NCERT Exemplar Chemical Kinetics - Chemistry Class 12 - NEET PDF Download

Table of Contents
1. Multiple Choice Questions
2.  Multiple Choice Questions
3. Short Answer Type Questions
4. Matching Type
5. Assertion And Reason Type
6. Long Answer Type Questions
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Multiple Choice Questions

Q.1. The role of a catalyst is to change ___________.
(i) Gibbs energy of reaction.
(ii) Enthalpy of reaction.
(iii) Activation energy of reaction.
(iv) Equilibrium constant.
Ans. (iii)
Solution.
A catalyst provides an alternative reaction pathway with a lower activation energy. It does not change the Gibbs free energy of reaction, the enthalpy change, or the equilibrium constant.

Q.2. In the presence of a catalyst, the heat evolved or absorbed during the reaction ___________.
(i) Increases.
(ii) Decreases.
(iii) Remains unchanged.
(iv) May increase or decrease.
Ans. (iii)
Solution.
A catalyst does not appear in the overall chemical equation and does not change the enthalpy change of the reaction. Therefore, the heat evolved or absorbed remains unchanged.

Q.3. Activation energy of a chemical reaction can be determined by ________.
(i) Determining the rate constant at standard temperature.
(ii) Determining the rate constants at two temperatures.
(iii) Determining probability of collision.
(iv) Using catalyst.
Ans. (ii)
Solution.
The activation energy Ea can be found from the Arrhenius relation by measuring rate constants k1 and k2 at two different temperatures T1 and T2 and using the form:

where Ea is the activation energy, T2 is the higher temperature, T1 is the lower temperature, k1 is the rate constant at T1 and k2 is the rate constant at T2. This expression is obtained from the Arrhenius equation.

Q.4. Consider Fig. 4.1 and mark the correct option.

(i) Activation energy of forward reaction is E₁ + E₂ and product is less stable than reactant.
(ii) Activation energy of forward reaction is E₁ + E₂ and product is more stable than reactant.
(iii) Activation energy of both forward and backward reaction is E₁+ E₂ and reactant is more stable than product.
(iv) Activation energy of backward reaction is E₁ and product is more stable than reactant.
Ans. (i)
Solution.
From the diagram, energy difference shows that reactants have lower energy than products, so products are less stable. The activation energy for forward reaction equals E₁ + E₂.

Q.5. Consider a first order gas phase decomposition reaction given below :
A(g) → B(g) + C(g)
The initial pressure of the system before decomposition of A was p_i. After lapse of time 't', total pressure of the system increased by x units and became 'p_t' The rate constant k for the reaction is given as _________.
(i)

(ii)

(iii)

(iv)

Ans. (ii)
Solution.

Consider the first order decomposition A(g) → B(g) + C(g).

Initial partial pressures: A = P_i, B = 0, C = 0.

After time t, let extent of decomposition = x; then

A = P_i - x, B = x, C = x.

Total pressure P_t = (P_i - x) + x + x = P_i + x.

So x = P_t - P_i and partial pressure of A after time t is

P_A = P_i - x = 2P_i - P_t.

Using first order integrated rate law ln(P_A/P_i) = -kt and substituting P_A, one obtains the expression shown in option (ii).

Q.6. According to Arrhenius equation rate constant k is equal to Ae^-Ea/RT. Which of the following options represents the graph of ln k vs 1/T ?
(i)

(ii)

(iii)

(iv)

Ans. (i)

Solution.

From the Arrhenius equation:

k = A e^-Ea/RT

Taking natural logarithm:

ln k = ln A - (Ea/R)(1/T)

This is a straight line of the form y = mx + c with y = ln k, x = 1/T, slope m = -Ea/R and intercept c = ln A. Thus ln k vs 1/T is a straight line with negative slope, as shown in option (i).

Q.7. Consider the Arrhenius equation given below and mark the correct option.

(i) Rate constant increases exponentially with increasing activation energy and decreasing temperature.
(ii) Rate constant decreases exponentially with increasing activation energy and decreasing temperature.
(iii) Rate constant increases exponentially with decreasing activation energy and decreasing temperature.
(iv) Rate constant increases exponentially with decreasing activation energy and increasing temperature.
Ans. (iv)
Solution.

