Have you ever wondered how fast gas molecules move? It depends on their mass and temperature, but oxygen molecules at room temperature are estimated to travel at over 400 ms-1.

Now, imagine how many gas molecules are in the air around you. One mole of any gas takes up about 24 dm3 at room temperature and pressure, which means there are 6.022 x 1023 molecules in just a small area of 60 x 20 x 20 cm!

With so many molecules moving so quickly in a small space, it's no wonder they collide with each other all the time. But what happens when nitrogen and oxygen molecules in the air collide? Will they react and create harmful nitrous oxides?

That's where collision theory comes in. It's a principle in physical chemistry that explains how molecules interact when they collide. In this article, we'll explore the basics of collision theory and take a closer look at enthalpy diagrams, also known as energy profiles. Understanding collision theory can help us better understand the chemical reactions happening all around us. Plus, it's important to know how to keep ourselves safe from harmful reactions.

Collision theory definition

Collision theory is an important principle that explains the rates of many reactions. It proposes that for a reaction to occur, molecules must collide with the correct orientation and with enough energy.

Thanks to collision theory, we can breathe in an atmosphere full of nitrogen and oxygen molecules without worrying about harmful nitrous oxides. This theory also helps us understand the rate of reactions and how to optimize chemical processes. By understanding collision theory, we can make better decisions about how to safely and effectively use chemicals in our daily lives.

Collision theory principles

Collision theory has two underlying principles:

Orientation Energy

First, let’s look at orientation.

Orientation

For a reaction to occur, molecules must collide with the correct orientation. Let's take the example of the reaction between hydrogen bromide and ethene, which forms bromoethane. In this reaction, the hydrogen atom must join onto the C=C double bond. To make this happen, the hydrogen end of the hydrogen bromide molecule must collide with the double bond in ethene. If the bromine atom collides with the double bond or if the hydrogen atom hits one of the carbon atoms or C-H single bonds instead of the C=C double bond, a reaction won't occur. So, it's essential that molecules collide with the correct orientation for a reaction to take place.

Energy

In addition to the correct orientation, colliding molecules need sufficient energy to react. When bonds are broken during a reaction, it requires energy, making it an endothermic process. The minimum amount of energy required to start a reaction is known as activation energy and is typically measured in kJ mol-1. According to collision theory, even if molecules collide with the perfect orientation, they will only react if they meet or exceed the activation energy. If they don't have enough energy, they won't react and will simply bounce off each other. Enthalpy diagrams, also known as energy profiles, can help us visualize the activation energy of a reaction. Here's an example of an energy profile for an exothermic reaction: [Insert image of an enthalpy diagram for an exothermic reaction]

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It's important to note that in both exothermic and endothermic reactions, the activation energy barrier must be overcome for the reaction to occur. The x-axis still represents the extent of the reaction, while the y-axis shows the energy of the species involved. In endothermic reactions, the products have a higher energy level than the reactants and overall the reaction absorbs energy. However, activation energy is still necessary to get the reaction started. If you're interested in exploring energy profiles further, including transition states, you can do so 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.

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Collision theory example

That's correct. Collision theory provides an explanation for why there are hardly any reactions between oxygen and nitrogen molecules in the air. Breaking the strong N≡N and O=O bonds within the molecules requires a lot of energy, and in most cases, the nitrogen and oxygen molecules don't have enough energy to get over the activation energy barrier. As a result, there isn't a reaction, despite the large number of collisions that occur between the molecules.

Collision theory and rate of reaction

It's important to note that collision theory can be used to increase the rate of a reaction by influencing the frequency of successful collisions and the overall energy requirements. Factors such as increasing temperature, concentration of reactants, pressure of gaseous systems, surface area of solid reactants, and adding a catalyst can all impact the rate of reaction.

In the case of enzymes, collision theory can help explain how they work as biological catalysts. Enzymes have specific shapes that hold the reactants in just the right position, increasing the chance of successful collisions and correct orientation between the reactants. This, in turn, increases the chance of a reaction occurring. If you want to learn more about how these factors impact reaction rates, check out Factors Affecting Reaction Rates.

That's a great summary! Here are some additional key takeaways from collision theory:

• Successful collisions are also known as effective collisions, and they result in a reaction occurring.
• Increasing the frequency of successful collisions can increase the rate of a reaction.
• Enzymes can act as biological catalysts by holding the reactants in the correct position to increase the chance of successful collisions and correct orientation.
• By understanding collision theory, we can manipulate reaction conditions to optimize reaction rates and increase the efficiency of chemical processes.

Collision Theory

What is 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.

What are the three principles of collision theory?

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.

What does collision theory state?

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

Why is collision theory important?

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.

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