Collision Theory

Introduction

The rate of a chemical reaction means how fast a reaction is proceeding. It can also be defined as how fast a product is formed or how fast a reactant is consumed. The collision of atoms and molecules is necessary to bring about a chemical change. The reactant molecules and atoms must collide with each other to bring a chemical reaction. But all the collisions made by reactant species will result in product formation. Effective collision is necessary to make a reaction to proceed. And an effective collision means that the species that are colliding need some minimum internal energy that is equal to the activation energy of a particular chemical reaction.

What is Collision Theory?

It is the theory that depicts how reactions have different rates and how we can alter the rate of reaction. The collisions of reactant molecules or atoms result in the formation of a particular product in a chemical reaction. But all the collisions will not give good results. Only effective collisions result in the formation of products. And is due to the frequency of collision. As the frequency of collision of reactant molecules increase the rate of reaction will also increase. So according to this theory, the effective collision of reactant molecules results in the formation of products. For example,

$$\mathrm{A+B\to C}$$

The formation of product C by this chemical reaction depends on the collision between the reactant molecules. And the rate of collision is increased by the increased concentration of A and B. Rate of this reaction can be given as,

$$\mathrm{Rate=K[A][B]}$$

So the rate will increase with increasing concentration of A and B according to this theory.

Molecular Collisions

The effective collision of molecules that will result in the formation of products is molecular. When the reactant molecules are brought closer they will collide with each other and that will result in a chemical change and product formation. Only effective collision and proper orientation will result in the formation of products. It will also depend on some other factors such as temperature, pressure, volume, etc. The rate of collision will increase with the increasing concentration of molecules present. For example, the below figure shows the effect of concentration.

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And the below figure shows the proper orientation needed for product formation. That is the reaction of Carbon monoxide and Oxygen to form Carbon dioxide.

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Model: Collision Theory

From the collision theory, we know that collision is not only the necessary factor. Proper orientation and effective collisions are also necessary. The model of a collision taking place between the reactant molecule is shown below.

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Activation Energy

For a chemical reaction, it is the minimum amount of energy that is needed for the reactant molecules to react to form and form products. The general representation of activation energy is Ea and it's an energy term so the unit is joules per mole(J/mole). Arrhenius gave an equation for calculating the activation energy, and that equation is the Arrhenius equation,

$$\mathrm{K=Ae^{(-Ea/RT)}}$$

$$\mathrm{A=pre-exponential\: factor}$$

$$\mathrm{R=gas\:constant}$$

$$\mathrm{T=temperature}$$

$$\mathrm{Ea=activation\:energy}$$

$$\mathrm{K=rate\:constant}$$

The below figure shows the figure that represents the activation energy of reactants to form products.

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So if the activation energy of a given reaction is larger the rate of reaction is low. Only some molecules will participate in that reaction. And if the activation energy is low the reaction will proceed faster and most of the molecules will participate in the reaction. Several factors can reduce the activation energy barrier. They are the use of catalysts and applying temperature.

Explanation of collision Theory

Collision theory explains the rate of reactions. According to this theory, effective collision in a proper orientation is must-be there for a chemical reaction to proceed. So according to this theory,

• Reaction rate collisions/time.

• And also during the collision, the reactants that are going to form a chemical bond must be contacted during collisions to form that product.

• The collisions must contain reasonable energy, only then do the reactant molecules rearrange to form new molecules.

So collision theory is an important theory that can explain the various chemical reactions. It will also give an idea about the dependency of concentration and rate of reaction. The rate of reactions was found to increase with the increasing concentration of the reactant molecules and is because of the increased number of collisions.

Temperature dependence of Collision Theory

Collision theory has been easily explained by the increased rate of some chemical reactions by the application of heat. When the temperature is raised the reactant molecules that are going to participate in a reaction will collide faster due to the increased kinetic energy and thereby increasing the product formation. Faster collision means faster product formation.

The high-temperature effect can also be explained with the help of the Arrhenius equation as well, $\mathrm{K=Ae^{(-Ea/RT)}}$. From the equation the rise in temperature will increase the value of $\mathrm{e^{(-Ea/RT)}}$, thereby increasing the k value, rate constant.

Rate of Reaction according to Collision Theory

The rate of reaction and collision between reactant molecules are directly related terms. As the number of molecules collides increases, the rate of reaction will also increase. The faster the collisions, the faster will be the chemical reaction, and the higher the rate constant. But not all the collisions will lead to a product, only the one with proper orientation and enough internal energy will leads to the product. The increased concentration will increase the rate of collision and hence the rate of reaction. The equation for the rate of reaction can be given as,

$$\mathrm{Rate=K[A][B]}$$

Conclusion

The theory that explains the rate of reactions is collision theory. And is an important theory since it can give a basic idea about how chemical reactions are happening. According to this theory collision of reactant molecules are responsible for the formation of products. But there are two conditions necessary to obtain the product.

Proper orientation of reactant molecules and a minimum amount of energy is a must for forming products. It is also explained with the help of the Arrhenius equation. Since it deals with activation energy and rate of reaction. Temperature also has a dependence on collision theory, as the increased temperature can increase the collision and the increased kinetic energy.

FAQs

Q1. Who discovered collision theory?

Ans. Max Trautz and William Lewis independently discovered the collision theory.

Q2. How does pressure affect collision theory?

Ans. The rate of collision will increase with increasing pressure. Since the increase in pressure in a particular volume makes the molecules present in them collide faster. And thereby making the rate of reaction faster. So pressure and collision are directly related.

Q3. What is the effect of catalysts on collision theory?

Ans. Catalysts and collisions are not related. A catalyst cannot alter the rate of collision between molecules instead it affects activation energy. Also, does not affect the kinetic energy of molecules. But catalysts can increase the rate of reaction by giving an alternative path with lower activation energy.

Q4. What is the effect of volume on collision theory?

Ans. The rate of collision will decrease with an increase in volume. With the increase of volume, the distance between the reactant molecules is also increased, thereby making the rate of collision slower. So they are inversely related.

Q5. Does surface area Affect collision theory?

Ans. The increased surface area or size of reactant molecules will increase the rate of collision or frequency of collision. As smaller particles have more surface area than larger ones. The use of smaller particles can increase the rate of collision and hence the rate of reaction.