Chemical Kinetics for NEET: Unit 8 Rates, Graphs and Traps
Chemical kinetics is the branch of chemistry that studies how fast reactions happen, what controls that speed and by which steps they run. In NEET it is Chemistry Unit 8, and it is still in the syllabus. This guide shows exactly what the unit asks, works every formula with numbers from your NCERT book and lists the traps that cost marks.
Is chemical kinetics in the NEET 2027 syllabus?
Yes. Chemical kinetics is Unit 8 of the Chemistry section in the NMC syllabus for NEET (UG) 2026, dated 22 December 2025. It is the last unit of the physical chemistry block, after Redox Reactions and Electrochemistry (Unit 7). No 2027 syllabus exists yet, so this file is the current text.
The NEET (UG) 2026 Information Bulletin says in its FAQ that there was no change in the syllabus of NEET (UG) 2026 compared with the previous year. As of 31 October 2026, the NEET (UG) public notices list nothing about the 2027 cycle. Some websites already print 2027 dates, but none of them is NTA. Learn the current wording and check it again the day NTA releases the 2027 bulletin.
Worried a chapter was cut? Our NEET 2027 deleted syllabus guide goes through the full list. Chemical kinetics is not on it.
What does Unit 8 ask you to know?
Unit 8 is short, and it names almost everything it wants. That makes it one of the easiest units to turn into a checklist. Here is the official text, unpacked line by line.
| Syllabus phrase (Unit 8) | What it means in practice | What to be able to do |
|---|---|---|
| Rate of a chemical reaction | Change in concentration per unit time | Write the rate for any balanced equation, dividing by the coefficients |
| Factors affecting the rate: concentration, temperature, pressure and catalyst | The four levers NCERT discusses | Predict the direction of each effect and explain it |
| Elementary and complex reactions | One-step reactions versus multi-step mechanisms | Spot an intermediate and the rate determining step |
| Order and molecularity of reactions | Two different numbers that students mix up | Find the order from a rate law; state the molecularity of an elementary step |
| Rate law, rate constant and its units | Rate = k[A]x[B]y, with x and y found by experiment | Derive the units of k for any order |
| Differential and integral forms of zero and first order reactions | The two integrated rate laws | Use [R] = [R]0 - kt and k = (2.303/t) log([R]0/[R]) |
| Their characteristics and half-lives | t1/2 = [R]0/2k and t1/2 = 0.693/k | Read a half-life off a graph or a table |
| Effect of temperature, Arrhenius theory, activation energy and its calculation | k = A e^(-Ea/RT) | Find Ea from k at two temperatures |
| Collision theory of bimolecular gaseous reactions (no derivation) | Collision frequency, threshold energy and orientation | Explain why only some collisions work |
Two details in that wording matter. First, the integrated laws stop at first order, so you will not be asked to integrate a second order rate law. Second, collision theory carries "no derivation" in brackets: learn the idea and the equation, not a proof. The pseudo first order reaction is not named in Unit 8, but NCERT teaches it inside the first order section, so keep it.
How is the rate of a reaction measured?
The rate of a reaction is the decrease in concentration of a reactant, or the increase in concentration of a product, per unit time. With concentration in mol L-1 and time in seconds, its unit is mol L-1 s-1. For gases it can be written with partial pressures, in atm s-1.
The NCERT Chemical Kinetics chapter, Unit 3 of Class 12 Chemistry Part 1, separates two kinds of rate:
- Average rate is the change over an interval, rav = -Δ[R]/Δt. In NCERT's butyl chloride table it falls from 1.90 × 10^-4 to 0.4 × 10^-4 mol L-1 s-1 as the reactant gets used up.
- Instantaneous rate is the slope of the tangent to the concentration curve at one moment, rinst = -d[R]/dt. NCERT reads 5.12 × 10^-5 mol L-1 s-1 at 600 s from its graph.
When the coefficients are not one, divide each rate by its coefficient so that every species gives the same number. For 2HI giving H2 and I2, the rate is -(1/2) Δ[HI]/Δt = Δ[H2]/Δt = Δ[I2]/Δt. NCERT's N2O5 example (Example 3.2) uses the same rule: the average rate of 2N2O5 giving 4NO2 and O2 is 6.79 × 10^-4 mol L-1 min-1, so NO2 forms four times faster, at 2.72 × 10^-3 mol L-1 min-1.
Rate law, order and molecularity: what is the difference?
The rate law links the rate to the concentrations of the reactants: Rate = k[A]x[B]y. The order is x + y, found by experiment. The molecularity is the number of species that collide in one elementary step. Order can be zero or a fraction; molecularity is always 1, 2 or 3.
