Chemical Kinetics JEE Main PYQs (2020–2026)

Solved Chemical Kinetics PYQs

3 recent questions with answers. The full set of 159 is free to practise.

1. JEE Main 2026EasyIntegrated Rate Equations for Zero, First and nth Order Reactions
   Q1

   Consider the first order reaction $R\to P$ .
   
   The fraction of molecules decomposed in the given first order reaction can be expressed as

   • A$1-e^{k_{1}t}$
   • B$1+e^{k_{1}t}$
   • C$1+e^{-k_{1}t}$
   • D$1-e^{-k_{1}t}$

   Show answer & solution

   Correct answer: (D)

   For a first order reaction
   
   $$R\to P$$
   
   the integrated rate law is
   
   $$[R{]}_t=[R{]}_0{e}^{-k_{1}t}$$
   
   where:
   
   $[R{]}_0$ = initial concentration of reactant
   
   $[R{]}_t$ = concentration after time $t$
   
   Now, the fraction of molecules left undecomposed after time $t$ is
   
   $$\frac{[R{]}_t}{[R{]}_0}=e^{-k_{1}t}$$
   
   So, the fraction of molecules decomposed is
   
   $$1-\frac{[R{]}_t}{[R{]}_0}=1-e^{-k_{1}t}$$
   
   Hence, the correct answer is
   
   $$\boxed{1-e^{-k_{1}t}}$$
   
   So, Option D is correct.
2. JEE Main 2026MediumTemperature Dependence of Rate: Arrhenius Equation & Activation Energy
   Q2

   At $27^{\circ }C$ in presence of a catalyst, activation energy of a reaction is lowered by $10kJ{mol}^{-1}$ . The logarithm of ratio of $\frac{k\text{ (catalysed) }}{k\text{ (uncatalysed) }}$ is….
   
   (Consider that the frequency factor for both the reactions is same)

   • A1.741
   • B0.1741
   • C17.41
   • D3.482

   Show answer & solution

   Correct answer: (A)

   Arrhenius equation (NCERT):
   
   $$k=A{e}^{-\frac{E_a}{RT}}$$
   
   Given: catalyst lowers activation energy by $10{\text{kJ mol}}^{-1}=10000{\text{J mol}}^{-1}$ and frequency factor $A$ is same.
   
   So,
   
   $$\frac{k_{\text{cat}}}{k_{\text{uncat}}}=\frac{A{e}^{-\frac{E_{a,\text{cat}}}{RT}}}{A{e}^{-\frac{E_{a,\text{uncat}}}{RT}}}=e^{\frac{E_{a,\text{uncat}}-E_{a,\text{cat}}}{RT}}=e^{\frac{10000}{RT}}$$
   
   At $27^{\circ }\text{C}$ , $T=300\text{K}$ and $R=8.314{\text{J mol}}^{-1}{\text{K}}^{-1}$ :
   
   $$\frac{10000}{RT}=\frac{10000}{8.314\times 300}\approx 4.009$$
   
   So,
   
   $${\log }_{10}\left(\frac{k_{\text{cat}}}{k_{\text{uncat}}}\right)={\log }_{10}\left(e^{4.009}\right)=\frac{4.009}{2.303}\approx 1.741$$
   
   Correct option: A (1.741)
3. JEE Main 2026EasyRate Law, Order and Molecularity of a Reaction
   Q3

   Observe the following reactions at $T(K)$ .
   
   I. $A\to$ products.
   
   $$5{Br}^{-}(aq)+{BrO}_3^{-}(aq)+6H^{+}(aq)$$
   
   $$\to 3{Br}_2(aq)+3H_{2}O(l)$$
   
   Both the reactions are started at 10.00 am . The rates of these reactions at 10.10 am are same. The value of $-\frac{\Delta \left[{Br}^{-}\right]}{\Delta t}$ at 10.10 am is $2\times 10^{-4}mol{L}^{-1}{min}^{-1}$ . The concentration of A at 10.10 am is $10^{-2}mol{L}^{-1}$ . What is the first order rate constant (in ${\min }^{-1}$ ) of reaction $I$ ?

   • A$4\times 10^{-3}$
   • B$2\times 10^{-3}$
   • C$10^{-3}$
   • D$10^{-2}$

   Show answer & solution

   Correct answer: (A)

   At $t=10$ minutes (i.e., at 10:10 am)
   
   For reaction II, $5{Br}^{-}$ are consumed when the reaction proceeds once. So, the rate of reaction is related to the rate of decrease of $\left[{Br}^{-}\right]$ as:
   
   $$\begin{array}{rl}& \text{ Rate of reaction }=-\frac{1}{5}\frac{\Delta \left[{Br}^{-}\right]}{\Delta t}=\frac{1}{5}\times \left(2\times 10^{-4}\right)\\ & =4\times 10^{-5}\end{array}$$
   
   Now consider reaction I: $A\to$ products.
   
   At $t=10$ minutes, both reactions have the same rate. So, the rate of reaction I at this time is also $4\times 10^{-5}mol{L}^{-1}mi{n}^{-1}$ .
   
   Since reaction I is first order, its rate law is:
   
   Rate of reaction $=k[A]$
   
   Given $[A]=10^{-2}mol{L}^{-1}$ at 10:10 am, we write:
   
   Rate of reaction $=4\times 10^{-5}=k[A]$
   
   So,
   
   $$k=\frac{4\times 10^{-5}}{10^{-2}}=4\times 10^{-3}{min}^{-1}.$$

Topic-wise question count

Where JEE Main has asked from Chemical Kinetics, most frequent first.

• Temperature Dependence of Rate: Arrhenius Equation & Activation Energy47
• Half-Life of a Reaction and Pseudo First Order Reactions33
• Integrated Rate Equations for Zero, First and nth Order Reactions31
• Rate Law, Order and Molecularity of a Reaction28
• Rate of Reaction (Average & Instantaneous) & Factors Affecting Rate10
• Radioactive Decay Kinetics, Half-Life & Nuclear Reactions10

Year-wise Chemical Kinetics questions in JEE Main

Number of questions from this chapter in each year's paper(s).