CBSE Class 12 Chemistry (2026–27)
Chapter 3: Chemical Kinetics
20 Important Questions and Answers
Chemical Kinetics in the current CBSE Class 12 syllabus covers rate of reaction, rate law, order and molecularity, integrated rate equations, half-life, pseudo-first-order reactions, and the Arrhenius equation.
1. What is the rate of a chemical reaction? Differentiate between average and instantaneous rate.
Answer:
The rate of a chemical reaction is defined as the change in concentration of reactants or products per unit time. It indicates how fast a reaction proceeds. Average rate is calculated over a specific time interval and is expressed as the change in concentration divided by the time taken. Instantaneous rate refers to the rate at a particular moment during the reaction and is determined from the slope of the concentration-time curve at that point. Average rate provides an overall measure of reaction speed, whereas instantaneous rate gives the exact speed at a specific instant. Both are generally expressed in mol L⁻¹ s⁻¹.
2. State the factors affecting the rate of a chemical reaction.
Answer:
Several factors influence the rate of a chemical reaction. Concentration of reactants affects the frequency of collisions; higher concentration generally increases the reaction rate. Temperature increases kinetic energy, leading to more effective collisions and faster reactions. Catalysts speed up reactions by providing an alternative pathway with lower activation energy. Surface area also affects reactions involving solids; finely divided solids react faster due to greater exposed area. The nature of reactants and reaction medium also influence reaction rates. Understanding these factors helps chemists control industrial processes and improve reaction efficiency.
3. What is a rate law? Explain with an example.
Answer:
A rate law is an experimentally determined equation that relates the rate of a reaction to the concentration of reactants. It is generally written as:
Rate = k[A]^m[B]^n
where k is the rate constant and m and n are reaction orders. For example, if the rate law is Rate = k[A]², doubling the concentration of A increases the rate four times. The rate law cannot be predicted from the balanced chemical equation and must be determined experimentally. It helps in understanding reaction mechanisms and predicting reaction behavior under different conditions.
4. Define order of a reaction. How is it determined?
Answer:
The order of a reaction is the sum of the powers of concentration terms appearing in the experimentally determined rate law. For a reaction with rate law Rate = k[A]²[B], the order is 2 + 1 = 3. Order can be zero, fractional, integral, or even negative. It provides information about the dependence of reaction rate on reactant concentrations. The order of a reaction cannot be obtained from the balanced chemical equation. It is determined experimentally using methods such as the initial rate method. Knowledge of reaction order helps in predicting rate changes when reactant concentrations are altered.
5. Distinguish between order and molecularity of a reaction.
Answer:
Order of a reaction is the sum of the exponents of concentration terms in the rate law, whereas molecularity is the number of reacting species participating in an elementary step. Order is determined experimentally and may be zero, fractional, or integral. Molecularity is always a positive whole number and is based on the reaction mechanism. Order can vary with reaction conditions, but molecularity remains fixed for a given elementary reaction. For complex reactions, molecularity cannot be assigned to the overall reaction, while order can always be determined experimentally through kinetic studies.
6. What is a zero-order reaction?
Answer:
A zero-order reaction is one whose rate is independent of the concentration of reactants. The rate law is expressed as:
Rate = k
where k is the rate constant. Such reactions generally occur when the catalyst surface becomes saturated with reactant molecules. The concentration decreases linearly with time. The integrated rate equation for a zero-order reaction is [A] = [A]₀ – kt. Examples include some photochemical reactions and catalytic reactions occurring on metal surfaces. The units of the rate constant for a zero-order reaction are mol L⁻¹ s⁻¹.
7. Write the characteristics of a first-order reaction.
Answer:
In a first-order reaction, the rate is directly proportional to the concentration of one reactant. The rate law is Rate = k[A]. The unit of the rate constant is s⁻¹. A characteristic feature is that the half-life period remains constant and does not depend on the initial concentration of reactant. The integrated rate equation allows determination of concentration at any time. Many decomposition and radioactive decay processes follow first-order kinetics. A plot of log[A] versus time gives a straight line with a negative slope. These properties make first-order reactions important in both chemistry and nuclear science.
8. What is the integrated rate equation for a zero-order reaction?
Answer:
The integrated rate equation for a zero-order reaction expresses the relationship between reactant concentration and time. It is given by:
[A]=[A]_0-kt
where [A]₀ is the initial concentration, [A] is the concentration after time t, and k is the rate constant. This equation shows that concentration decreases linearly with time. A graph of concentration versus time gives a straight line with slope equal to –k. The equation is useful for calculating the concentration of reactants at any stage of the reaction and predicting reaction completion time.
9. Write the integrated rate equation for a first-order reaction.
Answer:
For a first-order reaction, the integrated rate equation is:
k=\frac{2.303}{t}\log\frac{[A]_0}{[A]}
where [A]₀ is the initial concentration and [A] is the concentration after time t. This equation helps determine the rate constant and concentration at any time. The equation shows that the logarithm of concentration changes linearly with time. A plot of log[A] versus time gives a straight line. First-order kinetics is commonly observed in decomposition reactions and radioactive processes. The equation is widely used in solving numerical problems in board examinations.
10. Define half-life period. Why is it important?
Answer:
Half-life period is the time required for the concentration of a reactant to decrease to half of its initial value. It is represented by t½. Half-life is important because it provides a convenient measure of reaction speed. For first-order reactions, the half-life is independent of initial concentration, whereas for zero-order reactions it depends on concentration. The concept is extensively used in chemical kinetics, radioactive decay studies, medicine, and environmental science. Knowledge of half-life helps predict the duration of reactions and estimate the stability of substances over time.