From k = A e^-Ea/RT, a decrease in Ea increases e^-Ea/RT and hence increases k. An increase in temperature T reduces the magnitude of Ea/RT, increasing e^-Ea/RT and thus k. Therefore k increases when Ea decreases and T increases.

Q.8. A graph of volume of hydrogen released vs time for the reaction between zinc and dil. HCl is given in Fig. 4.2. On the basis of this mark the correct option.

(i) Average rate upto 40s is

(ii) Average rate upto 40 seconds is

(iii) Average rate upto 40 seconds is

(iv) Average rate upto 40 seconds is

Ans. (iii)

Solution.

Average rate up to 40 s is the change in volume divided by time. From the graph it corresponds to the value shown in option (iii).

Q.9. Which of the following statements is not correct about order of a reaction.
(i) The order of a reaction can be a fractional number.
(ii) Order of a reaction is experimentally determined quantity.
(iii) The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction.
(iv) The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression.
Ans. (iii)
Solution.

Order of reaction is determined experimentally and equals the sum of powers of concentrations in the rate law. It need not equal the sum of stoichiometric coefficients of reactants except for elementary reactions. Orders can be fractional.

Q.10. Consider the graph given in Fig. 4.2. Which of the following options does not show instantaneous rate of reaction at 40^th second?

(i)

(ii)

(iii)

(iv)

Ans. (ii)

Solution.

Instantaneous rate at t = 40 s is the slope of the tangent to the curve at that point. Option (ii) does not represent the correct tangent at 40 s.

Q.11. Which of the following statements is correct?
(i) The rate of a reaction decreases with passage of time as the concentration of reactants decreases.
(ii) The rate of a reaction is same at any time during the reaction.
(iii) The rate of a reaction is independent of temperature change.
(iv) The rate of a reaction decreases with increase in concentration of reactant(s).
Ans. (i)
Solution.

As reactants are consumed, their concentrations fall and, for most reactions, the rate decreases because rate depends on concentration and temperature.

Q.12. Which of the following expressions is correct for the rate of reaction given below?
5Br^-(aq) + BrO₃^-(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l)

(i)

(ii)

(iii)

(iv)

Ans. (iii)

Solution.

From stoichiometry and the definition of rate (change in concentration of species divided by its stoichiometric coefficient), the correct expression for the rate consistent for all species is the one given in option (iii).

Q.13. Which of the following graphs represents exothermic reaction?
(a)

(b)

(c)

(i) (a) only

(ii) (b) only

(iii) (c) only

(iv) (a) and (b)

Ans. (i)

Solution.

In an exothermic reaction the energy of products is lower than that of reactants (ΔH < 0). the diagram in (a) shows products at lower energy than reactants, so (a) corresponds to an exothermic.

Q.14. Rate law for the reaction A + 2B → C is found to be
Rate = k [A][B]
Concentration of reactant 'B' is doubled, keeping the concentration of 'A' constant, the value of rate constant will be______.
(i) The same
(ii) Doubled
(iii) Quadrupled
(iv) Halved
Ans. (i)
Solution.

Rate constant k is independent of reactant concentrations; changing [B] will change the rate but not k.

Q.15. Which of the following statements is incorrect about the collision theory of chemical reaction?
(i) It considers reacting molecules or atoms to be hard spheres and ignores their structural features.
(ii) Number of effective collisions determines the rate of reaction.
(iii) Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.
(iv) Molecules should collide with sufficient threshold energy and proper orientation for the collision to be effective.
Ans. (iii)
Solution.

Collision theory requires both sufficient energy and correct orientation for a collision to be effective. Statement (iii) is incomplete because it ignores the requirement of correct orientation, hence incorrect.

Q.16. A first order reaction is 50% completed in 1.26 × 10¹⁴ s. How much time would it take for 100% completion?
(i) 1.26 × 10¹⁵ s
(ii) 2.52 × 10¹⁴ s
(iii) 2.52 × 10²⁸ s
(iv) infinite
Ans. (iv)
Solution.

A first order reaction asymptotically approaches completion; it never reaches 100% in finite time. Complete 100% conversion would require infinite time.

Q.17. Compounds 'A' and 'B' react according to the following chemical equation.
A (g) + 2 B(g) → 2C (g)
Concentration of either 'A' or 'B' were changed keeping the concentrations of one of the reactants constant and rates were measured as a function of initial concentration. Following results were obtained. Choose the correct option for the rate equations for this reaction.

