Factors Affecting The Rate Of Reaction

"Chemical Kinetics" also known as "Reaction Kinetics" which helps us to understand the rate of chemical reactions (or reaction rate) and tells us how these rates influenced by certain conditions. 

For Examples: 

Some reactions are happened at a faster rate (like in few second) while others are happened at a very slow rate (like in many days). 

1. Rusting of iron - At a slow rate
2. Wood combustion - At a fast rate 
3. Nuclear explosion- At a fast rate

Hence, the rate of reaction (at which a reaction goes to completion) can change according to the condition through which the reaction occurs. In this article, we will learn about the factors which affects the rate of reaction.

Rate Law

Let us define a "Rate Law" which tell us about the rate of reaction and also tell us that these rates are affected by which parameters,

"A rate law or a rate equation is a relationship between the reaction rate and the concentration of its reactants".

Thus, a reaction rate is dependent on the concentration of the reactant and the reaction rate constant according to the rate law. But there are some other factors which affects the overall rate of reaction.

☛ Before studying the factors influencing the reaction rate in detail, we should be aware of some basis steps that occur before any reaction can proceed. And these are-

1. Reactant molecule must collide with each other (to break their bonds).
2. Reactant molecule must have enough energy so that they collide with each other and also overcome the energy barrier.
3. Reactant Reactive molecules must form new molecules of products.

Factors Affecting The Rate Of Reaction

Let's see the factors which affects the rate of reaction one by one;

1) Concentration of reactants

2) Temperature

3) Nature of reactant

4) Surface area

5) Presence of catalyst

6) Pressure

7) Electromagnetic radiation or 

     intensity of light

Let Us Now Look At All Of These Types In Detail

(1)Concentration Of Reactant

According to the rate law, the rate of reaction is directly proportional to the concentration of the reactant which means that if we increase the concentration of reactant, the rate of reaction also increases.

          On the other hand, concentration is generally defined as the number of moles of reactants per unit volume (n/V). So if we increase the concentration of the reactant then the number of the reactant molecules increase and the collision between the reactant molecules also increases, which increases the rate of reaction and reactions begin to occur rapidly.

Thus the rate of reaction increases if we increase the concentration of the reactant. Hence, 

[ Rate of reaction ∝ Concentration of reactant ]

(2)Temperature

If we increase the temperature at which the reaction is taking place, the rate of reaction will increase and the response will start happening fast. Typically when the temperature rises, the collision between the reactant molecules increases per second, which increases the rate of reaction.

      The detailed study of the temperature dependency on the reaction rate is done by "Collision Theory" which we will study further in detail. Hence, 

[ Rate of reaction ∝ Temperature ]

(3)Nature Of Reactant

Nature of the reactant molecules also affect the rate of reaction. Nature of reactant includes the physical state of the reaction and chemical nature of the reactant.

          For example, when all reactants are in the same phase (may be solid, liquid and gas phase), the collision between reactant molecules is more and when all reactants are in different phases then collisions between reactant molecules is less. Since the higher the collision between reactant molecules, the higher the rate of reaction. Therefore the rate of reaction is higher in homogeneous system than in heterogeneous system.

(The rate of reaction is slower in liquid than in gases. And the rate of reaction is higher in liquid than in solid.)

(4)Surface Area

The higher the surface area of reactant, the higher the rate of reaction. Hence, the rate of reaction is highly affected by the size of the reactant. 

     For example, if a solid reactant participates in a reaction then only the superficial particles of the solid participate in the reaction (or collide with another reactant particles) while the internal particles remain unaffected by these collision but if we takes these lumps break into small pieces. Then there is no change in the number of particles, but the number of surface particles of the reactant molecule increases, causing more collisions between these surface particles and other reactants and increasing the reaction rate. Hence, 

[ Rate of reaction ∝ Surface area ]

(5)Presence of Catalyst

We know that a catalyst is any substance that changes the rate of a reaction by changing its reaction path (or activation energy).

It enhances both the forward and backward reactions by providing an alternative pathway. Catalyst may increase or decrease the rate of chemical reaction. Hence, 

[ Rate of reaction ∝ Catalyst activity ]

(6)Pressure

We know that gases are compressible, while solids and liquids are not compressible. Therefore, pressure can only affect the rate of gaseous reaction, not the rate of solids and liquids.

         Usually if we increase the pressure of a reaction, then the volume of that reaction decreases and the concentration of the reactant present in the reaction increases. Since we know that the rate of reaction is directly proportional to the concentration. So if we increase pressure, the rate of reaction is also increases.

(Molecules of gases collide more often because they are highly compressible and if we increase the pressure, the collision frequency also increases.) Hence, 

[ Rate of reaction ∝ Pressure ]

(7)Electromagnetic Radiation Or Intensity Of Light 

Both are the forms of energy. As we know, reactant molecules must have enough energy to cross the transition state and convert into products. So in the presence of these forms of energy, reactant molecules gain enough energy and are quickly converted into product. Thus, the rate of reaction also increases. Hence, 

[ Rate of reaction ∝ Forms of energy supplies ]

For example, photosynthesis in plants occurs faster in the afternoon than in the morning or evening because there is more sunlight in the afternoon. 

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