This article is an attempt to introducing the basics of collision theory qualitatively. The theory and rates of reaction are related by the fundamental fact that all chemical reactions are a result of collisions between atoms, molecules, or ions. In the course of this discussion, we will also discuss the effect of concentration on reaction rate.
Collision theory is a quantitative theoretical construct for modeling the dynamics of a chemical reaction, based on principles of statistical mechanics and chemical energetics. The theory predicts the rate at which a chemical reaction may occur. That is, it can tell you how fast or how slow products may form in a chemical reaction, by knowing the initial conditions. It is one of the theoretical models of chemical kinetics which is the science that studies every aspect of dynamics of a chemical reaction.
Everything that happens in a chemical reaction, happens between atoms and molecules. Every material is made up of molecules and ultimately atoms. A chemical reaction involves collision between reactant atoms and molecules, resulting in breaking of some chemical bonds and creation of new ones, ultimately creating new products.
Who Developed Collision Theory?
This theory was developed during the course of World War I which was in itself a ‘Collision’ of imperialist nations vying for power. It was invented by two people independently on the opposite sides of the war, one a brilliant German chemist named Max Trautz in 1916 and the other a British professor of chemistry named William Lewis in 1918.
This theory models a chemical reaction as an inelastic and chaotic collision of millions of reactant molecules simultaneously. Some collisions just occur at the right energies and at right angles, to break certain old chemical bonds and make new ones. Breaking an old bond requires energy from a collision. Whereas, making a new bond, releases energy in the form of heat.
The exact energy required for a particular old bond to be broken and a new one to be formed, is called the ‘Activation Energy’. This is the threshold energy at or beyond which, if the collisions occur, new bonds form. However, the energies of molecules are not same and very few possess the energy to create a product.
Only the right activation energy is not sufficient for the process to occur. There are many more factors that contribute and determine the ultimate fate of a chemical reaction. A systematic study of all the multifarious factors that affect the rate and outcome of a chemical reaction, constitutes collision theory.
Explaining the Factors of Collision Theory
The Maxwell-Boltzmann distribution, named after two great physicists who discovered it, is a graph of the energy of reactant molecules against the number of molecules possessing that amount of energy. You cannot actually measure the energy and count every molecule in a solution. It is mathematically proven and observed fact that any random distribution over large numbers looks like a Gaussian distribution. So the graph of energy distribution of molecules is a Gaussian distribution. If you picture the graph, it is bell shaped, with the bell having a steep slope on the left which reaches a round peak and then a very slowly declining slope.
In this graph, if you mark the activation energy on the x-axis, you will observe that a very marginal slice of the area under the graph has energy, that is higher than the activation energy. It is only the last small triangular area at the high energy end of the graph. So majority of the reactant molecules do not have the required energy to make a reaction possible.
Collision theory has developed mathematical models that say that the rate of reaction is directly proportional to the temperature and concentration. Let us see how one can increase the probability of reaction of reactant molecules, which are below the threshold of activation energy. This can be done by studying every individual factor and its effect on the rate of reaction
Effect of Activation Energy
As already mentioned above, the activation energy is an energy hill that the reactants must scale in order to react and create reactants. The activation energy largely determines the rate of a reaction and the quantity of products created. This threshold energy is determined by the energy of the chemical bonds that need to be broken, in order to form new ones.
One way of increasing the fraction of molecules that undergo reaction, by crossing of the energy barrier is, heating and raising the temperature of the reaction solution. When the reactants are heated, they vibrate and collide more vigorously with other reactant molecules. This increases the rate of reaction and gives a higher yield invariably.
Effect of Concentration
Concentration is defined as the number of molecules of reactants per unit volume. The more the concentration of reactant molecules, more is the probability of collision due to their sheer number. Increasing the concentration of reactant molecules may or may not increase the rate of chemical reaction. It depends on the particular chemical reaction. These reactions happen in specific ratios of chemical reactants. Excess concentration may have no effect if one of the reactants is used up and the reaction goes into saturation.
There is one ingenious way of getting round the activation energy threshold or energy hill. Instead of going over the hill, one can tunnel through it. This is exactly what catalysts do. Catalysts are chemicals or substances that catalyze or promote a chemical reaction to occur and themselves remaining neutral in the end. They are like parts of an assembling mechanism that help making the final product but then detach themselves from it.
Effect of Pressure
If one is dealing with a chemical reaction involving gaseous reactants, an increase in pressure accelerates the rate of reaction. Compression of the gas due to increase in pressure, automatically increases the concentration of a gas. This causes the probability of gaseous reactant interaction to increase, which ultimately increases the rate of reaction.
Effects of Collision Angles
Not only the energy, but the angle of collision also matters. The reactant molecules or atoms must attack the right side of another molecule to break chemical bonds. The morphology and electron concentration centers of molecules matter. For a bond to break and new one to form, the appropriate parts of two molecules must bump into each other.
All these relationships of various factors to the rate of chemical reaction is formulated in terms of definite mathematical equations. By using the quantitative relations they represent, one can have a rough idea of the rate of reaction by knowing the reactant concentration, temperature, and other factors. This is an empirical theory. It cannot predict results accurately when the chemical reactions increase in complexity. However, that does not make it wrong, only inaccurate. Collision theory like many other physical theories is an approximation to the truth.