(i) Rate = k [A]²[B]
(ii) Rate = k [A] [B]²
(iii) Rate = k [A] [B]
(iv) Rate = k [A]² [B]⁰
Ans. (ii)
Solution.

Compare experiments: when [B] is doubled (exp.1 → exp.2) with [A] constant, rate increases 4-fold, so rate ∝ [B]². When [A] is doubled (exp.1 → exp.3) with [B] constant, rate doubles, so rate ∝ [A]. Hence Rate = k[A][B]².

Q.18. Which of the following statement is not correct for the catalyst?
(i) It catalyses the forward and backward reaction to the same extent.
(ii) It alters ∆G of the reaction.
(iii) It is a substance that does not change the equilibrium constant of a reaction.
(iv) It provides an alternate mechanism by reducing activation energy between reactants and products.
Ans. (ii)
Solution.

Characteristics of catalyst:

• It accelerates both forward and reverse reactions equally by lowering activation energies.
• It does not change Gibbs free energy change ΔG or the equilibrium constant K; ΔG depends on initial and final states only.
• It provides an alternate pathway with lower activation energy.

Q.19. The value of rate constant of a pseudo first order reaction ____________.
(i) Depends on the concentration of reactants present in small amount.
(ii) Depends on the concentration of reactants present in excess.
(iii) Is independent of the concentration of reactants.
(iv) Depends only on temperature.
Ans. (ii)
Solution.

In pseudo first order conditions, one reactant is kept in large excess so its concentration is effectively constant. The observed rate constant k(obs) depends on the concentration of the reagent present in excess (it multiplies the true second-order rate constant), and also on temperature.

Q.20. Consider the reaction A ⇌ B.  The concentration of both the reactants and the products varies exponentially with time. Which of the following figures correctly describes the change in concentration of reactants and products with time?
(i)

(ii)

(iii)

(iv)

Ans. (ii)

Solution.

Concentration of reactant decreases exponentially, while concentration of product increases exponentially toward equilibrium; option (ii) matches this behaviour.

 Multiple Choice Questions

Note : In the following questions two or more options may be correct.

Q.21. Rate law cannot be determined from balanced chemical equation if _______.
(i) Reverse reaction is involved.
(ii) It is an elementary reaction.
(iii) It is a sequence of elementary reactions.
(iv) Any of the reactants is in excess.
Ans. (i, iii, iv)
Solution.

Rate law can be written directly from the balanced equation only for an elementary reaction. If reverse reaction is involved, or the reaction is a sequence of elementary steps (complex), or if some reactant is present in large excess (leading to pseudo order), the simple balanced equation does not give the rate law; experimental determination or mechanism analysis is needed.

Q.22. Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?
(i) Order is same as molecularity.
(ii) Order is less than the molecularity.
(iii) Order is greater than the molecularity.
(iv) Molecularity can never be zero.
Ans. (i, iv)
Solution.

For an elementary reaction, the rate law reflects the molecularity, so order = molecularity. Molecularity counts colliding species and cannot be zero.

Q.23. In any unimolecular reaction ______________.
(i) Only one reacting species is involved in the rate determining step.
(ii) The order and the molecularity of slowest step are equal to one.
(iii) The molecularity of the reaction is one and order is zero.
(iv) Both molecularity and order of the reaction are one.
Ans. (i, ii)
Solution.

In a unimolecular step a single molecule undergoes transformation; for such an elementary rate-determining step molecularity = 1 and the rate law is first order with respect to that species.

Q.24. For a complex reaction ______________.
(i) Order of overall reaction is same as molecularity of the slowest step.
(ii) Order of overall reaction is less than the molecularity of the slowest step. 
(iii) Order of overall reaction is greater than molecularity of the slowest step. 
(iv) Molecularity of the slowest step is never zero or non integer.
Ans. (i, iv)
Solution.

For a multistep (complex) reaction, the overall rate is controlled by the slowest (rate-determining) step; the order of the overall reaction equals the molecularity of that slow step. Molecularity refers to the number of molecules colliding in an elementary step, hence it is a positive integer (≥1).

Q.25. At high pressure the following reaction is zero order.

