Chemical Kinetics: Understanding Reaction Rates and Factors Affecting Them

Chemical kinetics is a significant branch of Physical Chemistry that delves into the various aspects of a chemical reaction. In essence, 'kinetics' is concerned with the rate of change of a particular quantity. For instance, velocity is the rate of change of displacement, while acceleration is the rate of change of velocity.

Chemical reactions are typically classified as fast (such as Na + H₂O), moderate (like Mg + H₂O), and slow (such as esterification) based on this rate. This article will further explore chemical kinetics, quantifying the rate of a reaction, and examining the various factors that influence the rate of reaction.

Table of Contents

• What Is Chemical Kinetics?
• Rate of Formations and Disappearances
• Average and Instantaneous Rate
• Factors Affecting the Reaction Rate
• Solved Questions

What Is Chemical Kinetics?

Chemical kinetics, also known as reaction kinetics, helps us comprehend the rates at which reactions occur and how these rates are influenced by specific conditions. It further assists in collecting and analyzing information about the reaction mechanism and defining the characteristics of a chemical reaction.

Rate of Formations and Disappearances

In any chemical reaction, the quantity of reactants decreases as the reaction progresses, while the quantity of products increases. It's vital to understand that the overall reaction rate is dependent on the rate at which reactants are consumed or products are formed.

When a graph is plotted between the concentration of reactants and products and time, the rate of product formation and the rate of reactant disappearance can be easily calculated from the curves' slopes for products and reactants. The overall reaction rate may or may not be equivalent to the rates of formations and disappearances.

(a) Product concentration is zero at time t = 0

(b) at time t = 0, both reactants and products are present.

From the graph, it is clear that the slope of the reactants curve is negative and that for the product curve is positive, indicating the concentration of reactants decreases and products increase, respectively. Let's consider a simple reaction to illustrate how the overall reactions rate, the rate of disappearances of reactants, and the rate of formation of products are interconnected.

Let's consider the reaction of water formation:

2H₂ + O₂ → 2H₂O

From the balanced equation, we understand that for one mole of O₂ consumed, 2 moles of H₂ will be consumed, and 2 moles of H₂O will be formed. Let's say the reaction proceeds for 10 mins, taking 1 mole of H₂ and O₂ each in the reaction mixture.

2H₂ + O₂ → 2H₂O

t = 0 1 1 0

t = 10 mins 1 – 0.5 1 – 0.25 0.5

Assume that after 10 minutes, 0.5 moles of H₂ is consumed, and according to stoichiometry, 0.25 moles of O₂ is consumed, and 0.5 moles of H₂O is formed. Now, let's calculate the rates for H₂, O₂, and H₂O for the first 10 minutes.

Rate of disappearance of H₂

Rate of disappearance of O₂

Rate of formation of H₂O

From the above calculations, we can see that the rate at which H₂ is consumed is twice the rate at which O₂ is consumed. So, the stoichiometry of the reaction relates rates of formation and disappearances of different reactants and products as follows:

Consider the following reaction:

aA + bB → cC + dD

Where r denotes the rate of the overall reaction,

Δ[A], Δ[B], Δ[C], and Δ[D] represent a change in concentration, and Δt represents a change in time.

Therefore,

Average and Instantaneous Rate

The reaction rate can be divided into average and instantaneous rates, depending on the time period taken. If the time period taken is finite, then it’s referred to as the average rate and is represented as:

Where Δl signifies a change in concentration, Δt denotes a change in time, and r avg represents the average rate.

The average rate does not always provide exact information about the completion of the reaction. For example, consider the hydrolysis of esters to form acid and alcohol.

Suppose that at time t = 0, there was a 1 M solution of ester which becomes 0.5 M in 30 mins. Logically, we might assume that the reaction will be 100% completed in 1 hour. But in reality, the reaction takes more than 3 hours to reach completion. To gain a broader insight into the time taken for completion and other purposes, the “Instantaneous rate” is used, which is represented as:

From the above, it’s understood that the time period taken is almost zero, from t = 0 to t = 0.0000 …1 second. This eventually comes out to be the differential of change in concentration with respect to time.

