Rate Of Reaction

When we talk about chemical reactions, it is a given fact that rate at which they occur varies greatly especially in terms of speed. Some are nearly instantaneous, while others usually take some time to reach the final equilibrium. So today we will learn about and understand what exactly is rate of reaction for a given chemical compound.

As per the general definition, the speed with which a reaction takes place is referred to as the rate of a reaction. For example, wood combustion has a high reaction rate since the process is fast and rusting of iron has a low reaction rate as the process is slow.

Rate Of Reaction

Before going deep into the subject, let’s refresh a few of the basics. Any chemical reaction contains two things;

  1. Reactants

  2. Products

Reactants:

Substances which undergo chemical reactions are called reactants. In a chemical reaction, these reactants are converted to form a new substances.

Products:

The substances which are the end products of a chemical reaction are called products. In other words, new substances that are formed due to the chemical reactions are all called products.

Activation Energy:

Activation energy can be defined as the minimum amount of energy that is required to activate molecules or atoms so that they can undergo chemical transformation. This minimum energy is to overcome the energy barrier is called activation energy.

Similarly, chemical kinetics is a part of physical chemistry that is related to the study of reaction rates. It has many applications that include enzymology, chemical engineering, and environmental engineering.

In a chemical reaction, products are formed due to the collision between the reactant molecules.

Conditions for collisions to form products:

  1. Collisions should be effective.
  2. The right orientation of reactant molecules towards each other.
  3. All molecules should possess a minimum amount of energy to form product molecules.

As the chemical reaction advances, the concentration of reactants will decrease and the concentration of products will increase.

Rate Of Reaction

Factors influencing the rate of reaction

Nature of the reaction:

  • The rate of reaction highly depends on the type and nature of the reaction. As mentioned earlier, few reactions are naturally faster than others while some reactions are very slow. The physical state of reactants, number of reactants, complexity of reaction and other factors highly influence the reaction rate as well. The rate of reaction is generally slower in liquids when compared to gases and slower in solids when compared to liquids. Size of the reactant also matters a lot. The smaller the size of reactant, faster the reaction.

Change in Concentration:

  • According to collision theory, the rate of reaction increases with the concentration of the reactants. As per the law of mass action, the chemical reaction rate is directly proportional to the concentration of reactants. This implies that the chemical reaction rate increases with the increase in concentration and decreases with the decrease in the concentration of reactants. As discussed earlier, time plays a major role in changing the concentration of reactants and products. So, even the time factor has to be considered.

Pressure factor:

  • Pressure increases the concentration of gases which in turn results in the increase of the rate of reaction. The rate of reaction increases in the direction of less gaseous molecules and decreases in the reverse direction. By this we can say, pressure and concentration are interlinked and affects the rate of reaction.

Temperature:

  • According to collision theory, a chemical reaction that takes place at a higher temperature generates more energy than a reaction at a lower temperature. This is because colliding particles will have the required activation energy at high temperature and more successful collisions will take place. There are some reactions that are independent of temperature. Reactions without an activation barrier are independent of temperature.

Solvent:

  • The rate of reaction also depends on the type of solvent. Properties of solvent and ionic strength highly affect the reaction rate.

Order:

  • The order of reaction manages how the reactant pressure or concentration affects the rate of reaction.

Electromagnetic Radiation:

  • Electromagnetic radiation is a form of energy and its presence at the chemical reaction may increase the rate of reaction as it gives the particles of reactants more energy.

The intensity of light:

  • Even the intensity of light affects the rate of reaction. Particles absorb more energy with the increase in the intensity of light thereby increasing the rate of reaction.

Presence of Catalyst:

  • A catalyst can be defined as a substance that increases the rate of the reaction without actually participating in the reaction. The definition itself says its effect on chemical reactions. The presence of a catalyst increases the speed of reaction in both forward and reverse reaction by giving another pathway with lower activation energy.

