Chemical Kinetics Class 12 Notes

• Instantaneous rate is found from the slope of concentration vs time graph.
• Units of rate are mol L⁻¹ s⁻¹ (or pressure per time for gases).

• $\text{Order}:$
• the sum of the exponents in the rate law (x + y); it can be zero, fractional or negative and must be found by experiment.
• $\text{Molecularity}:$
• the number of reactant molecules in the slowest step; always a whole number and can never be zero or fractional.

• $\text{Zero order}:$
• $[A]=[A{]}_0-kt,t_{1/2}=\frac{[A{]}_0}{2k}$
• .
• $\text{Second order}:$
• $\frac{1}{[A]}=\frac{1}{[A{]}_0}+kt,t_{1/2}=\frac{1}{k[A{]}_0}$
• .

Radioactive decay and most unimolecular gas decompositions follow first-order kinetics, so the same equations apply.

Example: The half-life of a first-order reaction is 346.5 s. Find the rate constant and the time to complete 90% of the reaction.

Solution: $k=\frac{0.693}{346.5}=2\times 10^{-3}{\text{s}}^{-1}$. For 90% completion $\ln \frac{100}{10}=kt\Rightarrow t=\frac{\ln 10}{2\times 10^{-3}}\approx 1151\text{s}$.

Exercises (NCERT Q1–Q4)

• 1) For the reaction R → P, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds.
• 2) In a reaction, 2A → Products, the concentration of A decreases from 0.5 mol L⁻¹ to 0.4 mol L⁻¹ in 10 minutes. Calculate the rate during this interval.
• 3) The decomposition of NH3 on platinum surface is zero order reaction. What are the rates of production of N2 and H2 if k = 2.5 × 10⁻⁴ mol L⁻¹ s⁻¹?
• 4) The decomposition of dimethyl ether leads to the formation of CH4, H2 and CO and the reaction rate is given by Rate = k [CH3OCH3]^(3/2). If the pressure is measured in bar and time in minutes, what are the units of rate and rate constant?

Answers at a glance

1. 1
   Q1 → Average rate = 4.0 × 10⁻⁴ M min⁻¹ = 6.7 × 10⁻⁶ M s⁻¹
2. 2
   Q2 → Rate = 5.0 × 10⁻³ mol L⁻¹ min⁻¹
3. 3
   Q3 → Rate of N2 formation = 1.25 × 10⁻⁴ mol L⁻¹ s⁻¹; Rate of H2 formation = 3.75 × 10⁻⁴ mol L⁻¹ s⁻¹
4. 4
   Q4 → Unit of rate = bar min⁻¹; Unit of rate constant = bar^(-1/2) min⁻¹

MCQ Questions

1 Mark Questions

1. 1
   Define rate of a reaction.
2. 2
   What is the order of a reaction?
3. 3
   Write the integrated rate equation for a zero order reaction.

2 Mark Questions

1. 1
   For the reaction 2A + B → A2B, the rate = k[A][B]2 with k = 2.0 × 10−6 M−2 s−1. Calculate the initial rate when [A] = 0.1 M, [B] = 0.2 M.
2. 2
   Show that in a first order reaction, time required for completion of 99.9% is 10 times the half‑life.

3 Mark Questions

1. 1
   Derive the integrated rate equation for a first order reaction.
2. 2
   Explain the effect of temperature on the rate constant with the help of Arrhenius equation.

5 Mark Questions

1. 1
   The decomposition of dimethyl ether leads to the formation of CH4, H2 and CO and the reaction rate is given by Rate = k [CH3OCH3]3/2. If the rate is monitored by increase in pressure in a closed vessel, derive the expression for the rate in terms of total pressure of the system.
2. 2
   For the first order thermal decomposition of N2O5(g) at constant volume, 2N2O5(g) → 4NO2(g) + O2(g), calculate the rate constant when the initial pressure of N2O5 is 0.2 atm and after 50 s it becomes 0.182 atm.

Assignments walk the chapter's topics in order; the tests follow the current CBSE blueprint (MCQ/VSA, 2M, 3M and 5M questions) with answer keys. They will appear here as direct download links.