Which of the following options are correct for this reaction?
(i) Rate of reaction = Rate constant
(ii) Rate of the reaction depends on concentration of ammonia.
(iii) Rate of decomposition of ammonia will remain constant until ammonia disappears completely.
(iv) Further increase in pressure will change the rate of reaction.
Ans. (i, iii, iv)
Solution.

At very high pressure the reaction becomes zero order with respect to NH₃, so rate = k (independent of concentration). The rate will remain constant until [NH₃] becomes very small. Further pressure changes can shift equilibrium or change mechanism; by Le Chatelier's principle increasing pressure may affect the equilibrium and hence the observed rate.

Q.26. During decomposition of an activated complex
(i) Energy is always released
(ii) Energy is always absorbed
(iii) Energy does not change
(iv) Reactants may be formed
Ans. (i, iv)
Solution.

Activated complexes are short-lived transition states formed during collisions. Their decomposition can lead to products (releasing energy if the overall reaction is exergonic) or revert to reactants; not every activated complex proceeds to products.

Q.27. According to Maxwell Boltzmann distribution of energy, __________.
(i) The fraction of molecules with most probable kinetic energy decreases at higher temperatures.
(ii) The fraction of molecules with most probable kinetic energy increases at higher temperatures.
(iii) Most probable kinetic energy increases at higher temperatures.
(iv) Most probable kinetic energy decreases at higher temperatures.
Ans. (i, iii)
Solution.

With increasing temperature the Maxwell-Boltzmann curve broadens and the peak shifts to higher energy (most probable energy increases). Because the curve flattens, the fraction of molecules at the most probable energy decreases.

Q.28. In the graph showing Maxwell Boltzman distribution of energy, ___________.
(i) Area under the curve must not change with increase in temperature.
(ii) Area under the curve increases with increase in temperature.
(iii) Area under the curve decreases with increase in temperature.
(iv) With increase in temperature curve broadens and shifts to the right hand side.
Ans. (i, iv)
Solution.

The total area under the Maxwell-Boltzmann distribution curve equals 1 (the total probability) and is temperature independent. With increasing temperature the curve broadens and shifts rightwards.

Q.29. Which of the following statements are in accordance with the Arrhenius equation?
(i) Rate of a reaction increases with increase in temperature.
(ii) Rate of a reaction increases with decrease in activation energy.
(iii) Rate constant decreases exponentially with increase in temperature.
(iv) Rate of reaction decreases with decrease in activation energy.
Ans. (i, ii)
Solution.

From k = A e^-Ea/RT:

As T increases, -Ea/RT becomes less negative and k increases. As Ea decreases, e^-Ea/RT increases and k increases. Thus (i) and (ii) are correct; (iii) and (iv) are incorrect.

Q.30. Mark the incorrect statements.
(i) Catalyst provides an alternative pathway to reaction mechanism.
(ii) Catalyst raises the activation energy.
(iii) Catalyst lowers the activation energy.
(iv) Catalyst alters enthalpy change of the reaction.
Ans. (ii, iv)
Solution.

Catalysts provide an alternate pathway and lower activation energy; they do not raise activation energy and do not alter the overall enthalpy change of the reaction.

Q.31. Which of the following graphs is correct for a zero order reaction?
(i)

(ii)

(iii)

(iv)

Ans. (i, iv)

Solution.

For zero order reactions [R] = -kt + [R]₀, which is a straight line with slope -k when concentration is plotted against time; the integrated forms also give linear plots for appropriate variables, corresponding to options (i) and (iv).

Q.32. Which of the following graphs is correct for a first order reaction?
(i)

(ii)

(iii)

(iv)

Ans. (i, ii)

Solution.

For first order: ln[R] = -kt + ln[R]₀, so plot of ln[R] vs t is linear (option i). Also plots of rate vs [R] or appropriate transforms produce straight lines (option ii). The half-life for first order, t_1/2 = ln 2 / k, is independent of initial concentration, so the plot of t_1/2 vs [R]₀ is flat (option corresponding to ii in the original set).

Short Answer Type Questions

Q.33. State a condition under which a bimolecular reaction is kinetically first order reaction.
Ans. A bimolecular reaction behaves as pseudo first order when one reactant is present in large excess so its concentration remains effectively constant. Example: acid-catalysed hydrolysis of ethyl acetate where water is in large excess. The observed rate depends only on ethyl acetate concentration.

