Quick answer: A chemical kinetics NEET PYQ analysis shows that NTA asks about two questions a year from this chapter, and most are short numericals from four areas: the Arrhenius equation and activation energy, first order reaction half-life and time calculations, finding order from units of the rate constant or rate data, and the relation between rates of different species. The same formulas solve almost every past question.
This guide breaks down what NTA repeats, the formulas and graphs you must know, and solved PYQ-pattern numericals. For the full Physical Chemistry plan, see our NEET 2027 Physical Chemistry guide.
Want to test yourself first? Solve the Chemical Kinetics PYQs for free, then use this guide to fix the gaps.
Table of Contents
Why Chemical Kinetics Is a High-Return Chapter
The chapter has a small formula set and fixed question types, so a few hours of practice can secure its marks. If you want the concepts explained from scratch first, read our Chemical Kinetics study guide and our Chemical Kinetics class 12 notes. To see where it sits among other chapters, check the NEET 2027 chapter weightage.
Students often confuse kinetics with equilibrium: kinetics tells you how fast a reaction goes, equilibrium tells you how far. Our Equilibrium PYQ analysis covers the other half.
Chemical Kinetics NEET PYQ Analysis: What NTA Repeats
When you sort the past NEET chemistry PYQs from this chapter, five question types cover nearly everything.
| Question type | Frequency | What it tests |
|---|---|---|
| Arrhenius equation and activation energy | Most frequent | Two-temperature calculations, ln k vs 1/T graph, effect of catalyst |
| First order reaction | Very frequent | Half-life, time for a given fraction, integrated rate law |
| Order from units or rate data | Very frequent | Units of the rate constant, method of initial rates |
| Zero order reaction | Regular | Integrated law, half-life, graphs |
| Rate expressions and molecularity | Regular | Relating rates of reactants and products, order vs molecularity |
For a quick warm-up, try these five quick chemical kinetics questions.
Rate of Reaction and Rate Expressions
For a reaction aA + bB → cC + dD:
Rate = −(1/a) d[A]/dt = −(1/b) d[B]/dt = +(1/c) d[C]/dt = +(1/d) d[D]/dt
Example: for 2N₂O₅ → 4NO₂ + O₂,
−(1/2) d[N₂O₅]/dt = (1/4) d[NO₂]/dt = d[O₂]/dt
NTA often gives one rate and asks for another, so divide each rate by its coefficient.
Order vs Molecularity
| Feature | Order | Molecularity |
|---|---|---|
| Defined as | Sum of powers of concentration in the experimental rate law | Number of species colliding in an elementary step |
| Found by | Experiment | Theory (from the mechanism) |
| Can be zero or fractional? | Yes | No, always a whole number (1, 2 or rarely 3) |
| Applies to | Elementary and complex reactions | Elementary reactions only |
For a complex reaction, the slowest step is the rate-determining step, and its molecularity decides the order.
Units of the Rate Constant
General formula: units of k = (mol L⁻¹)^(1 − n) s⁻¹, where n is the order.
| Order | Units of rate constant |
|---|---|
| Zero | mol L⁻¹ s⁻¹ |
| First | s⁻¹ |
| Second | L mol⁻¹ s⁻¹ |
| Third | L² mol⁻² s⁻¹ |
Zero and First Order: The Formula Table
| Feature | Zero order | First order |
|---|---|---|
| Integrated rate law | [R] = [R]₀ − kt | k = (2.303/t) log([R]₀/[R]) |
| Half-life | t½ = [R]₀ / 2k | t½ = 0.693 / k |
| Half-life depends on [R]₀? | Yes, directly proportional | No, independent |
| Straight-line graph | [R] vs t, slope = −k | ln[R] vs t, slope = −k |
| Example | Decomposition of NH₃ on hot platinum at high pressure | Radioactive decay, decomposition of N₂O₅ |
Time-saving shortcuts for first order kinetics:
- Time for 75% completion = 2 × t½
- Time for 87.5% completion = 3 × t½
- Time for 99.9% completion ≈ 10 × t½
- A plot of log([R]₀/[R]) vs t is a straight line through the origin with slope k/2.303.
Pseudo first order reactions are second order reactions that behave as first order because one reactant is in large excess, such as the acid hydrolysis of ethyl acetate and the inversion of cane sugar.
