Chemical Kinetics – One Shot Lecture | CHAMPIONS – JEE/NEET CRASH COURSE 2022

PHYSICS WALLAH – ENGLISH

Chapter: Chemical Kinetics

Welcome to the champion series! Today, we will be covering the chapter chemical kinetics. In 11th grade, you learned about the thermodynamics chapter which gave you an idea of whether a reaction is possible or not. But have you ever thought about how much time it takes for a reaction to get completed if it’s spontaneous? Chemical kinetics answers this question. We will study how at what rate the products are formed or the reactants are consumed in a spontaneous reaction. Before starting a reaction, it’s essential to know its rate, and that’s what we will discuss in this chapter.

WHAT WE WILL COVER

• Rates of chemical reactions
• Factors that change the rate of a reaction
• Applications of kinetics in a reaction mechanism

All the reactions that we will discuss in this chapter will be single-direction reactions, and we will not discuss the concept of equilibrium.

DEFINING THE RATE OF A CHEMICAL REACTION

The rate of a chemical reaction is the amount of chemical change occurring per unit time. We can measure the amount of chemical change in terms of the decrease in the concentration of reactant or the increase in the concentration of products. The rate of a reaction with respect to a reactant is always a negative quantity since the reactant is getting consumed, and its rate is calculated with a minus sign. The rate of a reaction with respect to a product is always a positive quantity since the product is getting formed, and its rate is calculated with a plus sign.

CALCULATING RATE

The rate of a reaction can be calculated as the concentration of reactant consumed or product formed divided by the total time taken.

• Rate with respect to reactant: -[concentration of reactant used / total time taken]
• Rate with respect to product: [concentration of product formed / total time taken]

DEFINING RATE

Rate can be defined as the rate of a reaction over a long duration of time or the rate at a particular instant in time. Instantaneous rate is the rate at a specific time while average rate is the concentration change over a long period of time.

TYPES OF RATES

Rate can be classified into two types: instantaneous rate and average rate. Instantaneous rate is the rate at a particular instant in time while average rate is the change in concentration over a long period of time.

CALCULATING AVERAGE RATE

To calculate the average rate, you need to find the change in concentration of the reactant over a time period and divide it by the total time taken:

Average rate = (a2 – a1) / (t2 – t1)

CALCULATING INSTANTANEOUS RATE

To calculate the instantaneous rate, you need to plot a tangent at a specific time and find the slope of the tangent:

Instantaneous rate = -d[A] / dt

UNITS OF RATE

The unit of rate is mole per liter per second (mol/L/s).

Factors Affecting the Rate of a Chemical Reaction

The rate of a chemical reaction is dependent on various factors, such as:

• Concentration of the reactants
• Temperature
• Nature of the reaction
• Presence of a catalyst

The concentration of the reactants and temperature have a significant impact on the rate of a chemical reaction. The rate initially starts off fast but gradually decreases as the reactant concentration decreases. Similarly, an increase in temperature causes an increase in the energy of the reactants, leading to a faster reaction.

Instantaneous Rate of a Chemical Reaction

The instantaneous rate of a chemical reaction can be calculated with respect to any reactant or product. The rate expression is:

Rate = change in concentration of reactant or product / time taken

The stoichiometric coefficients are also taken into consideration while calculating the instantaneous rate. The rate is divided by the stoichiometric coefficient of the reactant or product. For example, if the stoichiometric coefficient is 2, then the rate is divided by 2.

Rate Law and Order of Reaction

The rate law shows how the rate of a chemical reaction is dependent on the concentration of the reactants. The rate is directly proportional to the concentration of the reactant, raised to the power of the stoichiometric coefficient. The order of the reaction is represented by the exponent, which can be equal to 2 or not. The order of reaction can be determined experimentally.

UNDERSTANDING THE RATE LAW AND RATE CONSTANT

When writing the rate law, we express how the rate of a reaction depends on the concentration of its reactants. The expression includes the order of the reaction, which may or may not be equal to the stoichiometric coefficient. For a single-step reaction, the order is equal to 2, but for complex reactions with multiple mechanisms, the order may differ. The rate constant is a constant value that depends only on the nature and temperature of the reaction, but not on the concentration of reactants. The unit of the rate constant, which is mole^(1-n) L^(n-1) s^(-1), is used to determine the order of the reaction.

• The rate constant k depends on the nature of the reaction and the temperature.
• The rate law is experimentally verified.
• The order of the reaction is an experimental quantity that depends on the reaction mechanism.

DETERMINING THE ORDER OF THE REACTION

The order of the reaction is an experimental quantity that depends on the reaction mechanism. It may not correspond to the stoichiometric coefficient in all cases. For a zero-order reaction, the order is 0 and the units of k are mole L^(-1) s^(-1). For a first-order reaction, the order is 1 and the units of k are s^(-1). The order can be determined using the units of the rate constant.

Order of Reaction

The order of a reaction is an experimental quantity that depends on the mechanism of the reaction. It cannot be equal to the stoichiometric coefficient. It is the number of concentration terms involved in the rate equation.

• The order is not necessarily a whole number and can be a fraction or negative value.
• Calculating the order of a reaction involves using the rate law equation, which relates the rate to the concentration of reactants.
• In complex reactions, the order may not correspond to the stoichiometric coefficients of the reactants.

