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Collision Theory

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Chemistry

Have you ever wondered how fast gas molecules move? It depends on lots of factors, such as their mass and the temperature. But as an example, scientists estimate that oxygen molecules in the air travel at room temperature at over .

Now, think about how many molecules of gas there are in the air around you. At room temperature and pressure, one mole of any gas takes up roughly . That’s molecules in an area just 60 x 20 x 20 cm!

If we put the two together, we get a very large number of molecules moving extremely quickly in a relatively small area. From this we can predict that there are a lot of collisions between the molecules each second.

But wait, the air around us is full of nitrogen and oxygen molecules. They can react to form harmful nitrous oxides! What if the molecules collide? Will they react? If they do, why isn’t the air around us filled with damaging nitrous oxides?

Before we take that thought any further, we need to look at collision theory.

Collision theory is an explanation for the rates of many reactions. It proposes two key ideas: molecules must collide with the correct orientation, and sufficient energy, in order for a reaction to occur.

  • This article is about collision theory in physical chemistry.
  • We'll begin by exploring the basic principles of collision theory.
  • We'll then look at enthalpy diagrams, also known as energy profiles.

The principles of collision theory

As we mentioned above, collision theory is an explanation for the rate of reaction. It helps us predict whether two molecules will react or not.

Collision theory has two underlying components:

  • Orientation

  • Energy

First of all, let’s look at orientation.

Orientation

Molecules must firstly meet with just the right orientation in order for a collision to occur. Take the reaction between the gases hydrogen bromide and ethene, for example. This forms bromoethane. This reaction involves the hydrogen atom joining on to the C=C double bond. To do this, the hydrogen end of the hydrogen bromide molecule must approach and collide with the double bond in ethene. If the bromine collides with the double bond, or the hydrogen hits one of the carbon atoms instead of the double bond, nothing will happen - a reaction won’t occur.

Collision Theory orientation StudySmarterMolecules must collide with the correct orientation to react. Anna Brewer, StudySmarter Originals

Energy

However, the correct orientation isn't the end of the story. In order to react, colliding molecules also need enough energy. This is because reactions all firstly involve breaking bonds, which is an endothermic process - it requires energy. The amount of energy needed varies depending on the species involved and the reaction itself. This energy is known as the activation energy.

Activation energy is the minimum amount of energy needed to start a chemical reaction. It takes the symbol .

This means that even if molecules collide with the perfect orientation, they’ll only react if they meet or exceed the activation energy. If they don't , they'll simply bounce off each other.

We can see the activation energy of a reaction using enthalpy diagrams. These are also known as energy profiles. Here's an example of an energy profile for an exothermic reaction:

Collision theory exothermic reaction enthalpy diagram StudySmarter

An example of an energy profile for an exothermic reaction. Anna Brewer, StudySmarter Originals

The reactants start off with a medium amount of energy. In order to react, they need more energy, as shown by the peak in the graph - enough energy to break the bonds in the reactants and reach the transition state. This energy needed is the activation energy, and we often call the peak in the graph the activation energy barrier. The molecules then lose energy as they form new bonds.

Because this is an exothermic reaction, the products have a lower energy than the reactants. However, we still come across the energy barrier in endothermic reactions, as shown below:

Collision theory endothermic reaction enthalpy diagram StudySmarter

An example of an energy profile for an endothermic reaction. Anna Brewer, StudySmarter Originals

In this case, the only difference is that the products have a higher energy level than the reactants. We still need activation energy to get the reaction started.

You can explore energy profiles in more depth, including transition states, in Chemical Kinetics.

Will they react?

We can think of the whole collision and reaction process like one big flow chart. Take two molecules. Firstly, do they collide? Secondly, are they orientated correctly? Thirdly, do they have enough energy? If the answer is 'no' at any stage, a reaction won’t occur.

Collision Theory flow chart reaction StudySmarterA flow chart for collision theory. Anna Brewer, StudySmarter Originals

Let’s go back to the problem at the start of the article. Although there may be many collisions between oxygen and nitrogen molecules in the air each second, there are hardly any reactions between them. Collision theory gives us a reason why. In this case, almost none of the molecules have sufficient energy to react. A reaction between nitrogen and oxygen would firstly require breaking the strong and bonds within the molecules. This requires a lot of energy. In most cases, the nitrogen and oxygen molecules don't have enough energy to get over the activation energy barrier, so there is no reaction.

Collision theory and rate of reaction

We now know that in order to react, molecules must collide with the correct orientation and sufficient energy. We call any collisions that result in a reaction successful collisions. The more successful collisions we have per second, the faster the rate of reaction.

It is important to remember that only a small proportion of collisions result in a reaction. Most collisions are unsuccessful - they are either orientated incorrectly or don't have enough energy.

How can we use collision theory to increase the rate of a reaction? Well, we can't change the orientation of the molecules when they collide. However, we can influence how often they collide, and their overall energy requirements. We can do this in the following ways.

