Factors Affecting Rate of a Reaction: Concentration, Temperature

Factors Affecting Rate of Reaction: When we add vitamin C tablets to the hot and cold-water solution, we observe that the colour of the solution changes very fast in hot solution than in cold solution. What is the reason behind this? Also, if we add hot water to the yeast, the reaction rate is speedy; why? Let us learn the reasons for these reactions in this article.

The reaction rate is determined by the activation energy of the reaction, which is a thermodynamics parameter. The activation energy, in turn, is determined by a number of characteristics of the process. In a chemical reaction, the rate of reaction refers to how quickly the products are generated from the reactants. It provides some insight into how quickly a reply can be performed. The reaction rate of cellulose combustion in fire, for example, is extremely high, and the reaction is finished in less than a second. Continue reading this article, to know more about Factors affecting rate of reaction.

Factors Influencing Rate of Reaction

Several factors influence the reaction rate, and these factors depend on the nature of the type of the reaction; the covalent or ionic or catalytic reaction.

The reaction rate depends on the reaction’s characteristic thermodynamics parameter known as the reaction’s activation energy. The activation energy, in turn, depends on several factors which characterise the reaction. These factors which influence the rate of a reaction can be summarised as 

1. Nature of the reactants 
2. Effect of concentration of the reactants
3. Effect of temperature on the reaction 
4. Effect of a catalyst 
5. Effect of the surface area of reactants
6. Effect of radiation

Nature of the Reactants 

Generally, the reactions occurring between ionic reactants take place much faster than the reactions occurring between covalent substances. For example, the reaction between the covalent substances \({{\rm{H}}_{\rm{2}}}\) and \({{\rm{O}}_2}\) to form \({{\rm{H}}_{\rm{2}}}{\rm{O}}\) is much slower than the ionic reaction between \({{\rm{H}}^{\rm{ + }}}\) ions and \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) in aqueous solutions to give \({{\rm{H}}_{\rm{2}}}{\rm{O}}\). This is because the reactants in the former reaction are covalent substances, while the latter are ionic substances. In addition, this is because the reactions between covalent molecules involve breaking and making covalent bonds and hence are slower than the ionic reactions that do not involve making and breaking of bonds.

Further, in the reactions between the covalent molecules, the reactions involving a significant number of cleavages and the formation of bonds are generally slower than the reactions with a lesser number of bond cleavages and formations. For example, consider the following two reactions.

(i) \(2{\rm{NO}} + {{\rm{O}}_2} \to 2{\rm{N}}{{\rm{O}}_2}\)

(ii) \({\rm{C}}{{\rm{H}}_4} + 2{{\rm{O}}_2} \to {\rm{C}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}}\)

The reaction (i) involves breaking of one bond in \({\rm{NO}}\) and the formation of two new bonds in \({\rm{N}}{{\rm{O}}_2}\) while the reaction (ii) involves breaking of four bonds in methane and the formation of two bonds in \({\rm{C}}{{\rm{O}}_2}\) and four bonds in water. The reaction (i) is much faster than the (ii) at the same temperature. 

Reactions involving lesser bond rearrangements are rapid at room temperature than those with more significant bond rearrangements.

Effect of Concentration of Reactants

The rate of reaction (except zero-order reactions) depends on the concentration of the reactants. The rate of reaction is directly proportional to the \({{\rm{(concentration}}\,{\rm{of}}\,{\rm{the}}\,{\rm{reactants)}}^{\rm{n}}}\) or \({{\rm{C}}^{\rm{n}}}\) where \({\rm{‘n’ = }}\) order of the reaction. Similarly, in gaseous reactions, the rate is directly proportional to the \({{\rm{(pressure}}\,{\rm{of}}\,{\rm{the}}\,{\rm{reactants)}}^{\rm{n}}}\).

The chemical reactions occur due to the collisions between the reacting molecules. Hence, the greater the number of these molecules in unit volume, the greater the possibility of their collisions and, therefore, the higher the rate of reaction. The dependence of the rate of reaction on the concentration of reactants can be mathematically expressed as:

\({\rm{ – }}\frac{{{\rm{dC}}}}{{{\rm{dt}}}}{\rm{ = k}}{{\rm{C}}^{\rm{n}}}\)

Here \({\rm{‘n’ = }}\) order of a reaction. This dependence is generally referred to as the law of mass action.