NCERT's Table 3.2 shows how the order is found. Doubling [NO] at fixed [O2] raises the initial rate from 0.096 to 0.384 mol L-1 s-1, four times, so the reaction is second order in NO. Doubling [O2] doubles the rate, so it is first order in O2. The rate law is Rate = k[NO]2[O2], third order overall. With the first row's numbers, k = 0.096 / (0.30² × 0.30) = 3.56 L2 mol-2 s-1 (our arithmetic).
That example happens to match the balanced equation. Most do not. NCERT gives CHCl3 + Cl2, whose rate law is k[CHCl3][Cl2]^1/2, order 1.5. And it gives KClO3 with FeSO4 and H2SO4: ten molecules in the equation, yet a second order reaction, because it runs in several steps. The rule to remember: the rate law cannot be read from the balanced equation; it must be found by experiment.
| Point | Order | Molecularity |
|---|---|---|
| Defined for | Elementary and complex reactions | Elementary reactions only |
| Found by | Experiment, from the rate law | The number of colliding species in one step |
| Possible values | 0, 1, 2, 3 or a fraction | 1, 2 or 3 only, never zero or a fraction |
| In a complex reaction | Set by the slowest step | Has no meaning for the overall reaction |
The slowest step is the rate determining step. In NCERT's decomposition of H2O2 with iodide, step one (H2O2 + I- giving H2O + IO-) is slow and decides the rate, and IO- is the intermediate that appears in neither the reactants nor the products.
The chemical kinetics formula sheet for NEET
These are the relations Unit 8 names, with the condition that decides when each one works. All of them come from the NCERT chapter linked above.
| Quantity | Formula | Condition or trap |
|---|---|---|
| Rate law | Rate = k[A]x[B]y | x and y from experiment, not from coefficients |
| Zero order, integrated | [R] = [R]0 - kt | Straight line of [R] against t, slope -k |
| Zero order, half-life | t1/2 = [R]0 / 2k | Grows with the starting concentration |
| First order, integrated | k = (2.303/t) log([R]0/[R]) | Same as [R] = [R]0 e^(-kt) |
| First order, two times | k = 2.303/(t2 - t1) × log([R]1/[R]2) | Use when t = 0 is not given |
| First order, half-life | t1/2 = 0.693/k | Independent of the starting concentration |
| First order gas reaction | k = (2.303/t) log(pi / (2pi - pt)) | For A(g) giving B(g) + C(g) at constant volume |
| Arrhenius equation | k = A e^(-Ea/RT) | Ea in J mol-1, so R = 8.314 J K-1 mol-1 |
| Arrhenius, two temperatures | log(k2/k1) = Ea/(2.303R) × (T2 - T1)/(T1T2) | Temperatures in kelvin, always |
| Collision theory | Rate = P ZAB e^(-Ea/RT) | P is the steric (orientation) factor |
What is R exactly? The molar gas constant listed by NIST is 8.314462618 J mol-1 K-1. NCERT uses 8.314, and any NEET option will give you the value it expects.
And the units of k follow from the order. NCERT's Table 3.3 gives the three you need:
| Order | Units of k |
|---|---|
| Zero | mol L-1 s-1 |
| First | s-1 |
| Second | L mol-1 s-1 |
So k = 2.3 × 10^-5 L mol-1 s-1 is a second order reaction, and k = 3 × 10^-4 s-1 is first order (NCERT Example 3.4). The general form is (mol L-1)^(1-n) s-1 for order n.
How do zero and first order reactions behave over time?
A zero order reaction loses the same amount of reactant every second, so its concentration falls in a straight line and its half-life shrinks as the reactant runs down. A first order reaction loses the same fraction every half-life, so it never quite reaches zero and its half-life stays constant.
The chart below starts both reactions at 100% with the same first half-life. It is drawn from NCERT equations 3.6 and 3.14; the points are our arithmetic.
Which graph gives a straight line? For zero order, [R] against t (slope -k). For first order, ln[R] against t (slope -k), or log([R]0/[R]) against t (slope k/2.303). NCERT's Table 3.4 lists these side by side, and graph-reading questions come straight from it.
Three NCERT examples are worth doing by hand:
- Find k from two concentrations. N2O5 falls from 1.24 × 10^-2 to 0.20 × 10^-2 mol L-1 in 60 minutes at 318 K. k = (2.303/60) × log 6.2 = 0.0304 min-1 (Example 3.5).
- Turn k into a half-life. With k = 0.0304 min-1, t1/2 = 0.693/0.0304 = about 22.8 minutes (our arithmetic). For k = 5.5 × 10^-14 s-1, NCERT gets 1.26 × 10^13 s (Example 3.7).