Q.34. Write the rate equation for the reaction 2A + B → C if the order of the reaction is zero.
Ans. If the overall order is zero then

Rate = k [A]⁰ [B]⁰ = k

so the rate is independent of concentrations of A and B.

Q.35. How can you determine the rate law of the following reaction?
2NO (g) + O₂ (g) → 2NO₂ (g)
Ans.

Experimentally vary initial concentrations and measure initial rates. For example:

(i) Keep [O₂] constant and double [NO]; if rate becomes four times then rate ∝ [NO]².

(ii) Keep [NO] constant and double [O₂]; if rate doubles then rate ∝ [O₂].

Therefore the rate law is Rate = k [NO]² [O₂].

Q.36. For which type of reactions, order and molecularity have the same value?
Ans. For elementary reactions order and molecularity are the same because the rate law follows directly from the single-step collision event represented in the balanced elementary equation.

Q.37. In a reaction if the concentration of reactant A is tripled, the rate of reaction becomes twenty seven times. What is the order of the reaction?
Ans.

Rate law: r = k[A]ⁿ.

If [A] → 3[A] then r → 27 r.

So k(3[A])ⁿ = 27 k[A]ⁿ ⇒ 3ⁿ = 27 ⇒ n = 3.

Order of the reaction with respect to A is three.

Q.38. Derive an expression to calculate time required for completion of zero order reaction.
Ans.

For a zero order reaction:

[R] = [R]₀ - kt

At completion [R] = 0, thus

0 = [R]₀ - kt

so t = [R]₀ / k, which is the time required for complete consumption of reactant at constant rate k.

Q.39. For a reaction A + B → Products, the rate law is - Rate = k[A][B]^3/2. Can the reaction be an elementary reaction? Explain.
Ans.

No. Molecularity of an elementary step must be an integer (1, 2, 3, ...). A fractional order of 3/2 with respect to B indicates a complex mechanism involving intermediate steps; hence the observed rate law cannot correspond to a single elementary collision step.

Q.40. For a certain reaction large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why?
Ans.

Even if many molecules have sufficient energy, reactions require proper orientation during collision for bonds to break and form. If correct orientation is rare, most collisions are ineffective and the overall rate remains slow.

Q.41. For a zero order reaction will the molecularity be equal to zero? Explain.
Ans.

No. Molecularity refers to the number of reactant molecules involved in an elementary step and must be a positive integer. Zero order refers to the dependence of rate on concentration in the observed rate law and does not imply zero molecules participate; molecularity cannot be zero.

Q.42. For a general reaction A → B, plot of concentration of A vs time is given in Fig. 4.3. Answer the following question on the basis of this graph.
(i) What is the order of the reaction?
(ii) What is the slope of the curve?
(iii) What are the units of rate constant?

Ans.

(i) The straight line plot of [A] vs t indicates a zero order reaction, since [A] = -kt + [A]₀.

(ii) Slope = -k.

(iii) Units of k for a zero order reaction are concentration/time, e.g., mol L^-1 s^-1.

Q.43. The reaction between H₂(g) and O₂(g) is highly feasible yet allowing the gases to stand at room temperature in the same vessel does not lead to the formation of water. Explain.
Ans.

The reaction is thermodynamically favourable but has a very large activation energy at room temperature. Without sufficient energy (or a suitable catalyst or ignition), molecules cannot overcome the activation barrier, so the reaction does not proceed appreciably.

Q.44. Why does the rate of a reaction increase with rise in temperature?
Ans.

Raising the temperature increases the fraction of molecules whose kinetic energy exceeds the activation energy (Arrhenius factor) and also increases collision frequency, so more effective collisions occur, increasing the rate.

Q.45. Oxygen is available in plenty in air yet fuels do not burn by themselves at room temperature. Explain.
Ans.

Combustion typically requires overcoming a significant activation energy (initiating bond breaking). At room temperature most molecules lack the necessary energy, so spontaneous ignition does not occur without a spark or other ignition source.

Q.46. Why is the probability of reaction with molecularity higher than three very rare?
Ans.

The simultaneous collision of more than three molecules with correct energies and orientations is statistically very unlikely, making elementary steps of molecularity greater than three extremely rare.