Effect of Temperature on Reaction

The rate of reaction increases with an increase in the temperature. In most cases, a rise of \({\rm{1}}{{\rm{0}}^{\rm{o}}}{\rm{C}}\) in temperature generally doubles the rate of the reaction.

An increase in temperature increases the number of collisions between the molecules and increases the reaction rate. 

Arrhenius proposed a simple empirical equation that relates the specific rate \(\left( {\rm{K}} \right)\) of a reaction to the temperature \(\left( {\rm{T}} \right)\).

The equation is written as:

\({\rm{k = A}}{\rm{.}}{{\rm{e}}^{{\rm{ – Ea/RT}}}}\)

or ln \({\rm{k = }}\) ln \({\rm{A}}{\rm{.}}{{\rm{e}}^{{\rm{ – Ea/RT}}}}\)

or \({\rm{2}}.{\rm{303}}\,{\rm{log}}\,{\rm{k}}\,{\rm{ = }}\,{\rm{2}}.{\rm{303}}\,{\rm{log}}\,{\mkern 1mu} {\rm{A – Ea/RT}}\)

\(\therefore {\rm{log}}\,{\rm{k = log}}\,{\mkern 1mu} {\rm{A – }}\left( {\frac{{\rm{1}}}{{{\rm{2}}.{\rm{303}}}}\frac{{{{\rm{E}}_{\rm{a}}}}}{{\rm{R}}}} \right) \times \frac{{\rm{1}}}{{\rm{T}}}\)

Here \({\rm{K}}\) represents the specific rate (rate per unit concentrations). \({{{\rm{E}}_{\rm{a}}}}\) is called activation energy. \({\rm{A}}\) is called frequency factor. \({\rm{A}}\) and \({{{\rm{E}}_{\rm{a}}}}\) are the characteristics of the reaction. \({\rm{log}}\,{\rm{k}}{{\rm{V}}_{\rm{s}}}\frac{{\rm{1}}}{{\rm{T}}}\) gives a linear graph with \({\rm{ – }}\frac{{{{\rm{E}}_{\rm{a}}}}}{{{\rm{2}}{\rm{.303R}}}}.\)

Effect of Catalyst

A catalyst increases the rate of reaction. It takes part in the reaction mechanism but remains unconsumed without undergoing any chemical change. Catalysts are therefore needed to enhance the rate of slow chemical reactions.

In some reactions, the reaction rate is directly proportional to the concentration of the catalyst. In contrast, there may be no direct proportionality between the rate and the concentration of the catalyst in many reactions. Generally, in acid catalysed hydrolysis reactions of esters, the reaction rate is proportional to the concentration of the acid catalyst. This is because a catalyst increases the reaction rate by changing the path for higher activation energy to the one with lower activation energy. The catalysts functioning in the biochemical processes are called enzymes.

Effect of Surface Area of Reactants

The factor has significance only in the case of heterogeneous reactions. For example, the larger surface area of solid reactants and catalysts tend to increase the reaction rate. Now since the surface area for the same mass increases with the decrease in particle size, the smaller particles will react more rapidly than the larger particles. Thus, for example, small wood chips of a given weight burn more rapidly than a single piece of wood of the same weight. 

Effect of Radiation

The absorption of light increases the rate of some reactions. Such reactions are called photo catalysed, or photo accelerated reactions or photosensitised reactions. Let us take an example to explain the effect of radiation on the rate of reaction.

Photosynthesis: These are some reactions that take place in the presence of sunlight.

\(6{\rm{C}}{{\rm{O}}_2} + 6{{\rm{H}}_2}{\rm{O}} \to {{\rm{C}}_6}{{\rm{H}}_{12}}{{\rm{O}}_6} + 6{{\rm{O}}_2}\)

In photosynthesis, carbon dioxide reacts with water in the presence of sunlight to form glucose.

Formation of \({\rm{HCl}}\): Hydrogen combines with chlorine in the presence of \({\rm{UV}}\) light or diffused sunlight to produce \({\rm{HCl}}\). 

\({{\rm{H}}_{\rm{2}}}\left( {\rm{g}} \right){\rm{ + C}}{{\rm{l}}_{\rm{2}}}\left( {\rm{g}} \right) \to {\rm{2HCl}}\)

Summary

In this article, we learned how the reaction rate depends on the nature of the reactants, concentration, temperature, catalyst, surface area, and radiation. We also studied the factors affecting the rate of a reaction in labs. Also, know about the factors that affect the rate of reaction.