11. Derive the half-life expression for a first-order reaction.
Answer:
For a first-order reaction, half-life is obtained by substituting [A] = [A]₀/2 into the integrated rate equation. The resulting expression is:
t_{1/2}=\frac{0.693}{k}
This equation shows that half-life depends only on the rate constant and is independent of the initial concentration. Therefore, the time required for successive halvings remains constant throughout the reaction. This property is unique to first-order reactions and is widely used in the study of radioactive decay and decomposition reactions. It simplifies calculations involving reaction progress and stability analysis.
12. What is a pseudo-first-order reaction? Give an example.
Answer:
A pseudo-first-order reaction is actually a higher-order reaction that behaves like a first-order reaction because one reactant is present in large excess. Since the concentration of the excess reactant remains nearly constant during the reaction, it is incorporated into the rate constant. An important example is the hydrolysis of ethyl acetate in the presence of excess water. Although the reaction is second order, it follows first-order kinetics with respect to ethyl acetate. Pseudo-first-order reactions simplify kinetic analysis and are commonly studied in laboratory experiments and industrial processes.
13. What is activation energy?
Answer:
Activation energy is the minimum amount of energy that reacting molecules must possess to undergo an effective collision and form products. It is represented by Ea. Molecules with energy lower than the activation energy cannot react successfully. Higher activation energy generally results in slower reactions. Catalysts increase reaction rates by providing an alternative pathway with lower activation energy. The concept explains why some reactions occur rapidly while others proceed slowly. Activation energy plays a vital role in determining temperature dependence of reaction rates and is a key concept in the Arrhenius theory.
14. State the Arrhenius equation and explain its terms.
Answer:
The Arrhenius equation relates the rate constant of a reaction to temperature. It is expressed as:
k=Ae^{-E_a/RT}
where k is the rate constant, A is the frequency factor, Ea is activation energy, R is the gas constant, and T is absolute temperature. The equation shows that the rate constant increases exponentially with temperature. Even a small rise in temperature can significantly increase the reaction rate. This equation is widely used to calculate activation energy and understand the effect of temperature on reaction kinetics.
15. How does temperature affect the rate of a reaction?
Answer:
Temperature has a significant effect on reaction rate. As temperature increases, the kinetic energy of molecules increases, leading to more frequent and energetic collisions. Consequently, a larger fraction of molecules acquires energy equal to or greater than the activation energy. According to the Arrhenius equation, the rate constant increases exponentially with temperature. For many reactions, the rate approximately doubles or triples with every 10°C rise in temperature. This effect is widely utilized in industrial chemical processes to increase productivity and reaction efficiency while maintaining control over reaction conditions.
16. Why does a catalyst increase the rate of a reaction?
Answer:
A catalyst increases the rate of a reaction by providing an alternative reaction pathway with lower activation energy. Because the energy barrier becomes smaller, a greater number of reactant molecules can participate in effective collisions. Catalysts do not change the equilibrium constant or the overall thermodynamics of a reaction; they only help the system reach equilibrium faster. Catalysts remain chemically unchanged after the reaction and can be reused. They are widely employed in industrial processes, automobile exhaust systems, and biological reactions involving enzymes to improve efficiency and reduce energy consumption.
17. What are the units of rate constant for different orders of reaction?
Answer:
The units of the rate constant depend on the order of the reaction. For a zero-order reaction, the unit is mol L⁻¹ s⁻¹. For a first-order reaction, it is s⁻¹. For a second-order reaction, the unit is L mol⁻¹ s⁻¹. In general, the units vary according to the overall order of the reaction so that the rate expression remains dimensionally correct. Determining the units of the rate constant is often useful in identifying the order of a reaction and solving kinetic problems accurately in examinations.
18. What is meant by effective collision?
Answer:
An effective collision is a collision between reacting molecules that results in product formation. Not all collisions lead to a reaction. For a collision to be effective, molecules must possess sufficient energy equal to or greater than the activation energy and must collide with proper orientation. The collision theory explains that the rate of a reaction depends on the number of effective collisions occurring per unit time. Increasing temperature increases the number of effective collisions because more molecules attain the required activation energy. Effective collisions are therefore responsible for the progress of chemical reactions.
19. Why is the rate law determined experimentally?
Answer:
The rate law is determined experimentally because the rate of a reaction depends on the actual reaction mechanism rather than the overall balanced chemical equation. The stoichiometric coefficients in the balanced equation do not necessarily represent the powers of concentration terms in the rate law. Experimental measurements reveal how changes in concentration affect reaction rate and help determine the reaction order. Methods such as the initial rate method are commonly used for this purpose. Determining the rate law experimentally provides valuable information about reaction pathways and the mechanism involved.
20. Explain the significance of Chemical Kinetics.
Answer:
Chemical Kinetics is important because it helps in understanding the speed and mechanism of chemical reactions. It enables scientists to determine factors affecting reaction rates and optimize industrial processes for maximum efficiency. Kinetic studies are useful in designing catalysts, manufacturing chemicals, preserving food, developing medicines, and understanding environmental processes. Knowledge of reaction rates also helps predict the stability and shelf life of substances. Unlike thermodynamics, which indicates whether a reaction is possible, chemical kinetics explains how fast it will occur. Therefore, it has immense practical and industrial significance.