10th Class Chemistry Notes 2026 — Chapter 5
Class 10 Chemistry — Chapter 05: Reaction Kinetics
Federal Board
1. Multiple Choice Questions (MCQs)
I. The rate of a reaction ______ as the reaction proceeds.
(a) Increases
(b) Decreases
(c) Remains the same
(d) May increase or decrease
Answer: (b) Decreases.
Concept: As a reaction proceeds, reactants are consumed, so their concentration decreases. This leads to fewer effective collisions per unit time and, consequently, a decreasing reaction rate.
II. The activation energy for a reaction can be:
(a) Increased by increasing temperature
(b) Increased by decreasing temperature
(c) Decreased by increasing concentration of reactants
(d) None of these
Answer: (d) None of these.
Concept: Activation energy (\(E_a\)) is an inherent energy barrier characteristic of a specific chemical reaction pathway. It is independent of temperature and concentration. It can be lowered by adding a suitable catalyst or changed by using a different reaction mechanism.
III. Reactions with high activation energy are usually:
(a) Fast
(b) Slow
(c) Exothermic
(d) Reversible
Answer: (b) Slow.
Concept: A high activation energy means that a larger energy barrier must be overcome for reactants to form products. Fewer molecules possess sufficient energy at a given temperature, resulting in a slow reaction rate.
IV. In a reversible reaction, a catalyst lowers the activation energy of the:
(a) Forward reaction
(b) Reverse reaction
(c) Forward as well as reverse reaction
(d) Forward reaction but increases it for the reverse reaction
Answer: (c) Forward as well as reverse reaction.
Concept: A catalyst lowers the activation energy of both forward and reverse reactions by providing an alternative reaction pathway. This accelerates both directions and helps equilibrium to be reached faster without changing the equilibrium position.
V. Which of the following is NOT a factor affecting the rate of reaction according to collision theory?
(a) Number of particles per unit volume
(b) Activation energy
(c) Presence of a catalyst
(d) Molar mass of reactants
Answer: (d) Molar mass of reactants.
Concept: Collision theory focuses on collision frequency, energy, and orientation. These depend on factors such as concentration, activation energy, and catalysts. Molar mass itself is not a direct factor in the basic collision theory framework.
VI. How does a catalyst increase the rate of reaction?
(a) By decreasing the number of particles per unit volume
(b) By increasing the activation energy
(c) By providing an alternate pathway with higher activation energy
(d) By providing an alternate pathway with lower activation energy
Answer: (d) By providing an alternate pathway with lower activation energy.
Concept: Catalysts offer an alternative reaction mechanism with a reduced activation energy barrier. This enables a larger fraction of reactant molecules to undergo effective collisions.
VII. Which physical parameter is NOT typically affected by the rate of reaction?
(a) Change in mass
(b) Temperature
(c) Formation of gas
(d) Colour of the reactants
Answer: (b) Temperature.
Concept: Reaction rates describe how fast reactants turn into products. This can be observed through changes in mass, gas volume, or colour intensity. Temperature is an external environmental factor that influences the reaction rate rather than being a direct result of the reaction rate itself.
VIII. Which factor can affect the rate of reaction involving gases?
(a) Change in solubility
(b) Change in pressure
(c) Change in volume
(d) Change in viscosity
Answer: (b) Change in pressure.
Concept: Changing the pressure of gaseous systems alters the concentration (number of moles per unit volume), affecting collision frequency and, consequently, the reaction rate.
IX. Increasing the surface area of solids generally:
(a) Decreases the rate of reaction
(b) Increases the rate of reaction
(c) Has no effect on the rate of reaction
(d) Makes the reaction irreversible
Answer: (b) Increases the rate of reaction.
Concept: A larger surface area exposes more reactant particles to collisions, increasing the frequency of effective collisions and accelerating the reaction rate.
X. How does temperature affect the rate of reaction according to collision theory?
(a) Higher temperature decreases the frequency of collisions
(b) Higher temperature increases the activation energy
(c) Higher temperature decreases the kinetic energy of particles
(d) Higher temperature increases the frequency of collisions and kinetic energy of particles
Answer: (d) Higher temperature increases the frequency of collisions and kinetic energy of particles.
