forming the product. Any reaction that ends at a lower energy state than it started is usually a reaction that will spontaneously happen. Our atmosphere is around 20% oxygen and 0.00005% hydrogen. Why don't they get together and form water all the time? It's because the reaction of oxygen and hydrogen to make water takes a huge amount of activation energy to get started. The hill it needs to climb to get to the activation state is huge.

Let's look at a graph of energy vs. the course of a reaction:

Activation energy is needed to climb the hill towards a chemical reaction.
Activation Energy Hill Graph

You can see this huge hill on the graph. The hill of energy that a reaction must climb is the activation energy hill.

Activated Complex

Now, activation energy will only get you so far. It gets you to the top of the hill. After that, there's a 50/50 chance of the reaction happening, either going over the hill and down the other side or not happening and falling back down the hill toward the starting point.

At that point, on the very top of the hill (just like the one in the graph above), the reactants are in a transition state. During this state, molecules are colliding with enough energy so that bonds are broken and new ones are formed. When they do this, they are making an activated complex. An activated complex is an unstable state that is between the reactants and the products in a chemical reaction.

During the transition state, molecules collide and break bonds.
Bonds Broken Transition State

The activated complex is very short-lived. The bonds are broken and remade and the reaction continues down either side of the hill. Not all collisions between molecules result in a product being made. It's a bit like love. You must not only meet the right person in your life, but the circumstances at the time of the meeting - current or recently failed relationships, age, and timing - all affect whether you fall in love.

In a chemical reaction, not only must the molecules collide with enough energy, but they must collide into the right other molecule at the right orientation with their electrons at the proper places in order for a reaction to occur.

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