- Use the 99.9% rule. 99.9% completion needs t = 6.909/k, which is 10 times the half-life (Example 3.8). By the same formula, 99% needs 4.606/k, about 6.6 half-lives (our arithmetic).
Where does first order show up in real life? NCERT names the hydrogenation of ethene, the decomposition of N2O5 and N2O, and every radioactive decay. Zero order is rarer: NCERT's examples are ammonia decomposing on hot platinum at high pressure and HI decomposing on gold, where the metal surface is already full. If you studied nuclear decay in Physics, our modern physics for NEET guide shows how the same first order law appears there.
Why does temperature speed up a reaction so much?
Temperature speeds up a reaction because more molecules then carry at least the activation energy, the minimum energy needed to form the activated complex. NCERT states that a 10 degree rise nearly doubles the rate constant. The Arrhenius equation, k = A e^(-Ea/RT), puts a number on it.
NCERT's own example shows how steep this is. The half-life of N2O5 is 10 days at 0°C, 5 hours at 25°C and 12 minutes at 50°C. The chart puts all three on one scale, which has to be logarithmic for the bars to fit.
Two worked examples from NCERT show how the two-temperature form is used:
- Find Ea. k is 0.02 s-1 at 500 K and 0.07 s-1 at 700 K. log(0.07/0.02) = 0.544, which gives Ea = 18,230.8 J mol-1 and A = 1.61 (Example 3.9).
- Find a new k. Ethyl iodide decomposes with k = 1.60 × 10^-5 s-1 at 600 K and Ea = 209 kJ mol-1. At 700 K, k = 6.36 × 10^-3 s-1 (Example 3.10), roughly 400 times faster for 100 K (our arithmetic).
The "10 degree doubling" rule has a number behind it too. If k doubles between 298 K and 308 K, then Ea = 2.303 × 8.314 × log 2 × (298 × 308)/10, about 52.9 kJ mol-1 (our arithmetic for NCERT intext question 3.8). And the factor e^(-Ea/RT) is the fraction of molecules with energy at least Ea: for HI with Ea = 209.5 kJ mol-1 at 581 K, that is about 1.5 × 10^-19 (our arithmetic for intext question 3.9). Tiny, which is why so few collisions react.
Plot ln k against 1/T and you get a straight line with slope -Ea/R and intercept ln A (NCERT Fig. 3.10). That plot is the graph question Unit 8 asks most often by name.
What does a catalyst change, and what does it not?
A catalyst raises the rate by giving the reaction a different path with a lower activation energy, and it comes out unchanged. It does not change ΔG or the equilibrium constant. It speeds the forward and backward reactions equally, so equilibrium is reached sooner but sits in the same place.
NCERT's example is MnO2 in the decomposition of KClO3 to KCl and O2. A substance that slows a reaction is called an inhibitor, not a negative catalyst. And a catalyst only speeds up reactions that are already spontaneous; it cannot make a non-spontaneous reaction run. That last line is a favourite assertion and reason question.
The same idea links to your practical chemistry. NCERT notes that potassium permanganate is decolourised by oxalic acid faster when warm. Oxalic acid against KMnO4 is one of the titrations named in Unit 20, and our electrochemistry for NEET guide explains the redox side of it.
Collision theory in two lines
Collision theory says molecules react only when they collide with at least the threshold energy and the right orientation. The rate is Rate = P ZAB e^(-Ea/RT), where ZAB is the collision frequency and P the steric factor. NCERT credits Max Trautz and William Lewis (1916 to 1918) and notes the theory treats molecules as hard spheres, which is its main weakness.
For NEET that is the whole of it: Unit 8 says "no derivation". Know that A in the Arrhenius equation is related to collision frequency, that threshold energy equals activation energy plus the energy the reactants already have, and that a collision with the wrong orientation simply bounces apart.
Which lines trap students in chemical kinetics?
Most marks lost in chemical kinetics come from a short list. Read it twice.
- Reading the order from the balanced equation. The order comes from experiment. KClO3 with FeSO4 looks like tenth order and is second order.
- Calling molecularity zero or a fraction. Only order can be zero or a fraction. Molecularity is 1, 2 or 3.
- Forgetting to divide by coefficients. For 2N2O5 giving 4NO2 and O2, the rate of NO2 formation is four times the reaction rate.
- Using the first order half-life for zero order. Zero order t1/2 = [R]0/2k depends on the start; first order 0.693/k does not.
- Mixing log and ln. k = (2.303/t) log([R]0/[R]) is the same as (1/t) ln([R]0/[R]). Use 2.303 only with log base 10.
- Putting °C into the Arrhenius equation. Always convert to kelvin; 25°C is 298 K.
