Introduction
Chemical kinetics studies chemical reaction rates or how quickly a reaction happens. It has been noted that certain responses happen in a fraction of a second, while others take years to complete.
Rate laws and reaction orders describe elementary chemical reactions.
These reversible reactions could include the following:
A single molecule breaks down to create new species.
Collision and reorganisation of two molecules or atoms to create a new species.
When two molecules or atoms collide under the influence of a third body, new species are created.
Rate
The reaction sequence indicates the number of chemical species acting as reactants.
A molecule disintegrating in a single reaction is known as a unimolecular reaction. An order of two and so on is used for reactions requiring the collision of two molecules or atoms.
Viva Questions
1. Define the Rate of reaction.
Ans. A chemical reaction's rate is the rate of change in a reactant or product's concentration divided by that reactant or product's coefficient from the balancing equation.
2. List the factors that influence a reaction's rate.
Ans. The four key factors determining reaction rate are reactant concentration, the physical state of the reactants, surface area, temperature, and the presence of a catalyst.
3. What is the unit of rate?
Ans. Since the rate of a reaction is the change in concentration during a specific period, the unit of reaction rate is a molar/second.
4. Define rate law.
Ans. The expression known as a rate law is one in which the reaction rate is expressed as the molar concentration of the reactants, each term being raised to a power. This expression may or may not be the same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.
5. What is a first-order reaction?
Ans. A first-order reaction is a chemical reaction in which only one of the reactants experiences variations in concentration, which causes the reaction rate to vary.
6. What is rate expression?
Ans. Rate expressions define reactions regarding the change in reactant or product concentrations over time. Any one of the reactants or products in a reaction can be used to express the pace of the reaction.
7. What is the unit of first-order rate constant?
Ans. First order reaction rate constant is expressed in sec⁻¹.
8. What do you understand about the Law of mass action?
Ans. According to the law of mass action, a chemical reaction's pace is proportional to the sum of the reactant concentrations. As an illustration, consider using [X] and [P] to represent the concentrations of X and P, as well as the rate of product production and the rate of reactant change in the reaction.
9. What do you understand about the temperature coefficient of a reaction?
Ans. The factor by which the rate constant of a reaction increases when the temperature is raised by 10°C is known as the temperature coefficient of reaction.
10. Does the rate constant depend on the catalyst?
Ans. The rate constant is not dependent on the presence of a catalyst. Catalysts, however, can affect the total rate of a reaction.
11. Specific reaction rate of a first order reaction depends on?
Ans. The temperature of the reaction affects the specific rate constant of a first order reaction.
12. Give the unit of rate constant of a second order reaction.
Ans. Rate constant (k) of a second order reaction is expressed as L mol⁻¹ s.
13. What is the unit of rate of a zero order reaction?
Ans. The rate constant of a zero-order reaction is expressed as concentration/time, or M/s, where M denotes the molarity and s is one second. The rate constant is measured in units of k = mol L⁻¹ s⁻¹.
14. State any two significance of rate constant.
Ans. The two significance of rate constants are as follows:
The reaction occurs more quickly the higher the rate constant value.
For every 10 K rise in absolute temperature from 298 K, the rate of the chemical reaction doubles.
15. What impact does temperature have on the rate constant of a reaction?
Ans. Both the reaction's rate and its rate constant increase with temperature.
16. What is molecularity?
Ans. The number of interacting molecules that collide simultaneously in order to initiate a chemical reaction is referred to as the molecularity of a reaction. In other words, the number of reactant molecules involved determines the molecularity of an elementary reaction.
17. What is the order of the hydrolysis of ester?
Ans. The hydrolysis of ester in alkaline medium is second order reaction with molecularity of this hydrolysis is 2.
18. Can the rate of a reaction be fractional?
Ans. Both integral values and fractional values can be used to represent the order of a reaction.
19. What is the rate-determining step?
Ans. The rate-determining step is that which proceeds very slowly in a chemical reaction. The rate of a chemical reaction is determined by the step that moves slowly. Constructing reaction mechanisms makes it possible to identify the slowest stage of the chemical reaction. Numerous responses occur in fundamental phases rather than all occurring at once.
20. What impact does the catalyst have on the rate?
Ans. A suitable catalyst can be added to a reaction to accelerating its rate. A catalyst is a substance that speeds up a chemical reaction without being consumed (remains chemically unchanged at the end). It offers a different, lower-activation-energy reaction pathway.
21. What is the equivalent mass?
Ans. An element's equivalent weight is calculated by dividing its gram atomic weight by its valence (combining power).
22. What is a pseudo-order reaction?
Ans. When a second-order or bimolecular reaction is made to act like a first-order reaction, it is known as a pseudo-first-order reaction. When one of the reactants is present in extremely high concentrations or is kept at a constant concentration about the other substance, this reaction will occur.
23. Which order reaction is independent of concentration?
Ans. In contrast to other orders of reactions, a zero-order reaction's rate is unaffected by the concentration of the reactant (s).
24. Define a Complex Reaction.
Ans. When an overall reaction takes more than one step, it is considered complex.
25. On increasing the concentration of reactants, the reaction rate does not change. What can you say about the order of the reaction?
Ans. The reaction order is zero, and the concentration of the reactants does not influence the rate of reaction. The rate will not vary whether we add or subtract reactants from the mixture.
26. What is the half-life period?
Ans. The half-life of a reaction is the time required for a reactant to reach one-half of its initial concentration or pressure.