The Arrhenius Equation
k = A e^(−Ea/RT)
| Form | Use |
|---|---|
| ln k = ln A − Ea/RT | A plot of ln k vs 1/T is a straight line with slope −Ea/R |
| log k = log A − Ea/(2.303RT) | Slope of log k vs 1/T = −Ea/(2.303R) |
| log(k₂/k₁) = [Ea/(2.303R)] × [(T₂ − T₁)/(T₁T₂)] | Two-temperature questions |
Facts that come back again and again:
- For many reactions, a 10 °C rise roughly doubles the rate.
- A catalyst lowers the activation energy, so it speeds up both forward and backward reactions. It does not change ΔG or the equilibrium constant.
- In collision theory, rate = P Z_AB e^(−Ea/RT), where P is the steric (probability) factor that accounts for proper orientation.
Worked PYQ-Pattern Questions
Q1. A first order process has k = 0.0693 min⁻¹. How long does it take to reach 75% completion?
t½ = 0.693/k = 0.693/0.0693 = 10 min. For 75% completion, t = 2 × t½.
Answer: 20 minutes.
Q2. The k of a reaction has units L mol⁻¹ s⁻¹. The order of the reaction is:
Using (mol L⁻¹)^(1 − n) s⁻¹: (mol L⁻¹)⁻¹ means 1 − n = −1, so n = 2.
Answer: Second order.
Q3. When the concentration of A is doubled, the rate increases four times. The order with respect to A is:
2ⁿ = 4, so n = 2.
Answer: 2.
Q4. The rate of a reaction doubles when the temperature rises from 300 K to 310 K. Find Ea (R = 8.314 J K⁻¹ mol⁻¹, log 2 = 0.301).
log 2 = [Ea/(2.303 × 8.314)] × [10/(300 × 310)]
Ea = 0.301 × 2.303 × 8.314 × 9300 ≈ 53,600 J mol⁻¹
Answer: About 53.6 kJ mol⁻¹.
Q5. For a zero order reaction, the half-life is: (a) Independent of initial concentration (b) Proportional to initial concentration (c) Inversely proportional to initial concentration (d) Proportional to the square of initial concentration
Answer: (b). t½ = [R]₀/2k, so it is directly proportional to the initial concentration.
Q6. In 2N₂O₅ → 4NO₂ + O₂, if d[O₂]/dt = 1.0 × 10⁻⁴ mol L⁻¹ s⁻¹, what is d[NO₂]/dt?
(1/4) d[NO₂]/dt = d[O₂]/dt, so d[NO₂]/dt = 4 × 1.0 × 10⁻⁴.
Answer: 4.0 × 10⁻⁴ mol L⁻¹ s⁻¹.
Common Traps in This Chapter
- Saying the first order half-life depends on initial concentration
- Thinking a catalyst changes the equilibrium constant or ΔG
- Giving molecularity as zero or a fraction
- Forgetting to divide by stoichiometric coefficients in rate expressions
- Using ln and log interchangeably without the 2.303 factor
- Writing the slope of ln k vs 1/T as +Ea/R instead of −Ea/R
Practise Every Chemical Kinetics PYQ
This chemical kinetics NEET PYQ analysis shows you where the marks are. Solving real past questions makes the formulas automatic. Every Chemical Kinetics previous year question is available chapter-wise, free:
👉 Solve all Chemical Kinetics NEET PYQs
Keep the formula table beside you for the first round, then solve the second round without it.
Conclusion
Chemical Kinetics rewards practice more than reading. Use this chemical kinetics NEET PYQ analysis to master the Arrhenius equation, first order half-life shortcuts, units of the rate constant and rate expressions. Then solve the past numericals until each one takes under a minute.
FAQ
Q: How many questions come from Chemical Kinetics in NEET? A: Usually about two questions a year, mostly short numericals.
Q: What is the most important topic in Chemical Kinetics for NEET? A: The Arrhenius equation and activation energy calculations are the most tested, followed by first order half-life and finding the order of a reaction.
Q: Does the half-life of a first order reaction depend on initial concentration? A: No. t½ = 0.693/k, so it is independent of the initial concentration. For a zero order reaction, it is directly proportional to it.
Q: Does a catalyst change the equilibrium constant? A: No. A catalyst lowers the activation energy and speeds up both forward and backward reactions equally, so the equilibrium constant and ΔG stay the same.
Q: How do I find the order of a reaction from the units of k? A: Use units of k = (mol L⁻¹)^(1 − n) s⁻¹. For example, s⁻¹ means first order and L mol⁻¹ s⁻¹ means second order.
Q: Where can I practise Chemical Kinetics NEET PYQs for free? A: You can solve every Chemical Kinetics previous year question chapter-wise for free using the practice links in this article.