Rate of Reaction - JEE Important Topic

The rate at which reactants change into products is known as the rate of reaction or reaction rate. It goes without saying that the rate at which chemical reactions take place varies greatly. While certain chemical reactions occur almost instantly, others typically take time to achieve their final equilibrium.

The rate of a reaction, according to the general definition, is the rate at which a reaction occurs. For instance, because it happens quickly, wood burning has a high reaction rate, whereas iron rusting has a low reaction rate because it happens gradually.

Rate of Reaction Formula

Consider a typical chemical process.

a A + b B → p P + q Q

Small letters (a, b, p, q) stand for stoichiometric coefficients while capital letters (A and B) denote reactants and (P and Q) denote products.

According to the IUPAC's Gold Book, the following is the definition of the rate of reaction (r) happening in a closed system under isochoric circumstances without the creation of reaction intermediates:

$r=-\frac{1}{a}\frac{d[A]}{dt}=-\frac{1}{b}\frac{d[B]}{dt}=\frac{1}{p}\frac{d[P]}{dt}=\frac{1}{q}\frac{d[Q]}{dt}$

In this case, the reactant concentration is decreased and is denoted by a negative sign. The units for the rate of reaction is typically measured in moles per litre per second.

Factors Affecting the Rate of Reaction

This part lists the different factors that can influence the speed of a chemical reaction.

• Nature of The Reaction

The kind and nature of the reaction have a significant impact on the rate of reaction. As was previously indicated, some reflexes are incredibly slow while some are inherently quicker than others. The amount of reactants, the complexity of the reaction, and other factors all have a significant impact on the reaction rate. In general, the rate of reaction is slower in liquids than in gases and slower in solids than in liquids. The size of the reactant is really important. The rate of the reaction increases with the size of the reactant.

• Effect of Concentration on Reaction Rate

The collision theory states that as the concentration of the reactants rises, so does the rate of the reaction. The pace of a chemical reaction is exactly proportional to the concentration of reactants, according to the law of mass action. This suggests that the rate of a chemical reaction increases with increasing reactant concentration and decreases with decreasing reactant concentration. Changes in the concentration of reactants and products are largely influenced by time. Time, thus, plays an important role in determining reaction rate.

• Pressure Factor

Gas concentration rises under pressure, which in turn accelerates the rate of reaction. The reaction rate rises when there are fewer gaseous molecules present and falls when the opposite is true. It follows that pressure and concentration are related and that both have an impact on the rate of reaction.

• Presence of Catalyst

A chemical that speeds up a reaction without actually taking part in the reaction is known as a catalyst. Its impact on chemical reactions is described in the term itself. Through the provision of a different pathway with a lower activation energy, a catalyst accelerates both forward and reverse reactions.

• Order

The rate of reaction is controlled by how the reactant pressure or concentration is used.

Rate Law

A chemical reaction's rate law is an equation that shows how the concentrations of the reactants involved in the reaction relate to each other. If a reaction is provided by

$aA+bB\to cC+dD$

(where a, b, c, and d stand for the stoichiometric coefficients of the reactants and products.)

The rate equation for the reaction is given by 

$Rate\propto {{[A]}^{x}}{{[B]}^{y}}$

$Rate=k{{[A]}^{x}}{{[B]}^{y}}$

where,

The reactant side A & B's concentrations are shown by ${[A]} and {[B]}$

The reaction's rate constant ‘k’ is the proportionality constant.

The partial reaction orders for reactant side A and B are indicated by the symbols x and y. (This may or may not be equal to their stoichiometric coefficients a & b.)

Rate Constant

Given that rate constant

$Rate=k{{[A]}^{x}}{{[B]}^{y}}$

So, 

$k=\frac{Rate}{[A]{}^{x}{{[B]}^{y}}}$

The following equation is used to calculate the units:

$k=(M\cdot {{s}^{-1}})\times ({{M}^{-n}})={{M}^{(1-n)}}\cdot {{s}^{-1}}$

The concentration is represented in $mol/L$ or M and time is represented in seconds.

Unit of Rate Constant for First Order Reaction

Let R represent the reaction rate.

First-order response,

$R = k{{[A]}^{1}}$

$K = R{{[A]}^{-1}}$

while K and [A] stand for the rate constant and the starting reactant concentration, respectively.

Therefore, unit of rate constant $={{(mol{{L}^{-1}})}^{(1-n)}}{{\sec }^{-1}}$

For first order reaction, n=1

Unit of rate constant $={{\sec }^{-1}}$

Therefore, sec^-1 is the unit of the first-order reaction's rate constant.

Conclusion

The pace at which a chemical reaction takes place, sometimes referred to as the reaction rate or rate of reaction, is proportional to the rise in product concentration per unit time and the fall in reactant concentration per unit time. The reaction might move at many different speeds. The chemicals that affect the rate of a reaction often come from one or more of the reactant's sides, although occasionally they can also be products. The rate of a reaction can also be influenced by catalysts, which do not appear in the balanced chemical equation.