27. On which factor does the half-life of a first-order reaction depend?
Ans. The half-life for the first-order reaction is concentration-independent and constant over time.
28. What is the radioactive decay rate usually considered to be?
Ans. All the radioactive decays are generally considered to be of First order.
29. Why are reactions limited to a maximum molecularity of three?
Ans. There is minimal possibility that more than three molecules will collide at once. Therefore, the probability of molecularity having more than three molecules is relatively low.
30. The order and molecularity of ester is?
Ans. The acid hydrolysis of ester is a first-order, bimolecular, and pseudo-unimolecular reaction. When an ester (such as ethyl acetate) is mixed with water, it is converted into alcohol and acid according to the following equation: $CH_{3}COOC_{2}H_{5}+H_{2}O\rightarrow CH_{3}COOH+C_{2}H_{5}OH$ .
Summary
Chemical kinetics studies the process and rate of chemical reactions that occur under predetermined temperatures, pressure, concentration, etc.
The rate of a reaction is defined as the change in species concentration per unit of time in a chemical reaction.
The rate of a reaction often increases as temperature rises. But there are a very small number of exceptions. For various reactions, this rate increase's size varies. As a general rule, the reaction rate doubles for many reactions occurring close to room temperature when the temperature is raised by 10 ⁰C.
FAQs on Viva Questions on Chemical Kinetics With Answers
1. What are the most important topics in Chemical Kinetics for the CBSE Class 12 Board Exam 2025-26?
For the 2025-26 board exams, the most crucial topics from which questions are frequently asked are:
- Integrated Rate Equations: This includes derivations for zero and first-order reactions, along with numerical problems based on them.
- First-Order Reactions: Concepts like half-life (t₁/₂) and its proof are very important.
- Arrhenius Equation: Expect numerical problems to calculate activation energy (Ea) and the effect of temperature on the rate constant (k).
- Difference between Order and Molecularity: This is a classic 2-mark conceptual question.
2. How do you differentiate between the order and molecularity of a reaction?
The key differences are important for board exams:
- Basis: Order of reaction is an experimental value determined from the rate law. Molecularity is a theoretical value determined by the number of species colliding in an elementary reaction.
- Values: Order can be an integer, zero, or a fraction. Molecularity can only be a positive integer (usually 1, 2, or 3).
- Applicability: Order is applicable to both elementary and complex reactions. Molecularity is defined only for elementary reactions and has no meaning for complex reactions.
3. What types of numerical problems are commonly asked from Chemical Kinetics for 3 or 5 marks?
For higher marks, you should focus on numerical problems involving:
- Calculation of the half-life (t₁/₂) or rate constant (k) for a first-order reaction. A typical question might state that a reaction is 'x%' complete in 'y' minutes.
- Using the Arrhenius equation (log(k₂/k₁) = Ea/2.303R [1/T₁ - 1/T₂]) to find activation energy or a rate constant at a new temperature.
- Determining the order of a reaction and the rate constant from a given table of experimental data (initial concentrations and initial rates).
4. Derive the integrated rate equation for a first-order reaction, as this is an important question for the board exam.
For a first-order reaction R → P, the rate law is: Rate = -d[R]/dt = k[R]¹. Rearranging the equation gives: d[R]/[R] = -k dt. On integrating both sides, we get: ln[R] = -kt + I, where I is the constant of integration. At time t=0, concentration [R] = [R]₀. Therefore, I = ln[R]₀. Substituting this value back gives: ln[R] = -kt + ln[R]₀. This can be rearranged to give the final integrated rate equation: k = (2.303/t) log([R]₀/[R]). This derivation is essential.
5. Explain the role of a catalyst in a chemical reaction with reference to activation energy and collision theory.
A catalyst increases the rate of a reaction by providing an alternate reaction pathway that has a lower activation energy (Ea). According to collision theory, this lower energy barrier allows a greater number of reactant molecules to have enough energy to form products upon collision. This increases the frequency of effective collisions, thereby speeding up the reaction. A catalyst does not get consumed in the reaction and does not alter the overall enthalpy (ΔH) or equilibrium constant.
6. Can the order of a reaction be zero? What does it signify for the reaction rate?
Yes, the order of a reaction can be zero. A zero-order reaction signifies that the rate of the reaction is independent of the concentration of the reactant(s). The rate law is expressed simply as Rate = k. This implies that the reaction proceeds at a constant rate as long as some reactant is present. A common example asked in exams is the decomposition of gaseous ammonia on a hot platinum surface.
7. How can you graphically determine the activation energy (Ea) of a reaction?
The activation energy can be determined graphically using the Arrhenius equation, ln k = -Ea/RT + ln A. This equation is analogous to the equation of a straight line, y = mx + c. By plotting a graph of ln k (on the y-axis) versus 1/T (on the x-axis), you will obtain a straight line. The slope (m) of this line is equal to -Ea/R. Therefore, you can calculate the activation energy using the formula: Ea = - (slope) × R, where R is the universal gas constant (8.314 J K⁻¹ mol⁻¹).
8. Why is it incorrect to define the molecularity for a complex reaction?
It is incorrect because a complex reaction occurs through a sequence of multiple simple steps, called elementary reactions. Each elementary step has its own distinct molecularity. The overall rate of the complex reaction is governed by the slowest step in this sequence, known as the rate-determining step. Therefore, assigning a single molecularity value to the entire multi-step reaction is meaningless as it does not accurately represent the reaction mechanism.