FAQs

Q.1. How does catalyst affect the rate of reaction?
Ans: A catalyst increases the rate of reaction. It takes part in the reaction mechanism but remains unconsumed without undergoing any chemical change. Catalysts are therefore needed to enhance the rate of slow chemical reactions.
In some reactions, the reaction rate is directly proportional to the concentration of the catalyst. In contrast, there may be no direct proportionality between the rate and the concentration of the catalyst in many reactions. Generally, in acid catalysed hydrolysis reactions of esters, the rate is proportional to the concentration of the acid catalyst. This is because a catalyst increases the reaction rate by changing the path for the reaction of higher activation energy to the one with the lower activation energy. The catalysts functioning in the biochemical processes are called enzymes.

Q.2. How does temperature affect the rate of reaction?
Ans: The reaction rate increases with an increase in the temperature. In most cases, a rise of \({\rm{1}}{{\rm{0}}^{\rm{o}}}{\rm{C}}\) in temperature generally doubles the specific rate of the reaction.
An increase in temperature increases the number of collisions between the molecules and increases the rate of reaction.
Arrhenius proposed a simple empirical equation that relates the specific rate \(\left( {\rm{K}} \right)\) of a reaction to the temperature \(\left( {\rm{T}} \right)\).
The equation is written as
\({\rm{k = A}}{\rm{.}}{{\rm{e}}^{{\rm{ – Ea/RT}}}}\)
or ln \({\rm{k = }}\) ln \({\rm{A}}{\rm{.}}{{\rm{e}}^{{\rm{ – Ea/RT}}}}\)
or \({\rm{2}}.{\rm{303}}\,{\rm{log}}\,{\rm{k = 2}}.{\rm{303}}\,{\rm{log}}{\mkern 1mu} {\rm{A – Ea/RT}}\)
\(\therefore {\rm{log}}\,{\rm{k = log}}{\mkern 1mu} {\rm{A – }}\left( {\frac{{\rm{1}}}{{{\rm{2}}.{\rm{303}}}}\frac{{{{\rm{E}}_{\rm{a}}}}}{{\rm{R}}}} \right) \times \frac{{\rm{1}}}{{\rm{T}}}\)
Here \({\rm{k}}\) represents the specific rate (rate per unit concentrations). \({{\rm{E}}_{\rm{a}}}\) is called activation energy. \({\rm{A}}\) is called frequency factor. \({\rm{A}}\) and \({{\rm{E}}_{\rm{a}}}\) are the characteristics of the reaction. \({\rm{log}}\,{\rm{k}}{{\rm{V}}_{\rm{s}}}\frac{{\rm{1}}}{{\rm{T}}}\) gives a linear graph with a negative slope.

Q.3. How does concentration affect the rate of reaction?
Ans: The reaction rate (except zero-order reactions) depends on the concentration of the reactants. The rate of reaction is directly proportional to the \({{\rm{(concentration of the reactants)}}^{\rm{n}}}\) or \({{\rm{C}}^{\rm{n}}}\) where \({\rm{‘n’ = }}\) order of a reaction. Similarly, in gaseous reactions, the rate is directly proportional to the \({{\rm{(pressure}}\,{\rm{of}}\,{\rm{the}}\,{\rm{reactants)}}^{\rm{n}}}\). The chemical reactions occur due to the collisions between the reacting molecules. Hence, the greater the number of these molecules in unit volume, the greater the possibility of their collisions and, therefore, the higher the rate of reaction. The dependence of the rate of reaction on the concentration of reactants can be mathematically expressed as
\({\rm{ – }}\frac{{{\rm{dC}}}}{{{\rm{dt}}}}{\rm{ = k}}{{\rm{C}}^{\rm{n}}}\)
Here \({\rm{‘n’ = }}\) order of the reaction. This dependence is generally referred to as the law of mass action.

Q.4. How does surface area affect the rate of reaction?
Ans: The factor has significance only in the case of heterogeneous reactions. The larger surface area of solid reactants and catalyst tend to increase the rate of reaction. Now since the surface area for the same mass increases with the decrease in particle size, the smaller particles will react more rapidly than the larger particles. Thus, for example, small wood chips of a given weight burn more rapidly than a single piece of wood of the same weight.

Q.5. What are the six factors that affect the rate of reaction?
Ans: These six factors which influence the rate of a reaction can be summarised as
1. Nature of the reactants
2. The concentration of the reactants
3. The temperature of the reaction
4. Presence of a catalyst in the reaction
5. Effect of the surface area of reactants
6. Effect of radiation

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