Q.47. Why does the rate of any reaction generally decreases during the course of the reaction?
Ans.

As the reaction proceeds reactant concentrations decrease, so fewer effective collisions occur per unit time; consequently the rate generally decreases.

Q.48. Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.
Ans.

Conversion of diamond to graphite is thermodynamically favourable, but the process is extremely slow because of a very high activation energy. Thus thermodynamic favourability does not guarantee a fast rate.

Q.49. Why in the redox titration of KMnO₄ vs oxalic acid, we heat oxalic acid solution before starting the titration?
Ans.

Heating increases the reaction rate by increasing the fraction of molecules able to overcome activation energy, ensuring the redox reaction proceeds at a measurable rate during titration.

Q.50. Why can't molecularity of any reaction be equal to zero?
Ans.

Molecularity counts the number of molecules participating in an elementary step; at least one molecule must be involved, so molecularity cannot be zero.

Q.51. Why molecularity is applicable only for elementary reactions and order is applicable for elementary as well as complex reactions?
Ans.

Molecularity pertains to a single elementary collision event and is a statement about mechanism; complex reactions consist of multiple elementary steps so an overall molecularity is meaningless. Order is an experimentally observed relationship between rate and concentrations and can be defined for both elementary and complex reactions (for complex reactions it corresponds to the rate-determining step).

Q.52. Why can we not determine the order of a reaction by taking into consideration the balanced chemical equation?
Ans.

The balanced equation gives stoichiometry, not mechanism. Many reactions are complex and proceed via multiple elementary steps; the observed order is governed by the kinetics (rate-determining step) and must be determined experimentally. For example the oxidation of KClO₃ with FeSO₄ and H₂SO₄ appears to be tenth order by stoichiometry but is in fact second order experimentally due to a multistep mechanism.

Matching Type

Note : In the following questions match the items of Column I with appropriate item given in Column II.
Q.53. Match the graph given in Column I with the order of reaction given in Column II.
More than one item in Column I may link to the same item of Column II.

Column I	Column II
(i)	(a) 1st order
(ii)	(b) Zero order
(iii)
(iv)

Ans. (i) → (a) (ii) → (b) (iii) → (b) (iv) → (a)
Solution.

Zero order integrated law: [R] = -kt + [R]₀, which is linear in [R] vs t. First order integrated law: ln[R] = -kt + ln[R]₀, linear in ln[R] vs t. The matching follows the shapes shown in the figures.

Q.54. Match the statements given in Column I and Column II

Ans. (i) → (c) (ii) → (a) (iii) → (d) (iv) → (f) (v) → (b) (vi) → (e)
Solution.

1. Catalyst alters the rate by lowering activation energy (c).
2. Molecularity cannot be fractional or zero (a).
3. Second half-life for first order equals the first (d).
4. e^-Ea/RT gives fraction of molecules with energy ≥ Ea (f).
5. Energetically favourable reactions can be slow due to improper orientation, giving ineffective collisions (b).
6. Area under the Maxwell-Boltzmann curve is constant because total probability = 1 (e).

Q.55. Match the items of Column I and Column II.

Ans. (i) → (b) (ii) → (a) (iii) → (c)
Solution.

1. Diamond converts to graphite extremely slowly under ordinary conditions (b).
2. Instantaneous rate refers to rate at a very short time interval (a).
3. Average rate is measured over a long duration (c).

Q.56. Match the items of Column I and Column II.

Ans. (i) → (b) (ii) → (a) (iii) → (d) (iv) → (c)
Solution.

1. Mathematical expression for rate of reaction is the rate law (b).
2. For zero order rate = k, so rate equals the rate constant (a).
3. Units of k for zero order are same as units of rate (d), e.g., mol L^-1 s^-1.
4. Order of a complex reaction is determined by the molecularity of the slowest step (c).

Assertion And Reason Type

Note: In the following questions a statement of assertion followed by a statement of reason is given.
Choose the correct answer out of the following choices.

1. (i) Both assertion and reason are correct and the reason is correct explanation of assertion.
2. (ii) Both assertion and reason are correct but reason does not explain assertion.
3. (iii) Assertion is correct but reason is incorrect.
4. (iv) Both assertion and reason are incorrect.
5. (v) Assertion is incorrect but reason is correct.