Concept: Raising the temperature increases particle velocity, collision frequency, and the proportion of molecules possessing energy equal to or greater than the activation energy.
2. Short Questions
I. Draw energy diagrams that represent the activation energy and show the effect of a catalyst.
Answer: An energy profile diagram plots potential energy on the y-axis against reaction progress on the x-axis.
The uncatalyzed curve rises from the reactants to a high peak, representing the uncatalyzed activation energy (\(E_{a(\text{uncatalyzed})}\)), before dropping to the products.
The catalyzed reaction curve shows an alternative pathway with a lower peak, representing the reduced activation energy (\(E_{a(\text{catalyzed})}\)).
II. What is the effect of a catalyst on the following?
(a) The rate of reaction
A catalyst increases the rate of reaction by providing an alternative pathway with lower energy requirements.
(b) The energy of activation
A catalyst lowers the activation energy barrier for the reaction.
III. Why does powdered Zn react faster with acid than a piece of Zn? Give reason.
Answer: Powdered zinc has a much greater surface area than a solid piece of zinc. According to collision theory, a larger surface area exposes more reactant particles to acid molecules, increasing the frequency of successful collisions and thus accelerating the reaction rate.
IV. List physical parameters which are affected by reaction rates.
Answer: Parameters reflecting the progress of a reaction over time include:
Change in reactant or product concentration or molarity.
Change in mass or weight of the reaction mixture.
Volume or pressure of gas produced or consumed.
Colour intensity or optical absorbance changes.
pH changes (if acids or bases are involved).
V. Explain collision theory and its key components.
Answer: Collision theory states that for a chemical reaction to occur, reactant particles must collide with each other.
Its key components are:
Collision Frequency: The total number of collisions per unit time per unit volume.
Activation Energy (\(E_a\)): Colliding particles must possess a minimum threshold kinetic energy to break existing bonds and initiate a reaction.
Proper Orientation: Particles must collide with the correct geometric alignment so that reactive atoms touch and bond appropriately.
VI. Discuss the effect of changing the concentration of a reactant on the rate of reaction.
Answer: Increasing the concentration of reactants increases the number of particles per unit volume. This leads to a higher frequency of collisions between reactant molecules, which increases the rate of the chemical reaction.
VII. Describe how graphs are used to interpret the rate of reaction.
Answer: Reaction rates are interpreted using concentration-time graphs.
The slope (gradient) of a concentration-time curve at any given point represents the instantaneous rate of reaction.
A steeper slope indicates a faster reaction rate.
A flattening curve indicates that the reaction is slowing down or has reached completion.
VIII. Why is activation energy important in determining the rate of reaction?
Answer: Activation energy acts as an energy barrier that reactant molecules must overcome to transform into products. A higher activation energy means fewer molecules have sufficient thermal energy at a given temperature to react, resulting in a slower reaction rate.
IX. If you increase the temperature of a reaction, how does this affect the kinetic energy of the particles?
Answer: Increasing the temperature increases the average kinetic energy of the particles. This causes particles to move faster, increasing both the frequency of their collisions and the proportion of particles whose kinetic energy equals or exceeds the activation energy.
X. Why do only a fraction of collisions lead to a reaction according to collision theory?
Answer: Not all collisions are effective collisions. For a collision to be successful, particles must possess energy equal to or greater than the activation energy (\(E_a\)) and collide with the correct spatial orientation.
XI. Evaluate how increasing activation energy affects the rate of reaction.
Answer: Increasing the activation energy raises the energy barrier that reactants must overcome. Consequently, at a constant temperature, fewer molecules possess the required minimum energy, resulting in a lower proportion of effective collisions and a slower reaction rate.
XII. Suggest a way to identify if a catalyst has been effective in a reaction.
Answer: A catalyst’s effectiveness can be identified by:
Observing an increase in the reaction rate, such as faster gas evolution or quicker colour change, under identical conditions.
Measuring a lower apparent activation energy using temperature-dependent rate studies.
