Chemical Kinetics

Rate of a Chemical Reaction

In a chemical reaction, a substance converts into another substance under certain given conditions in a given time. It is important to know the rate of a chemical reaction to completely understand the reaction. Which are the slowest and the fastest reactions in the world? The study of the rate of a reaction is helpful in our day to day life as well. Let’s learn how to calculate the rate of a chemical reaction.

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Inside a Chemical Reaction

It is very important to find out the rate of a chemical reaction and the factors controlling the rate of reaction. This information is useful in our daily life as well. For example, we need to the know the rate at which food cooks to know when we have to stop the cooking process and prevent burning of food. Doctors need to design a rapidly filling setting material for a dental filling. We need to know at what rate the fuel in a vehicle burns.

The branch of chemistry which deals with the study of reaction rates and their mechanisms is called Chemical Kinetics. Thermodynamics can only tell the feasibility of a chemical reaction whereas chemical kinetics tells about the rate of a chemical reaction. Chemical kinetics not only helps in finding the rate of a chemical reaction but also the factors influencing the rate such as concentration, temperature, pressure and a catalyst.

Rate of a Chemical Reaction

Some reactions happen very fast like the precipitation of silver chloride. It occurs immediately after mixing aqueous solutions of silver nitrate and sodium chloride. On the other hand, some reactions are very slow like the rusting of iron in the presence of air and moisture. Some chemical reactions are neither slow nor fast but take place at a moderate rate.

chemical reaction

The speed or rate of a chemical reaction is the change in concentration of a reactant or product per unit time. To be specific, it can be expressed in terms of:

  • the rate of decrease in concentration of any of the reactants
  • the rate of increase in the concentration of any of the products

Let’s understand the rate of a chemical reaction better. Consider a reaction in which the total volume of the system remains constant. Let R be the reactants and P be the products i.e. R → P

Thus, one mole of reactant R produces one mole of product P. Let the initial concentration of R be [R]1 and the concentration of P be [P]1 at time t1. Therefore at time t2, the concentrations of R and P are [R]2 and [P]2 respectively. Therefore,

Δt = t–  t1

Δ[R] = [R]– [R]1

Δ[P] =  [P]– [P]1

(The square brackets represent the molar concentrations)

Equation of Rate of a Chemical Reaction

Rate of disappearance of R = Decrease in concentration of R/time taken = -Δ[R]/Δt

and, rate of appearance of P = Increase in concentration of P/time taken = + Δ[P]/Δt

Δ[R] is a negative quantity because the concentration of the reactants is decreasing while Δ[P] is a positive quantity because the concentration of the products is increasing. While performing calculations on the rate of a chemical reaction, we multiply Δ[R] by -1 to make it a positive quantity. The above equations give the average rate of a chemical reaction which is, rav.

Therefore, the average rate of a reaction depends upon the change in concentrations of the reactants or products and the time taken for that change to occur.

Unit of Rate of a Chemical Reaction

It is clear from equations I and II that the unit of rate of a reaction is concentration time-1. But if the concentration is in mol L-1  and time is in second then the unit will be mol Ls. In case of gases, the rate of a chemical reaction will be atm s-1  when the concentration of gases is expressed in their partial pressures.

Solved Questions For You

Q1: Define average rate of a chemical reaction.

Solution: The average rate of a chemical reaction is the change in the concentration of the reactant or product divided by the time taken for that reaction to occur.

Q2: Derive formula of the rate of reaction for the reaction given below:

2Na + Cl→ 2NaCl

Solution: The rate of a reaction is the change in concentration of the reactant or product divided by the change in time.Therefore, the formula of rate of reaction for the above reaction would be:

Rate of reaction =  -(1/2)Δ[Na]/Δt = -Δ[Cl]/Δt = +(1/2)[NaCl]/Δt

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