There is a chapter 17 (Reaction Kinetics) solved exercise for Class 10 Chemistry with the most important short questions. All notes for class 10 chemistry are here, and All Video lectures are here. Watch and Subscribe, please.
A. Multiple Choice Questions (solved exercise)
1. The number of collisions per unit volume of the reaction mixture is called:
(a) Collision energy
(b) Activation energy
(c) Collision frequency (correct)
(d) Collision force
2. If the reactants possess an energy higher than the activation energy, then the reaction will be:
(a) Slow
(b) Fast (correct)
(c) Not affected
(d) Instantaneous
3. Which of the following explains the increase in the rate of reaction in the presence of a catalyst?
(a) Catalyst provides extra energy to the reactant molecules
(b) Catalyst provides an alternative pathway that lowers the activation energy. (correct)
(c) Catalyst increases the collision frequency
(d) Catalyst decreases the collision frequency
4. Which of the following statements is correct?
(a) Collisions with energy equal to or greater than the activation energy lead to a reaction. (correct)
(b) Collision frequency is not related to the reaction rate
(c) All collisions lead to a reaction.
(d) Collisions with energy less than the activation energy lead to the reaction.
5. When a reaction proceeds ahead, how do the concentrations of reactants and products change?
(a) Concentration of reactants increases, and that of the products decreases.
(b) Concentration of reactants decreases, and that of the products increases. (correct)
(c) Concentration of both reactants and products decreases.
(d) Concentration of both reactants and products increases.
B. Short Answer Questions (solved exercise)
17.1 What is a successful collision?
Answer: A collision between reactant particles that successfully results in the formation of products is called a successful (or effective) collision. For a collision to be successful, the colliding particles must possess:
- A minimum amount of energy, known as activation energy (Ea).
- The correct orientation at the moment of impact.
17.2 How does an increase in temperature increase the rate of a reaction?
Answer: Increasing the temperature increases the kinetic energy of the reactant particles, causing them to move faster. This results in:
- An increase in the total frequency of collisions per second.
- A significantly larger fraction of molecules possesses energy equal to or greater than the activation energy (Ea).
Consequently, the number of successful collisions increases, which elevates the rate of reaction.
17.3 Define activation energy.
Answer: The minimum amount of energy required by colliding reactant particles to break existing bonds and initiate a chemical reaction is called activation energy (Ea). It acts as an energy barrier that reactants must overcome to convert into products.
17.4 Why does the burning of sulphur proceed slower in air than in pure oxygen?
Answer: The rate of a reaction depends directly on the concentration of the reactants. Air contains only about 21% oxygen by volume, whereas pure oxygen has a 100% concentration. The higher concentration of oxygen molecules in pure oxygen leads to a greater frequency of collisions between sulphur and oxygen particles, causing sulphur to burn much faster in pure oxygen than in air.
17.5 Why is a catalyst used in a reaction preferably taken in the powdered form?
Answer: A catalyst is preferably taken in powdered form because finely divided particles offer a much larger surface area than a solid lump. A larger surface area provides more active sites for the reactant molecules to come into contact and interact, thereby increasing the rate of reaction more efficiently.
17.6 Why is the rate of a reaction often very fast at the beginning of the reaction?
Answer: At the beginning of a reaction, the concentration of the reactant molecules is at its maximum. According to collision theory, a higher concentration results in a maximum frequency of collisions per unit time. As the reaction proceeds, reactants are consumed, their concentration decreases, and the reaction rate slows down.
17.7 Magnesium does not react with air at room temperature but it reacts very fast at high temperature giving intense white light. Explain.
Answer: At room temperature, the colliding particles of magnesium and oxygen do not possess enough kinetic energy to overcome the high activation energy barrier required for the reaction. Heating provides the necessary thermal energy, allowing a large fraction of particles to cross the energy barrier, resulting in a rapid exothermic reaction that emits an intense white light.
17.8 What happens to the reactants after they climb the energy hill during a reaction?
Answer: When the reactants absorb enough energy to climb to the top of the energy hill (the activation energy barrier), they form an unstable, temporary high-energy species called the activated complex (or transition state). From this state, the old bonds fully break, new bonds form, and the complex quickly decomposes to slide down the hill to form stable products.
