Chemical Kinetics (Class 12 NCERT) Revision Notes

 over 100 short question-and-answer pairs covering the essential concepts from the NCERT Class 12 Chemistry Chapter on Chemical Kinetics

Short Q&A for Chemical Kinetics (Class 12 NCERT)

Part 1: Rate of Reaction and Rate Laws

Q. No.	Question	Answer
1	What is Chemical Kinetics?	The branch of chemistry that deals with the rate of reactions and the factors affecting them.
2	Define the Rate of a Reaction.	The change in concentration of a reactant or product per unit time.
3	What is the unit of the Rate of Reaction?	mol L−1 s−1 or M s−1.
4	How is the rate expressed for a reactant (R)?	Rate=−ΔtΔ[R]​ (negative sign indicates decreasing concentration).
5	How is the rate expressed for a product (P)?	Rate=+ΔtΔ[P]​ (positive sign indicates increasing concentration).
6	For the reaction A→2B, how is the overall rate expressed?	Rate=−ΔtΔ[A]​=+21​ΔtΔ[B]​.
7	Define the Rate Law (or Rate Expression).	The mathematical expression that relates the reaction rate to the molar concentration of reactants.
8	Write the general form of the Rate Law for aA+bB→Products.	Rate=k[A]x[B]y (where x and y are experimentally determined).
9	Define the Rate Constant (k).	The rate of reaction when the concentration of each reactant is unity (1 M).
10	Define the Order of a Reaction.	The sum of the exponents (x+y) of the concentration terms in the rate law expression.
11	Can the order of a reaction be fractional?	Yes, it is an experimentally determined value.
12	Can the order of a reaction be zero?	Yes (Zero Order Reaction).
13	What is the unit of the rate constant (k) for a Zero Order Reaction?	mol L−1 s−1 (same as the rate).
14	What is the unit of the rate constant (k) for a First first-order reaction?	s−1 (or time−1).
15	What is the unit of the rate constant (k) for a Second Order Reaction?	L mol−1 s−1.
16	Define the Molecularity of a reaction.	The number of reacting species (atoms, ions, or molecules) taking part in an elementary step.
17	Can molecularity be fractional or zero?	No, it must be a whole number (typically 1,2, or 3).
18	For a complex reaction, what determines the molecularity?	Molecularity is defined only for elementary steps.
19	For an elementary reaction, how are order and molecularity related?	They are equal (Order = Molecularity).
20	What is the Rate Determining Step (RDS)?	The slowest step in a reaction mechanism, which governs the overall reaction rate.
21	What is a Pseudo First Order Reaction?	A second-order reaction made to behave like a first-order reaction by taking one reactant in large excess (e.g., hydrolysis of ester).

Part 2: Integrated Rate Equations and Half-Life

Q. No.	Question	Answer
22	Write the integrated rate equation for a Zero Order Reaction.	[R]=−kt+[R]0​ (where [R]0​ is initial concentration).
23	Write the integrated rate equation for a First Order Reaction.	k=t2.303​log[R][R]0​​ (or ln[R]=−kt+ln[R]0​).
24	What is the graphical representation of a Zero Order Reaction?	A straight line when [R] vs t is plotted (slope =−k).
25	What is the graphical representation of a First Order Reaction?	A straight line when log[R] vs t is plotted (slope =−k/2.303).
26	Define Half-Life (t1/2​)	The time required for the concentration of a reactant to be reduced to one half of its initial value.
27	Write the half-life equation for a Zero Order Reaction.	t1/2​=2k[R]0​​.
28	Is the t1/2​ of a zero order reaction dependent on initial concentration?	Yes, it is directly proportional to [R]0​.
29	Write the half-life equation for a First Order Reaction.	t1/2​=k0.693​.
30	Is the t1/2​ of a first order reaction dependent on initial concentration?	No, it is independent of [R]0​.
31	How many half-lives are required for a first order reaction to be 99.9% complete?	Approximately ten half-lives (≈10t1/2​).
32	What is the formula to calculate the time for 99% completion of a first-order reaction?	t99%​=2×t90%​ (since log100≈2log10).
33	What is the order of radioactive decay reactions?	First Order.

