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Rate of Reaction


A chemical reaction can be defined as the change in one of the substance to another by breaking and formation of chemical bonds. It results the formation of new substances which have different chemical and physical properties from starting substance. Here starting substance is called as reactant whereas the new substance formed is called as product. So overall a chemical reaction is conversion of reactant to product. A chemical reaction can be represented as the chemical equation in which reactants and products are written as their molecular formulae with reagent and reaction conditions. There are two aspects of a chemical reaction; thermodynamic and kinetic aspects. Thermodynamic concepts explain the heat change during the reaction. 

On the basis of this reactions are widely classified as exothermic and endothermic reactions. Exothermic reactions can be defined as the reaction which occurs with release of heat like combustion whereas endothermic reactions occurs with absorption of heat during reaction. Here we deal with change in enthalpy, entropy and free energy during the reaction. Gibb’s equation of free energy is used to explain the relation between three important parameters of thermodynamics. Kinetic concepts explain us the speed of a reaction with respect to time. Here we discuss about the time taken for the change of reactant to product molecules and also about path which reaction will follow to form desired compound. Both thermodynamic and kinetic concepts are related to each other. As the energy change during a chemical reaction is always related to speed of reaction.

Reaction Rate Definition

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The rate of reaction can be defined as the change in concentration of reactant with time. It is expressed as the concentration change per unit time.

We know that as reaction proceeds, reactant consumes to form product molecules. Therefore with time, the rate of reaction decreases. Rate of reaction can be expressed in terms of change in concentration with respect to time. With time, the concentration of reactant decreases whereas concentration of product increases. Overall the rate of reaction indicates the speed at which the reaction occurs. 

Reaction Rate Units

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Let’s take an example of chemical reaction;
x A + y B → p P + z Q

Here A and B are reactant molecules whereas P and Q are newly formed substances during the reaction. ‘x’,’y’,’p’ and ‘z’ indicates the coefficients with A,B, P and Q respectively.

The rate of reaction canbe expressed in terms of reactants as well as products as shown below.

r = - $\frac{1}{X} \frac{d[A]}{dt}$ = - $\frac{1}{Y} \frac{d[B]}{dt}$ = $\frac{1}{P} \frac{d[P]}{dt}$ = $\frac{1}{Z} \frac{d[Q]}{dt}$

This is called as rate equation that expressed the change in concentration with time. Here d[A] indicates small change in concentration with respect to time. Negative and positive sign indicate decrease and increase in concentrations. Hence the unit of rate of reaction can be written as mole / L / sec. Here we can take time in minute or hours also. 

Reaction Rate Constant

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The rate of reaction can be expressed in terms of rate law. Rate law is based on law of active mass. The law of active mass states that the rate of chemical reaction is directly proportonal to the active mass of reactants. Hence we can write rate law for a reaction with A and B reactants as given below.

R α [A] [B]
R = k [A] [B]

Reaction Rate Constant

Here ‘k’ is rate constant of reaction and a, b represents the order of reaction. The unit of reaction rate constant depends on the order of reaction. The unit of reactant rate constant with different order of reactions are listed below. 

Reaction order  Differential Rate Law  Units of Rate Constant 
 Zero  $\frac{-d[A]}{dt}$ = k  mole L-1 sec-1
 First  $\frac{-d[A]}{dt}$ = k[A]  sec-1
 Second  $\frac{-d[A]}{dt}$ = k[A]2  L mole-1 sec-1

Order of a Reaction

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The number of molecules of reactant and products take part in a chemical reactions is called as molecularity of reaction. We can determine the molecularity of reaction with the help of balanced chemical equation. The sum of number of reactants and products in the balanced chemical equation is equal to molecularity of reaction.

Order of reaction is little different from molecularity of reaction. It is a practically determined value which indicates the number of reactant molecules take part in slowest step of mechanism. The slowest step of reaction mechanism is called as rate determining step. The order of reaction determines with the help of rate determining step. It can be defined as the exponent to which the reactant’s concentrations term in the rate law is raised. 

Factors that Affect Rate of Reaction

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Physical state of reactant and product

The nature of reaction highly affects the rate of reaction. A reaction in gaseous state will move faster than a reaction in solid state. This is because of high kinetic energy of gaseous molecules compare to solid molecules. Since the rate of reaction depends on effective collisions between reactant molecules, hence more kinetic energy of molecules causes more effective collisions that increases the rate of reaction.

Concentration of reactant

The rate law of reaction states that the rate of reaction depends on the concentration of reactant molecules. In other words as the concentration of reactant molecules increases the rate of reaction also increases. Even collision theory of react Even collision theory of reaction supports this as the concentration of reactant molecules increases it increases the effective collisions between molecules that leads to formation of more products.

Effect of pressure

Pressure effects only gaseous reactions as with increases the concentration of reactants the pressure increases. Pressure and concentration of reactant molecules affect the rate of reaction with more pressure, the rate of reaction increases. 

Effect of Temperature

In general, temperature increases the collision of reactant molecules that enhance the rate of reaction. This is because of increase in the collisions of reactant molecules. For endothermic reaction, the rate of reaction increases with increase in the temperature whereas for exothermic reaction the rate of reaction decreases with increasing the temperature. Since reactant molecules require activation energy to form products therefore high temperature causes more successful collisions that converts reactant to product molecules.

Effect of presence of catalyst

Catalyst is the substance that increase the rate of reaction without participating in the reaction. In other words, it’s regenerate at the end of the reaction. They speed up the reaction by decreasing the activation energy of reaction through a parallel path of low activation energy. 

Effect of surface area

As the surface area of reactant molecules increases the rate of reaction increases for heterogeneous reactions.