  • Increasing the temperature of a system increases the kinetic energy of all the molecules within it. The molecules move faster, resulting in more collisions, and on average they have higher energy. This means that the molecules have an increased chance of meeting the activation energy requirements when they collide.

  • Increasing the concentration of the reactants in a system, and increasing the pressure of a gaseous system, both increase the number of collisions per second.

  • Increasing the surface area of the solid reactants increases the number of exposed particles that are able to react with a surrounding liquid or gas. This also increases the number of collisions.

  • Adding a catalyst reduces the activation energy of the reaction. This means that an increased number of molecules meet or exceed the activation energy requirements when they collide.

If you want to find out more about how these factors change reaction rates, check out Factors Affecting Reaction Rates.

Collision theory can help explain how enzymes work. Enzymes are biological catalysts, meaning that they increase the rate of reaction. There are a few different hypotheses explaining how. One idea is that they could simply lower the reaction's activation energy.

Another idea looks at the shapes of enzymes. Enzymes have specific shapes. Scientists hypothesise that they hold some of the reactants in just the right position, so when two reactants collide, there is an increased chance of the pair being orientated correctly. This increases the chance of a reaction.

Collision Theory - Key takeaways

  • Collision theory is an explanation for the rates of many reactions.
  • Collision theory has two key ideas: molecules must collide with the correct orientation and sufficient energy in order for a reaction to occur.
  • The minimum amount of energy needed for a reaction to occur is known as the activation energy.
  • We can show activation energy using energy profiles.
  • Most collisions are unsuccessful because they have an incorrect orientation or lack enough energy to react.

Collision Theory

Collision theory is an explanation for the rates of many reactions. It proposes two key ideas: molecules must collide with the correct orientation and sufficient energy in order for a reaction to occur.


There are three important parts to collision theory. First, the reacting substances must collide. Secondly, they must collide with the correct orientation. Thirdly, they must collide with enough energy. If all of this occurs, then the molecules will react.

Collision theory states that molecules must collide with the correct orientation and sufficient energy in order for a reaction to occur. 

Collision theory is important because it helps us influence the rate of reaction. By changing how often molecules collide and their average energy, we can increase the rate of a reaction.

Final Collision Theory Quiz

Question

What is collision theory?

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Answer

For a reaction to occur, the reactant particles, atoms or molecules, must collide and interact with each other in some way: this is collision theory. Collision theory states that the rate of a chemical reaction is proportional to the number of collisions between reactant molecules. The more often reactant molecules collide, the more often they react with one another, and the faster the reaction is.


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Question

What is an effective collision?

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Answer

Only a small fraction of collisions result in a chemical reaction: these are called effective collisions. Inversely, ineffective collisions do not result in a chemical reaction. 


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Question

How does temperature affect a chemical reaction?

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Answer

At lower temperatures molecules have less kinetic energy. As the temperature of the molecules increases, so does their kinetic energy, meaning the molecules are moving faster. A slight increase in temperature causes a large increase in the number of molecules with kinetic energy greater than the activation energy, meaning that the rate of reaction increases. 


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Question

What is an exothermic reaction?

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Answer

When energy is transferred out to the surroundings, this is called an exothermic reaction, and the temperature of the surroundings increases. The energy level decreases in an exothermic reaction. This is because heat energy is given out to the surroundings. Because of this, a reaction is also said to be exothermic when the energy of the product(s) is less than that of the reactants.


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Question

What is an endothermic reaction?

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Answer

When energy is taken in from the surroundings, this is called an endothermic reaction and the temperature of the surroundings decreases. The energy level increases in an endothermic reaction. This is because heat energy is taken in from the surroundings. Because of this, a reaction is also said to be endothermic when the energy of the product(s) is greater than that of the reactants. 


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Question

What is enthalpy?  

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Answer

Enthalpy is a measure of the amount of heat absorbed or released during a chemical reaction. It is calculated by subtracting the energy of the reactants from the energy of the products. 


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Question

What is a catalyst ?

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Answer

Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst. Catalysts are not consumed in the reaction and remain unchanged after it.


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Question

How does a catalyst lower the activation energy of a reaction? 


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Answer

A catalyst can lower the activation energy for a reaction by:


  • Providing a surface for reactions to take place on. Reactant molecules are then oriented at a favourable angle for collisions to occur, increasing the likelihood of successful collisions, thus speeding up the reaction. 
  • Reacting with one or more reactants to form intermediates that require lower energy to form the final reaction product, in the process regenerating the catalyst, and increasing the rate of reaction. 


Show question

Question

Molecules always react when they collide. True or false? 

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Answer

False

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Question

What are the two main principles of collision theory?

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Answer

Molecules must collide with the correct orientation and enough energy for a reaction to occur.

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Question

Exothermic reactions have no activation energy. True or false?

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Answer

False

Show question

Question

What factors affect the rate of reaction?

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Answer

  • Pressure
  • Concentration
  • Temperature
  • Surface area
  • Catalysts

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