Part 3: Temperature Dependence and Collision Theory

Q. No.	Question	Answer
34	How does increasing the temperature generally affect the rate of a reaction?	Increases the rate (often doubles for every 10 K rise).
35	Define Temperature Coefficient.	The ratio of the rate constant at temperature T+10 K to the rate constant at temperature T.
36	Define Activation Energy (Ea​).	The minimum extra amount of energy absorbed by the reactant molecules to form the activated complex.
37	Define the Threshold Energy.	The minimum energy that the colliding molecules must possess for the reaction to occur.
38	Write the relationship between Ethreshold​,Ea​, and Ereactant​.	Ethreshold​=Ea​+Ereactant​.
39	Who proposed the relationship between k and T?	Arrhenius.
40	Write the Arrhenius Equation.	k=Ae−Ea​/RT (A is Arrhenius factor).
41	In the Arrhenius equation, what does 'A' represent?	Frequency Factor or Pre-exponential Factor (related to collision frequency).
42	What is the graphical representation of the Arrhenius equation?	A straight line when lnk (or logk) vs 1/T is plotted (slope =−Ea​/R or −Ea​/2.303R).
43	How can Ea​ be determined experimentally?	By plotting logk vs 1/T and calculating the slope.
44	State the Collision Theory.	Reactant molecules must collide with sufficient kinetic energy (>Ea​) and in the proper orientation to react.
45	Define Collision Frequency (Z).	The number of collisions per second per unit volume of the reaction mixture.
46	Define the Probability Factor (P) or Steric Factor.	The factor that accounts for the requirement of proper orientation during collision.
47	How is the rate constant k expressed according to the Collision Theory?	k=PZAB​e−Ea​/RT (where ZAB​ is collision frequency).
48	What is the effect of a catalyst on Ea​ and the reaction rate?	It lowers Ea​ by providing an alternative path, thereby increasing the reaction rate.
49	Does a catalyst affect the ΔH (enthalpy) of the reaction?	No.
50	Does a catalyst shift the equilibrium constant (K)?	No, it speeds up both forward and reverse reactions equally.

Part 4: Numerical and Conceptual (Mixed)

Q. No.	Question	Answer
51	If the rate triples when the concentration of A doubles, what is the order of the reaction w.r.t A?	log3/log2≈1.58 (Fractional order).
52	A reaction is second order in A and first order in B. What is the overall order?	3 (2+1).
53	For a first-order reaction, what fraction remains after 3t1/2​?	1/8 (Fraction remaining=(1/2)n).
54	A reaction has a rate constant k=10−2 s−1. What is its half-life?	69.3 s (t1/2​=0.693/k).
55	What is the slope of the plot logk vs 1/T?	−Ea​/2.303R.
56	If logk2​−logk1​=0.5 and 1/T1​−1/T2​=10−3, calculate Ea​ using R=8.314 J.	Ea​≈95.7 kJ/mol (Using log(k2​/k1​)=Ea​/2.303R⋅(T2​−T1​)/T1​T2​).
57	What is the value of the rate when [R]=[R]0​/2 for a zero order reaction?	k (Rate is always constant, independent of concentration).
58	What is the value of the rate when [R]=[R]0​/2 for a first order reaction?	k[R]0​/2 (Rate is halved).
59	How can we experimentally determine the order of a reaction?	Using the Initial Rate Method or Integrated Rate Method.
60	What happens to the half-life of a second order reaction when the initial concentration is doubled?	Half-life is halved (t1/2​∝1/[R]0​).
61	What is the significance of the fact that the order of a reaction is zero?	The reaction rate is independent of the concentration of that reactant.
62	In a gaseous phase reaction, how is the rate expressed in terms of pressure?	Rate=kPx (Concentration is proportional to pressure).
63	Name a technique used to monitor the rate of a reaction over time.	Spectrophotometry (measuring color change) or Titration.

Part 5: Detailed Concepts and Complex Reactions

Q. No.	Question	Answer
64	What does a molecularity greater than three indicate?	Such reactions are rare/impossible due to low probability of simultaneous collision.
65	What is an Elementary Reaction?	A reaction that occurs in one single step.
66	What is a Complex Reaction?	A reaction that occurs in a sequence of elementary steps.
67	What is the overall rate law determined by in a complex reaction?	The rate of the slowest step (RDS).
68	What is the common example of a reaction that is bimolecular but pseudo-first order?	Hydrolysis of an ester (e.g., CH3​COOC2​H5​).
69	Why is H2​+Cl2​→2HCl a zero order reaction w.r.t H2​ under photochemical conditions?	It proceeds via a chain mechanism, and the rate is limited by light intensity.
70	In the rate law, what does the exponent of a reactant generally indicate?	The number of moles of that reactant involved in the rate-determining step.
71	What is the effect of changing the catalyst concentration on the reaction rate?	No effect (the catalyst is regenerated and usually not consumed).
72	What is the physical basis for the Arrhenius frequency factor (A)?	It is the rate at which collisions occur at infinite temperature.
73	Which reaction order is suitable for determining the concentration of reactants after a fixed time?	First Order (since k is independent of [R]0​).
74	What is the graphical area under the distribution curve of molecular energies that is relevant to kinetics?	The area corresponding to molecules having energy greater than Ea​.
75	How does increasing temperature affect the number of effective collisions?	It increases the number of molecules possessing energy ≥Ea​, thus increasing effective collisions.

