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Collision Theory: Definition & Significance

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  • 0:01 What Is Collision Theory?
  • 1:29 What Is a Chemical Reaction?
  • 2:34 What Affects Reaction Rates?
  • 4:08 Using Collision Theory
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

This lesson will define and explain collision theory and explore how it is related to chemical reactions and reaction rates. So if you've ever wondered how two particles combine to form a new product, read on!

What Is Collision Theory?

Collision theory. It sounds exciting, unless you're a dinosaur. I mean, you say the word 'collision', and they are likely to imagine a giant asteroid or comet hurtling towards the earth. The collision that changed everything! And I guess the dinosaur would be partially right; collision theory does have something to do with collisions, but it's not about asteroids. Instead, we're talking about the tiny particles, or atoms and molecules, that make up that asteroid, as well as everything around you.

All of these particles occasionally collide, or bump into each other, which leads us to collision theory. This theory helps scientists make predictions about how fast chemical reactions occur. In order to understand this definition, we need to understand chemical reactions and what affects the speed of a chemical reaction. So, like a t-rex chasing an herbivore, let's go!

Let's go back to our asteroid, our lake, and our air from a moment ago. In the solid asteroid, particles are closely packed together and vibrating. In the liquid lake, particles are moving a little more and are farther apart. Finally, in the gas, or air, particles are even farther apart and bouncing around. Let's focus on those gas particles. Even though they're far apart, they sometimes collide with one another. Just think of a room full of dinosaurs running around. On occasion, these dinosaurs will collide. Other times, they'll run right past each other.

What Is a Chemical Reaction?

When two particles collide, sometimes a chemical reaction can occur, which means the bonds between two or more particles are broken and reformed, creating one or more new substances. The particles are called reactants and the substances that are formed are called products.

Let's take a quick look at a chemical reaction before we go on. Over there, behind the lake is a giant fern. Inside that Cretaceous fern, photosynthesis is taking place, just like it is in today's ferns. The reactants are carbon dioxide, water, and light, and the products are sugar, water, and oxygen. But in order for a chemical reaction to occur, several things must happen:

  1. The particles must come into contact with one another, or collide.
  2. When they collide, the particles must be lined by correctly so the right parts of the atoms or molecules are exposed. If they aren't oriented correctly, no chemical reaction will take place.
  3. The particles must collide with enough energy to break their chemical bonds. This energy is often referred to as the activation energy.

What Affects Reaction Rates?

Okay, we know what is required for a chemical reaction to occur, but let's look at what impacts the reaction rates, or how quickly reactants are turned into products. Believe it or not, some reactions can take a really long time. Hey, maybe so long that the dinosaurs were around. Okay, maybe not, but some really can take thousands of years, whereas others can happen in seconds.

Here are the factors that can affect reaction rate:

  1. The number of particles - The more particles you have, the more likely they are to collide. For example, the more dinosaurs you have running around in a room, the more likely they are to collide with one another. But if you have a bunch of dinosaurs running around in a room, you have bigger problems than the collision theory, I guess.
  2. Temperature - When things get hotter, particles start bouncing around with greater energy. This increases the chance that they will collide with enough energy to initiate a chemical reaction.
  3. Pressure - When pressure is increased, particles are closer together and are more likely to collide. I know the dinosaurs aren't particles, but let's pretend. If you decrease the volume of the room they're in, they will all be closer together and more likely to collide.
  4. Catalyst - A catalyst is an enzyme that helps reactions occur faster. A catalyst helps bring reactants together.
  5. Size - The bigger the particles, the greater the chance they will collide with another particle. Just think about a room full of giant dinosaurs versus a room full of tiny dinosaurs. Which group is more likely to collide?

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