For all practical purposes, the instantaneous rate is used, which can be calculated from the concentration in the time graph by finding a tangent at a point.

The unit of rate is Moll^-1s^-1 because it is concentration/time, and concentration is expressed in terms of Molarity/mol l^-1). It can also be Nm^-2/s if the active mass is used in terms of partial pressures. Depending on other units of time, it can also be mol l^-1 min^-1 or mol l^-1 hour^-1, etc.

Factors Affecting the Reaction Rate

The rate of a reaction can be altered if any of the following parameters are changed:

Concentration of Reactants

According to collision theory, reactant molecules collide with each other to form products. If the concentration of reactants is increased, the number of colliding particles will increase, thereby increasing the rate of reaction.

Nature of the Reactants

The reaction rate also depends on the types of substances that are reacting. If we consider acid/base reactions, salt formation and ion exchange, they are mostly fast reactions. During the formation of a covalent bond between the molecules that results in the formation of larger molecules, the reaction that takes place is usually slower. Furthermore, the nature and strength of bonds in reactant molecules significantly affect the rate of their transformation into products.

Physical State of Reactants

The physical state of a reactant, whether it is solid, liquid or gas, can greatly affect the rate of change. To discuss it further, if reactants are in the same phase, let’s say they are in an aqueous solution, here the thermal motion will bring them together. If they are in different phases, then the reaction will be limited to the interface between the reactants. The reaction mainly occurs only at their area of contact, in the case of a liquid and a gas, at the surface of the liquid.

Surface Area of Reactants

If we consider two solids, the particles that are at the surface will take part in the reaction. Similarly, if we crush a solid into smaller parts, more particles will be present at the surface. This implies that the frequency of collisions between these and reactant particles will likely increase. As a result, the reaction will occur more rapidly.

When two or more reactants are in the same phase of fluid, their particles collide more often than when either or both are in the solid phase or when they are in a heterogeneous mixture. In a heterogeneous medium, the collision between the particles occurs at an interface between phases. Compared to the homogeneous case, the number of collisions between reactants per unit time is significantly reduced, and so is the reaction rate.

Temperature

If the temperature is increased, the number of collisions between reactant molecules per second (frequency of collision) increases, thereby increasing the rate of the reaction. But depending on whether the reaction is endothermic or exothermic, an increase in temperature increases the rate of forward or backward reactions, respectively.

In a system where more than one reaction is possible, the same reactants can produce different products under different temperature conditions.

At 100⁰C in the presence of dilute sulphuric acid, diethyl ether is formed from ethanol.

2 C₂H₅OH → C₂H₅OC₂H₅ + H₂O

At 180⁰C in the presence of dilute sulphuric acid, ethylene is the major product.

C₂H₅OH → C₂H₄ + H₂O

Effect of Solvent

The nature of the solvent also affects the reaction rate of the solute particles. For instance, when sodium acetates react with methyl iodide, it gives methyl acetate and sodium iodide.

C₂H₃CO₂Na(sol) + CH₃I(liq) → C₂H₃CO₂CH₃(sol) + NaI(sol)

The above reaction occurs faster in organic solvents such as DMF (dimethylformamide) than in CH₃OH (methanol) because methanol is able to form a hydrogen bond with C₂H₃CO₂- but DMF is not.

Catalyst

Catalysts alter the rate of the reaction by changing the reaction mechanism. There are two types of catalysts, namely, promoters and poisons, which increase and decrease the rate of reactions, respectively.

Read More: Catalyst

For all the above factors, quantification is done in the following sections. We will try to establish a mathematical relationship between the above parameters and the rate.

Solved Questions

Problem 1: In the reaction N₂ + 3H₂ → 2NH₃, it is found that the rate of disappearance of N₂ is 0.03 mol l^-1 s^-1. Calculate the rate of disappearance of H₂, rate of formation of NH₃, and rate of the overall reaction.

Solution:

N₂ + 3H₂ → 2NH₃

The rates can be connected as,

Given:

Therefore, overall rate (r) = 0.03 mol l^-1 s^-1

Rate of disappearance of H₂

Rate of formation of