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The collision theory was first proposed by Arrhenius and later developed independently by William Lewis and Max Trautz. They used the kinetic theory of gasses to work upon the collision theory of reaction rates. While the kinetic theory explains the behavior of gases by considering them to be a sea of constantly moving atoms and molecules, The collision theory of bimolecular reaction aims to calculate the rates of various types of bimolecular reactions.
This article will focus our study on the collision theory of bimolecular reaction and the various postulates of collision theory in detail.
As per the collision theory of bimolecular gaseous reactions, the reactant molecules are assumed to be hard spheres. The reaction is said to occur successfully only when these hard spheres collide. These spheres collide to produce enough energy to initiate a reaction. This energy is known as activation energy.
Why is collision important in for chemical reaction?
As discussed earlier, the collision is vital as it produces energy. Some of these collisions reach the minimum amount of energy called the threshold energy (ET), which helps break the pre-existing bonds of reactants to form new bonds. In such a case, the reaction is successful, and the new bonds form new substances called products.
In other collisions, the molecules collide but do not reach threshold energy. According to collision theory, all the collisions do not contribute to the formation of products. These collisions are called “normal collisions.”
Rate of the reaction is given by expression:
rate = Z.e-Ea/RT.
What are the main requirements for a reaction to be successful?
the collision theory puts forward two essential requirements for a successful chemical reaction, which are mentioned below:
The collision theory of bimolecular reactions says that a reaction occurs only when the reactant molecules collide.
The collisions that occur when the molecule is in proper orientation lead to the production of threshold energy (ET) and the formation of products. ET refers to the minimum energy required by the reactants to produce products.
Take an example,
A reaction between the molecules of ethene and hydrogen chloride
ethene is CH2 = CH2
hydrogen chloride is HCl, reactions takes place as shown below:
CH2 = CH2 + HCl → CH3CH2Cl
The above reaction can only happen when the hydrogen end of the HCl molecule approaches the C=C. This occurs because the double bond has a high concentration of negative charge around it, and if the partially negative Cl approaches C=C, it will experience repulsion. The reaction will not give the same effect if the two molecules approach each other in any other way. Thus, the collision theory says that how two asymmetrical species hit each other in any collision is fundamental.
The activation energy (Ea) is the threshold and average energy difference.
Ea = ET – average energy
[where Average energy is the amount of energy produced by normal molecules under STP (standard temperature and pressure)]
The molecules having activation energy are called activated molecules
The collision theory says that the reactions between the activated molecules are effective collisions or activated collisions. The reaction happens, and the products are fusion.
Let us consider the following general equation of normal collision
For the chemical change to occur, for the chemical change to occur A + A → A* + A*
Where,
A = normal molecule
A* = activated molecule
The general equation of effective collision will be
A* + A* = products
The effective collisions have a small fraction of total collisions. It is why all the reactions do not occur in a fraction of a second.
What is the rate of a bimolecular reaction?
Take, for example,
An elementary bimolecular reaction
A + B → products
Rate is given by
Rate = Z.e-EaRT
Factors affecting the rate of collision?
Collision frequency
It is defined as the number of collisions per second per unit volume of the reacting mixture in a defined system. The collision frequency is affected by the reactants’ temperature, density, and size. The increase in any or all three of these factors will increase collision frequency.
Concentration
The collision theory of bimolecular reaction states that two molecules must collide for a reaction to occur. For this, they need to come in contact. Therefore, when the reactants are more concentrated, they are more likely to come in contact and collide. Thus, increasing the rate of reaction.
Phase
The phase of the reactants also matters in determining the rate of reaction. For example, the particles can move around more freely than solids in a liquid or gaseous state. Therefore, according to collision theory, the reaction rate is higher in a liquid or gaseous state.
Cross-section
A collision cross-section is an area available for a collision of molecules. The more the area of cross-section, the more the chances of molecules colliding with each other and the more the reaction rate.
Temperature
Collision theory states that the reaction rate also depends on the conditions available for the reaction, like temperature. Higher temperature means the molecules have higher energy, leading to more collisions and faster achievement of activation energy.
Arrhenius created a mathematical relation between the rate constant and the temperature. This equation is called the Arrhenius equation.
K = A e-EaRT
- A: frequency factor.
- Ea: activation energy.
- R: gas constant
- T: absolute temperature
Presence of a catalyst
When there is a catalyst present, it supplements the collision process of the reactant molecules. Therefore, less energy will be required to make more collisions to attain threshold energy for the chemical change to occur.
Conclusion
Collision theory is an easy way to predict the rates of chemical reactions. It also puts forward several postulates and lays down the foundation of necessary conditions required for the collision theory to be valid for a particular reaction. But at the same time, it also has some drawbacks which are to be kept in mind during the application of this theory.