Part 6: Application and Specifics

Q. No.	Question	Answer
76	Name an industrial process whose rate is controlled by a catalyst.	Haber Process (NH3​ synthesis) or Contact Process (H2​SO4​ synthesis).
77	How is the shelf life of a medicine related to t1/2​?	Longer t1/2​ usually means a longer shelf life (slow decay).
78	What term describes the decay rate in a radioactive sample?	Activity (proportional to the number of radioactive nuclei).
79	What is the relationship between the slope of logk vs 1/T and the exothermic/endothermic nature of the reaction?	No direct relationship; the slope depends only on Ea​.
80	What happens to the rate constant k if the concentration of the reactant is doubled?	Remains unchanged ( k is constant at fixed T).
81	Why must an effective collision possess proper orientation?	To ensure the atoms meant to form new bonds come close to each other.
82	If Ea​ for the forward reaction is 50 kJ/mol and ΔH=−10 kJ/mol, what is Ea​ for the backward reaction?	60 kJ/mol (Ea​(b)=Ea​(f)−ΔH).
83	What are the typical physical states of reactants in heterogeneous catalysis?	Reactants are typically gas/liquid, and the catalyst is a solid.
84	What kind of process is the adsorption of reactants onto a solid catalyst surface?	Exothermic (heat is released).
85	What type of process is the desorption of products from a solid catalyst surface?	Endothermic (energy is required).
86	What does the integrated rate law allow us to calculate?	The concentration of reactants at any given time t.
87	How is the Arrhenius Factor (A) related to the Entropy of Activation?	A is proportional to eΔS‡/R.
88	What is the maximum order a reaction can theoretically have?	No theoretical limit, but practically rarely exceeds 3.
89	Is the hydrolysis of sugar in dilute acid a zero, first, or second order reaction?	Pseudo-First Order.
90	What are the units for the reaction rate in terms of pressure?	bar s−1 or atm s−1.
91	How does an increase in surface area affect the rate of heterogeneous reactions?	Increases the reaction rate.
92	Define the term Reaction Intermediate.	A species formed during the course of a reaction that is highly unstable and generally cannot be isolated.
93	What is the relationship between k and Ea​?	Smaller Ea​ means larger k and a faster reaction.
94	What does the slope of the k vs T graph look like for most reactions?	It increases (rate constant increases faster with T).
95	What is the half-life used for in Archaeology?	Carbon Dating (C−14 radioactive decay).
96	Does the initial concentration affect the rate constant k?	No.
97	Does the concentration affect the half-life of a second order reaction?	Yes.
98	Is molecularity always defined for a zero order reaction?	No, only for its elementary steps.
99	How can a zero order reaction be identified graphically?	The plot of [R] vs t is a straight line.
100	What is the significance of the y-intercept in the logk vs 1/T plot?	It is equal to logA (Arrhenius factor).
101	If Ea​=0, how does the rate constant change with temperature?	Rate constant does not change with temperature (k=A).
102	What is the overall order of the decomposition of HI on a gold surface?	Zero Order (since it's a surface reaction, rate depends on surface coverage).
103	What is the physical significance of the constant 0.693 in the t1/2​ formula?	It is equal to ln2.
104	Give an example of a unimolecular elementary reaction.	Decomposition of N2​O5​ (N2​O5​→NO2​+NO3​).
105	What is the condition for an elementary reaction to be termed termolecular?	Three molecules must simultaneously collide (very rare).
106	Why is the order of reaction determined experimentally and not from the balanced equation?	The balanced equation does not show the mechanism or the rate-determining step.
107	What is the term for the process of determining the overall order from the sum of exponents in the rate law?	Summing the exponents (Order =x+y+…).
108	Is the concentration of the catalyst included in the final rate law expression?	Yes, if the catalyst participates in the rate-determining step.
109	What is the relationship between the half-life and the rate of decay for a first-order process?	Inverse relationship (k∝1/t1/2​).
110	Why are complex reactions generally of lower order than their molecularity?	Because the overall rate is limited by the slowest step, not the total number of colliding species.

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