17.9 How does a catalyst lower the activation energy of a reaction?
Answer: A catalyst increases the reaction rate by providing an alternative reaction pathway or mechanism. This new path has a lower activation energy barrier compared to the uncatalyzed route, enabling a greater number of reactant particles to successfully react at the same temperature.
17.10 Give any two features of the catalytic action of an enzyme.
Answer:
- High Specificity: Enzymes are highly specific in nature; a particular enzyme can catalyze only one specific reaction or a specific class of reactions.
- Highly Efficient / Temperature Sensitive: Enzymes operate with extreme efficiency at an optimum body temperature (around 37°C in humans) and can be denatured (inactivated) at high temperatures.
C. Constructed Response Questions (solved exercise)
17.1 In what different ways can you increase the successful collisions between the particles of the reactants?
Answer: The number of successful collisions can be increased by:
- Increasing the Temperature: Elevates the kinetic energy of particles, making more collisions energetic enough to surpass the activation energy.
- Increasing the Concentration/Pressure: Packs more particles into a given volume, leading to more frequent collisions.
- Increasing Surface Area (for solids): Powdering solid reactants exposes more particles to immediate collision.
- Adding a Catalyst: Introduces an alternative pathway with a lower activation energy barrier, making existing collisions more likely to succeed.
17.2 Give an example of a reaction which proceeds with the gain in mass.
Answer: When a metal is heated or burnt in an open system, it combines with oxygen from the air, resulting in an apparent gain in mass due to the formation of a solid metal oxide.
- Example: Burning a magnesium ribbon in an open crucible produces Magnesium Oxide (MgO), which weighs more than the initial magnesium metal.
2Mg(s) + O2(g) → 2MgO(s)
17.3 From where do molecules get energy to attain a higher energy state?
Answer: Molecules obtain the energy needed to attain a higher energy state (or activation energy) from:
- Thermal Energy: Continuous environmental heating or intentional application of heat.
- Intermolecular Collisions: Random, continuous kinetic collisions where kinetic energy is transferred between moving particles, allowing some molecules to accumulate high energy.
17.4 How does the presence of V2O5 catalyst lower the activation energy of the following reaction?
2SO2(g) + O2(g) → 2SO3(g)
Answer: Vanadium pentoxide (V2O5) provides an alternative, multi-step reaction pathway on its solid surface. Instead of SO2 and O2 directly colliding in the gas phase (which requires high energy), the molecules adsorb onto the catalyst’s surface, weakening their internal bonds. This temporary chemical interaction lowers the energy barrier (Ea), speeding up the conversion into SO3.
17.6 Explain the catalytic action of an enzyme.
Answer: Enzymes function via a “Lock and Key” mechanism. The surface of an enzyme features a specific 3D geometric shape known as the active site. The reactant molecules (substrates) fit precisely into this active site like a key into a lock, forming a temporary enzyme-substrate complex. This placement correctly orients the substrates and strains their chemical bonds, dramatically lowering the activation energy needed to form products. Once the reaction completes, the products leave, and the enzyme remains unchanged.
17.7 If you desire to stop a reaction going on at 60 °C, what action you will take?
Answer: To stop or significantly halt the reaction, you can:
- Rapidly Cool the Mixture (Quenching): Immersing the reaction vessel in an ice bath drastically lowers the temperature, diminishing the kinetic energy of the particles so they can no longer cross the activation energy barrier.
- Add a Negative Catalyst (Inhibitor): Introducing an inhibitor destroys or blocks the active pathways/catalysts driving the reaction.
Most Important short questions (Internal).
All questions are explained in Sir Umair Khan’s lecture.
- Define rate or reaction?
- Write the collision theory of reaction rate.
- Define Effective collision.
- Write the factors affecting effective collision and the rate of reaction.
- Define activation energy.
- Explain the change in mass during chemical reactions.
- What is the change in temperature during a chemical reaction?
- Name the factors affecting the rate of reaction. Discuss the effect of the concentration of reactants.
- How do the gas-phase reactants affect the rate of reaction?
- How does the surface area of a solid affect the rate of reaction?
- Write the effect of temperature on the rate of reaction with a reason.
- Write the effect of the catalyst on the rate of reaction.
- How does an enzyme affect the rate of reaction?
- Write a few lines on the importance of chemical kinetics in the food industry.