Calculating the Order of a Reaction

To calculate the order of a reaction, you are given different rates at different concentrations of reactants. You use the rate law equation to find the value of the order with respect to each reactant and then add them up to find the overall order.

Types of Reactions

A one-step reaction takes place in a single step, whereas a multistep reaction involves several intermediate steps before reaching the final product. Complex reactions have a more elaborate mechanism.

Single Step vs. Multistep Reactions

For complex reactions, the order can only be determined experimentally. For single step reactions, the order is equal to the stoichiometric coefficient of the reactant. A transition state is the state when bond fission and bond formation occur simultaneously. A single step reaction has only one transition state, whereas a multistep reaction has multiple transition states. Molecularity is the number of reactants colliding simultaneously in a one step chemical reaction. It can only be defined for elementary reactions and cannot be more than three due to the difficulty of molecules colliding directly with each other. The molecularity of a single step reaction is equal to the stoichiometric coefficients of the reactants.

DIFFERENTIATING SINGLE STEP AND MULTISTEP REACTIONS

Single step reactions have one transition state, whereas multistep reactions have multiple transition states. Multistep reactions are complex and can only be defined experimentally.

TRANSITION STATE

A transition state is the state when bond fission and bond formation occur simultaneously. It is highly unstable and occurs during the conversion of reactants to products.

MOLECULARITY

Molecularity is the number of molecules colliding simultaneously in a one step chemical reaction. It can only be defined for elementary reactions and cannot be more than three due to the difficulty of molecules colliding directly with each other. Molecularity is a theoretical concept and is always a positive quantity.

EXAMPLES

The molecularity of a single step reaction is equal to the stoichiometric coefficients of the reactants. For example, H2 + I2 → 2HI has a molecularity of 2. H2/2 + 1/2 I2 → HI has a molecularity of 1. 2H2 + 2 → 4HI has a molecularity of 4.

To determine the correct molecularity of a reaction, it should be changed to the most stable balanced reaction. The molecularity can only be defined in the simplest form of a balanced chemical reaction. The perfect molecularity is obtained from the smallest integer value of the stoichiometric coefficients of the individual reactants, and the sum of the stoichiometric coefficients.

For an elementary reaction, the order and molecularity are numerically equal. For a complex reaction, only the order can be defined, not the molecularity. The order of a one-step reaction can never be a fraction, as it is equal to the molecularity.

In a multistep reaction, the slowest step is the rate determining step, and the rate law is written according to this step. The intermediate formed and consumed in the reaction cannot be present in the rate law. To eliminate the intermediate, the equilibrium constant is used, and the rate law can be simplified to a new constant value.

For example, the reaction 2NO + Cl2 → 2NOCl has a mechanism with a rate determining step of NO + Cl2 → NOCl2, and the rate law would be rate = k’ [NO]^2 [Cl2]. The overall order of the reaction is 3.

CALCULATING ORDER OF REACTIONS

The order of a reaction can be calculated by finding the rate determining step, writing the rate law, replacing the intermediate with the equilibrium constant value, and simplifying the expression. The overall order may be different from the order given in the reaction equation.

RATE LAW FOR MULTISTEP REACTIONS

• The second step is an elementary process with equal order and molecularity.
• The intermediate cannot be present in the overall rate equation.
• The rate of the overall reaction is equal to the rate of the rate determining step, but the rate constant may be different.
• If the order corresponds to the stoichiometric coefficients, the reaction may or may not be a one-step reaction.

SAMPLE QUESTIONS

1. Determine the differential rate law equation for the given elementary reaction. (Answer: Option C)
2. Identify the reaction given the following expressions for the rate. (Answer: Option B)
3. If the initial rate of the reaction A + 2B → 6C + 2D is 2.6 × 10-2 M/s, what is the value of -dB/dt? (Answer: 5.2 × 10-2 M/s)
4. What is the unit of rate constant k? (Answer: mole1-n Ln-1/s)
5. Identify the incorrect statement. (Answer: Option A)

Understanding Zero Order Reaction and Integrated Rate Law

Let’s start with understanding the unit of the rate of disappearance. It is molar second inverse. So, which of the following statements is incorrect?

• The unit of rate constant, k, depends on the order of the reaction.
• Unit of k for first order would be molar second inverse.
• Unit of rate of a reaction is molar per second inverse.
• The unit of rate constant for a first order reaction would be molar second inverse.

The incorrect statement is option d.

Next, we have to determine the factors on which the rate of a reaction depends. The rate constant of a reaction depends on temperature, not on pressure, extent of the reaction, or initial concentration of the reactant.

Now, let’s move on to the different types of order of reaction, starting with zero order reaction. If a reaction is zero order, it means n would be equal to zero. So, the rate law would be given by rate = k[A]^0, which simplifies to rate = k.

Therefore, the rate of the reaction is constant and does not depend on the concentration of the reactant. The graph of rate vs concentration of reactant will be a straight line with a constant value of rate.

The integrated rate law of zero order can be derived by using the formula 𝑟=−d[A]/dt=k. Integrating this formula gives us [A] = -kt + [A]0, where [A]0 is the initial concentration of reactant.

Thus, for a zero order reaction, the concentration of reactant decreases linearly with time.

Free Limit Exceeded! 🚀 Please Upgrade to Pro to view the full length

How would you rate this summary?