Q.57. Assertion : Order of the reaction can be zero or fractional.
Reason : We cannot determine order from balanced chemical equation.

Ans. (ii)
Solution.

Both statements are true: order can be zero or fractional and the order must be determined experimentally or from mechanism. The reason does not directly explain why order can be zero or fractional, so (ii) is correct.

Q.58. Assertion : Order and molecularity are same.
Reason : Order is determined experimentally and molecularity is the sum of the stoichiometric coefficient of rate determining elementary step.

Ans. (v)
Solution.

The assertion is not generally true - order and molecularity may or may not be the same. The reason is correct: order is experimental, while molecularity pertains to an elementary step. Hence (v).

Q.59. Assertion : The enthalpy of reaction remains constant in the presence of a catalyst.
Reason : A catalyst participating in the reaction, forms different activated complex and lowers down the activation energy but the difference in energy of reactant and product remains the same.

Ans. (i)
Solution.

Both assertion and reason are correct and the reason explains the assertion: catalysts change the pathway and activation energies but not the initial and final states, so ΔH remains unchanged.

Q.60. Assertion : All collision of reactant molecules lead to product formation.
Reason : Only those collisions in which molecules have correct orientation and sufficient kinetic energy lead to compound formation.

Ans. (v)
Solution.

The assertion is false; not all collisions lead to products. The reason is correct: only collisions with adequate energy and correct orientation are effective. Thus (v).

Q.61. Assertion : Rate constants determined from Arrhenius equation are fairly accurate for simple as well as complex molecules.
Reason : Reactant molecules undergo chemical change irrespective of their orientation during collision.

Ans. (iii)
Solution.

The assertion is correct in the sense that Arrhenius fits often provide good estimates, but the reason is incorrect: orientation matters for complex molecules. Therefore (iii).

Long Answer Type Questions

Q.62. All energetically effective collisions do not result in a chemical change. Explain with the help of an example.
Ans.

For a collision to be effective and produce products two main conditions must be satisfied:

• Colliding molecules must possess sufficient kinetic energy (≥ activation energy).
• They must have the proper orientation so that bonds can break and new bonds form.

Example: nucleophilic substitution on bromomethane depends on orientation of the nucleophile approach. If approach is improper, even energetic collisions fail to produce product.

To include the orientation probability a steric factor P is used in collision theory:

Rate, k = P Z_AB e^-Ea/RT

where Z_AB is the collision frequency between A and B and P accounts for the fraction of collisions with correct orientation.

Q.63. What happens to most probable kinetic energy and the energy of activation with increase in temperature?
Ans.

Most probable kinetic energy increases with temperature; the Maxwell-Boltzmann distribution shifts to higher energies and broadens.

Activation energy Ea is a characteristic of the reaction and does not change with temperature. However, the Arrhenius expression k = A e^-Ea/RT shows that the rate constant k increases with temperature. Thus Ea itself remains constant for a given pathway (unless mechanism or catalyst changes).

Q.64. Describe how does the enthalpy of reaction remain unchanged when a catalyst is used in the reaction.
Ans.

A catalyst provides an alternate mechanism involving intermediate complexes that lowers the activation energy(s). The potential energy diagrams for catalysed and uncatalysed reactions have different maxima (activated complexes) but the energies of reactants and products - the initial and final states - are the same. Because enthalpy change ΔH is a state function that depends only on initial and final states, ΔH is unchanged by the catalyst.

Q.65. Explain the difference between instantaneous rate of a reaction and average rate of a reaction.
Ans.

The differences are summarised below:

Q.66. With the help of an example explain what is meant by pseudo first order reaction.
Ans.

Pseudo first order reactions are those where one reactant is present in large excess so its concentration remains effectively constant during the reaction. The observed rate law appears first order in the limiting reactant.

Example (i): Acid-catalysed hydrolysis of ethyl acetate in large excess water:

True rate law: Rate = k' [CH₃COOC₂H₅] [H₂O]

When [H₂O] is very large and effectively constant, k(obs) = k' [H₂O], so Rate = k(obs) [ethyl acetate], which is pseudo first order.

Example (ii): Inversion of cane sugar in excess water:

Rate = k' [sucrose] [H₂O] ≈ k(obs) [sucrose] with k(obs) = k' [H₂O].