The surface area of reactants:

  • The surface area of reactants affects the rate of reaction. If the size of a particle is small, the surface area will be more and this increases the speed of heterogeneous chemical reactions.

Rate of Reaction formula or equation

Let’s take a traditional chemical reaction.

a A + b B → p P + q Q

Capital letters (A&B) denote reactants and the (P&Q) denote products, while small letters (a,b,p,q) denote Stoichiometric coefficients.

As per IUPAC’s Gold Book, the rate of reaction r occurring in a closed system without the formation of reaction intermediates under isochoric conditions is defined as:

Rate Of Reaction

Equation for Rate Of Reaction, r

Average rate of reaction

Now let us consider the following reaction to understand even more clearly.

A→ B

In this reaction a reactant A undergoes a chemical reaction to give a product B. It is a general convention to represent the concentration of any reactant/product as [reactant/product]. So the concentration of can be represented as [A] and that of B as [B] Let the time at which the reaction begins be the start time, that is t=0.

Let’s consider the following situation:

At t=t1,

Concentration of A=[A]1

Concentration of B=[B]1

At t=t2,

Concentration of A=[A]2

Concentration of B=[B]2

Now we want to know the rate at which A (reactant) is disappearing and rate at which the product B is appearing in the time interval t1 and t2. So,

Rate of disappearance of A =\( \frac{[A]_2 – [A]_1}{t_2 – t_1} = – \frac {\triangle [A]}{\triangle t} \)

The negative sign shows that the concentration of A is decreasing.

Similarly

Rate of disappearance of B = \( \frac {[B]_2 – [B]_1}{t_2 – t_1} = – \frac {\triangle [B]}{\triangle t} \)

Since A is the only reactant involved in the reaction and B is the only product that is formed and as mass is conserved, the amount of A disappeared in the time interval Δt will be same as the amount of B formed during the same time interval. So we can say that

Rate of reaction = – Rate of disappearance of A = Rate of appearance of B

\( \Rightarrow Rate\;of\; reaction = –  \frac{\triangle [A]}{\triangle t} = \frac {\triangle [B]}{\triangle t} \)

The above terms for the rate of disappearance of A and rate of appearance of B are average rates of reaction. These rates give the rate of reaction for the entire time interval Δt and hence are called average rates of reaction.

Instantaneous rate of reaction

What if we want to know the rate at which the reaction discussed above is proceeding at any instant of time and not for a given period of time? The average reaction rate remains constant for a given time period so it can certainly not give any idea about the rate of reaction at a particular instant.

Here comes instantaneous rate of reaction into the picture. Instantaneous rate of reaction is the rate at which the reaction is proceeding at any given time. Suppose the term Δt becomes infinitesimally small, tending to zero. So now we have an infinitesimally small Δt which is very small period and can be considered a particular instant of time and the average reaction rate will be the instantaneous rate of reaction.

Mathematically,

\( \Rightarrow average\; rate \;of\; reaction = – \frac {\triangle [A]}{\triangle t} = \frac {\triangle [B]}{\triangle t} \)

When Δt →0

\( \Rightarrow instantaneous\; rate \;of \;reaction = – \frac {\triangle [A]}{dt} = \frac {\triangle [B]}{dt} \)

\( \Rightarrow instantaneous \;rate \;of \;reaction = – \frac {d [A]}{dt} = \frac {d [B]}{dt} \)

The unit of rate of reaction is given by concentration/time that is (mol/L)/sec.

Meanwhile, in today’s world chemical kinetics has gained a critically significant role. The reaction rate (both average and instantaneous) is enabling engineers and scientists around the globe to optimize the process parameters in order to get the most desired results from a chemical reaction in the most economical and safe way. Chemical kinetics along with its critical role in the manufacturing industry has also served as a base for further advances in the fields of reaction engineering and biochemical engineering.

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Practise This Question

A balanced chemical equation obeys the law of conservation of mass.
State true or false.