XIII. Assess the impact of catalysts on industrial chemical processes.
Answer: Catalysts are vital in industry because they lower operating temperatures and pressures, saving energy costs. They also increase production efficiency, enhance reaction rates, and improve selectivity to minimize unwanted side products.
XIV. Evaluate the impact of adding an enzyme to a biochemical reaction.
Answer: Enzymes are biological catalysts that dramatically increase biochemical reaction rates under mild physiological conditions by providing alternative reaction pathways with exceptionally low activation energies.
XV. Analyse how temperature control during transportation can affect the quality of the product.
Answer: Temperature control is critical because rates of chemical degradation, such as food spoilage and oxidation, increase with temperature. Maintaining low temperatures during transport slows down these reactions, preserving product quality and freshness.
3. Long Questions
I. Examine the graph depicting:
(a) The concentration of reactants over time
The curve starts high at \(t = 0\) and decreases non-linearly. It is steep at first and then gradually flattens as reactants are consumed and converted into products. Eventually, it approaches a plateau when the reaction reaches completion.
(b) The effect of temperature on the rate of reaction
Higher temperature shifts the kinetic energy distribution curve to the right, increasing the fraction of molecules exceeding \(E_a\) and resulting in a steeper concentration-time curve.
(c) The effect of the catalyst on reaction rate
The catalyzed reaction graph shows a steeper decline in reactant concentration over time compared to the uncatalyzed reaction. This reflects a faster reaction rate due to lowered activation energy.
II. An increase in surface area increases the reaction rate. Evaluate this statement in the light of collision theory to support your answer.
Answer: When a solid reactant is divided into smaller pieces or powdered, its exposed surface area increases significantly.
In heterogeneous reactions, more surface area provides a greater contact area for reactant molecules to collide with solid particles. This leads to a higher frequency of effective collisions per unit time and a faster reaction rate.
III. Interpret the role of a catalyst in a chemical reaction.
Answer: A catalyst is a substance that increases the rate of a chemical reaction without being consumed. It provides an alternative reaction pathway that has a lower activation energy (\(E_a\)), allowing a larger fraction of reactant collisions to be effective at a given temperature.
IV. Discuss the impact of temperature on the rate of reaction.
Answer: Temperature profoundly affects reaction rates. Often, the rate doubles for every 10°C rise in temperature.
Raising the temperature increases the average kinetic energy of molecules, shifting the Maxwell-Boltzmann energy distribution curve and increasing both collision frequency and the fraction of molecules with energy equal to or greater than the activation energy (\(E_a\)).
V. Justify the role of chemical kinetics in the food industry.
Answer: Chemical kinetics allows food scientists to model spoilage rates, lipid oxidation, and microbial growth. This helps establish accurate expiration dates, design effective preservation techniques such as refrigeration and pasteurization, and optimize food processing parameters.
VI. Propose a hypothesis for how temperature and surface area affect the reaction rate. Design a set of experiments to test your hypothesis, detailing the methods and measurements.
Answer:
Hypothesis: Both increasing temperature and increasing the surface area of solid reactants increase the reaction rate by raising collision frequency and the proportion of effective collisions.
Experiment Design:
React calcium carbonate (\(CaCO_3\)) with dilute hydrochloric acid (\(HCl\)) and measure the evolution of carbon dioxide (\(CO_2\)) gas over time.
Surface area test: Compare coarse calcium carbonate chips with fine calcium carbonate powder.
Temperature test: Carry out reactions at 20°C, 40°C, and 60°C.
Measurement: Measure the volume of gas at regular time intervals using a gas syringe.
VII. You are investigating a new catalyst for a reaction that produces a valuable pharmaceutical product.
Answer: Comparative trials are conducted with and without the catalyst under identical conditions.
Key parameters measured include:
Reaction yield.
Reaction time.
Purity of the pharmaceutical product.
Turnover number (TON).
Selectivity to avoid hazardous byproducts.
VIII. Explain the role of the catalyst in the reaction mechanism, including its effect on activation energy.
Answer: A catalyst actively participates in the mechanism by forming temporary intermediate species with reactants in early steps. These intermediates then break down to release products and regenerate the catalyst.
