The rate of chemical reaction is related to the theory of chemical kinetics. In simple words, chemical kinetics refers to the study of the rates of chemical reactions occurring. The rate of reaction can be explained by two theories. One is chemical kinetics, and the other is collision theory. 

Theory of reaction rate

The primary condition of the collision theory is that the molecules of the reactant elements must collide with each other. This is the central theme of collision theory. In reality, very few collisions are effective. The collision theory states that the rate of chemical reaction is directly proportional to the number of collisions that occur between different reactant molecules. The greater the frequency with which molecules collide, the faster the rate of chemical reaction. As mentioned earlier, very few collisions are effective collisions, and only effective collisions result in chemical reactions. To create an effective collision, the reactant molecules must possess some kind of energy. This energy is called “activation energy.” The word “activation” indicates that this energy initiates the chemical reaction. For every reactant element, some of its molecules possess activation energy. The larger the sample size is, the number of effective collisions will also increase and the rate of reaction will also increase. The number of molecules possessing activation energy is dependent on the temperature. If the reactant molecules don’t possess the necessary activation energy, then an effective collision will not take place and molecules will bounce back. Some chemical reactions also require their reactant molecules to be in a particular orientation to make an effective collision. The reaction of ozone with nitrogen monoxide is one such example where the reactant molecules must maintain a particular orientation.

Factors influencing rate of reaction 

The rate of reaction can be influenced by various factors. For example, if the product of the reaction is gas, then the rate of reaction is directly proportional to the rate of production of the gas. Another such example can be given with respect to the coloured reactants. The rate of reaction is directly proportional to the rate of discolouration. The rate of reaction is also influenced by the factors mentioned below.

1) Nature of the reaction: The rate of reaction is highly influenced by the nature of the reaction. As mentioned earlier, if the product is gas, then the reaction is directly proportional to the rate of production. Also, some reactions are naturally fast-occurring reactions, and some are slow.

2) The state of chemical reactions : The rate of reaction is also influenced by the state of chemical reactions, i.e., their physical state, the complexity of the reaction and some other factors as well. 

3) Effect of concentration on reaction rate: As mentioned earlier in the collision theory, the rate of reaction is directly proportional to the concentration of the reacting elements. As concentration does differ with time, so does time play a vital role in affecting reaction rate.

4) Pressure Effect: Pressure is directly related to concentration, which means concentration increases with increasing pressure. So, pressure is directly related to the rate of reaction. 

5) Effect of temperature on reaction rate: As mentioned earlier, according to the collision theory, activation energy is generated at higher temperatures. So, temperature is also directly proportional to the rate of reaction. Though there are some reactions that are independent of temperature, which means reactions without activation barriers are independent of temperature. 

6) Effect of solvent on reaction rate: The rate of reaction is also highly influenced by the property of the solvent used. Mainly, the ionic strength of the solvent plays a vital role in determining the rate of reaction. 

7) The presence of a catalyst and its effect on the reaction: A catalyst is a substance that accelerates the rate of a reaction. Though there are some negative catalysts present, those are used to decrease the rate of reaction. Catalysts, in general, increase the rate of reaction by providing a lower activation energy alternative pathway.

8) The surface area of the reactants and their effect on the rate of reaction: If the size of a reactant element is small, the surface area is bigger and it leads to a faster chemical reaction. 

Rate of reaction formula

The conventional formula for the rate of reaction is

a A + b B – p P + q Q

The capital A and B denote the reactant elements and the P and Q denote the products. The small letters denote the stoichiometric coefficients. 

As per IUPAC’s gold book, the rate of reaction (denoted by r) is defined as,

r = – 1/a d[A]/dt = – 1/b d[B]/dt = 1/p d[P]/dt = 1/q d[Q]/ dt

In this equation, the negative sign signifies the decreasing rate of the reactants. 

Conclusion

Chemical kinetics, reaction rates, and so on are all important in today’s world. These concepts are very important for GATE aspirants. These concepts are pretty easy to grasp and also quite easy to mathematically apply. Aspirants can have a clear idea about which components affect chemical reactions and the nature of their impact.