Thermodynamically, it bypasses the high-energy transition state by providing a new pathway with a lower activation energy (\(E_a\)).
IX. Design an experiment to compare the reaction rates with and without the catalyst.
Answer:
Aim: Compare the decomposition rate of hydrogen peroxide (\(H_2O_2\)) with and without manganese dioxide (\(MnO_2\)).
Procedure:
Measure the volume of oxygen gas collected from 50 cm³ of 1 M hydrogen peroxide every 30 seconds without a catalyst.
Repeat the exact procedure after adding 0.5 g of manganese dioxide powder.
Compare the results using a volume-versus-time graph.
The catalyzed trial will show a much steeper initial slope, indicating a faster reaction rate.
X. A chemical reaction between substances A and B follows collision theory. You are tasked with maximizing the reaction rate. Describe how you would modify the number of particles per unit volume, temperature, and activation energy to achieve this goal.
Answer:
1. Particles per unit volume: Increase the concentration of reactants A and B (or increase pressure if the reactants are gases) to maximize collision frequency.
2. Temperature: Raise the reaction temperature to increase particle kinetic energy and the proportion of effective collisions.
3. Activation energy: Introduce a highly efficient catalyst to lower the activation energy barrier, enabling a greater number of successful collisions.
XI. Compare and contrast the effects of temperature and concentration on the rate of reaction.
Answer:
Similarities: Both increasing temperature and concentration increase the reaction rate by increasing the number of effective collisions per unit time.
Differences:
Concentration: Increasing concentration increases collision frequency by packing more particles into the same volume without changing their energy distribution.
Temperature: Increasing temperature raises both collision frequency and average kinetic energy, increasing the fraction of molecules exceeding the activation energy (\(E_a\)).
XII. Discuss the significance of activation energy in chemical reactions and its relationship to reaction rates.
Answer: Activation energy (\(E_a\)) determines the kinetic stability of reactants. Reactions with high activation energy do not proceed appreciably at room temperature.
According to the Arrhenius equation:
The rate constant and reaction rate depend exponentially on activation energy, determining how quickly a chemical system proceeds.
Where:
\(k\) = Rate constant
\(A\) = Frequency factor
\(E_a\) = Activation energy
\(R\) = Gas constant
\(T\) = Absolute temperature in Kelvin
4. Think Tank
1. Analyse a scenario where a catalyst is not effective in increasing the rate of reaction.
Answer: A catalyst may be ineffective due to several reasons:
Catalyst poisoning: Impurities adsorb onto active sites and block reactants.
Wrong catalyst: The selected catalyst may not be suitable for the specific reaction mechanism.
Poor dispersion: A heterogeneous catalyst may have a low active surface area because of poor dispersion.
Mass transfer limitations: Diffusion of reactants to the catalyst surface may be slower than the chemical reaction itself.
2. Evaluate the role of chemical kinetics in optimizing food production processes in the food industry.
Answer: Chemical kinetics allows engineers to model spoilage rates and microbial death kinetics as rate laws.
This helps optimize thermal processing times, such as UHT pasteurization and retort sterilization, to ensure food safety while minimizing nutrient degradation and undesirable flavour changes.
3. Predict the effect of doubling the surface area of a solid reactant on the rate of reaction, providing reasoning based on collision theory.
Answer:
Prediction: Doubling the surface area of a solid reactant roughly doubles the initial rate of reaction.
Reasoning: In heterogeneous reactions, collisions occur at the solid-fluid interface. Doubling the available surface area doubles the number of exposed surface sites available for reactant molecules to collide with per unit time, directly increasing the frequency of effective collisions.
4. Explain why the formation of gas is often associated with an increase in the rate of reaction.
Answer: The evolution of gas, such as \(H_2\), \(CO_2\), or \(O_2\), is a visible indicator of product formation in many reactions.
As the reaction proceeds, rapid gas evolution reflects a high frequency of effective collisions and a fast rate of conversion. Therefore, measuring gas volume is a standard experimental method in chemical kinetics.
Notes by Sir Dilawar
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