- Saying a catalyst shifts equilibrium. It does not change K or ΔG. It only gets there faster.
- Getting the units of k wrong. Zero order mol L-1 s-1, first order s-1, second order L mol-1 s-1.
A one-week revision plan for chemical kinetics
Chemical kinetics rewards a fixed order of revision, because each idea builds on the last. Here is a plan that fits into one normal study week.
- Copy the scope onto one page. Write the nine Unit 8 phrases from the table above. That page is everything this chapter can ask.
- Drill rate expressions. Ten balanced equations a day, each written as a rate with every coefficient divided out, until the HI and N2O5 forms feel automatic.
- Work Table 3.2 yourself. Find the order in each reactant from initial rates, then k and its units. Repeat with any data table from a past paper.
- Learn the two integrated laws as graphs. Sketch [R] against t for zero order and ln[R] against t for first order from memory, with the slopes and intercepts marked.
- Do Arrhenius with numbers. Redo Examples 3.9 and 3.10 and the two intext questions on temperature, writing the kelvin conversion every time. Put each trap above on a card; our guide to NEET flashcards shows how.
- Test under time. Mix chemical kinetics questions with Units 6 and 7 and sort your wrong answers by trap number. A wrong answer costs five marks against a right one at plus four and minus one, which our NEET negative marking guide explains.
How many chemical kinetics questions will NEET 2027 have? Nobody can tell you honestly. NTA publishes the subject-wise pattern, 45 Chemistry questions for 180 marks, not a chapter-wise split. For where Unit 8 sits in a sensible revision order, futureMBBS's NEET 2027 chapter-wise weightage guide is a good next read, and the NEET syllabus overview on futureMBBS puts all three subjects on one page.
For the whole subject plan, start with our NEET 2027 Chemistry preparation strategy, and for practice by unit see the NEET Chemistry questions guide. Our NEET previous year question papers guide shows how to use old papers without burning them, and futureMBBS explains why NEET leans on the NCERT book and sets out how to prepare for NEET from start to finish.
Want to see whether chemical kinetics is where your Chemistry marks leak? NEETsheet's chapter-wise Chemistry sets and timed mocks show which trap you keep falling into, so your next revision goes there first. Start at neetsheet.in.
Sources
Every syllabus line and number above comes from these documents, opened and read on 31 October 2026.
- National Medical Commission (UGMEB), Syllabus for NEET (UG) 2026, public notice of 22 December 2025, linked from neet.nta.nic.in (Chemistry Units 7, 8 and 20): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/01/202601081066816297.pdf
- National Testing Agency, Information Bulletin NEET (UG) 2026 (pattern: Chemistry 45 questions and 180 of 720 marks; plus four and minus one marking; syllabus FAQ stating no change for 2026): https://cdnbbsr.s3waas.gov.in/s37bc1ec1d9c3426357e69acd5bf320061/uploads/2026/02/202602231394640855.pdf
- National Testing Agency, NEET (UG) public notices, checked on 31 October 2026 for any NEET (UG) 2027 notice: https://neet.nta.nic.in/public-notices/
- NCERT, Chemistry Class 12 Part 1, Unit 3 Chemical Kinetics, reprint 2026-27 (Tables 3.1 to 3.4, Examples 3.2 to 3.10, sections 3.1 to 3.5, intext questions 3.8 and 3.9): https://ncert.nic.in/textbook/pdf/lech103.pdf
- National Institute of Standards and Technology, CODATA value of the molar gas constant, 8.314462618 J mol-1 K-1 (exact): https://physics.nist.gov/cgi-bin/cuu/Value?r
Frequently Asked Questions
Chemical kinetics is the branch of chemistry that studies the rate of chemical reactions, the factors that change it (concentration, temperature, pressure and catalyst) and the steps a reaction runs through. Thermodynamics tells you whether a reaction can happen; chemical kinetics tells you how fast it actually does.
Yes. Chemical kinetics is Chemistry Unit 8 in the NMC syllabus for NEET (UG) 2026, the current file. The 2026 bulletin says the syllabus did not change from the previous year, and no 2027 syllabus had been published by 31 October 2026. Check again when NTA releases the 2027 bulletin.
Order is the sum of the powers of concentration in the experimental rate law, and it can be zero or a fraction. Molecularity is the number of species colliding in one elementary step, so it is always 1, 2 or 3. Order applies to any reaction; molecularity only to elementary steps.
The half-life of a first order reaction is t1/2 = 0.693/k, the time for the reactant to fall to half its starting concentration. It does not depend on the starting concentration. For a zero order reaction, t1/2 = [R]0/2k, so it does depend on how much you start with.
Chemical kinetics is Unit 3 of NCERT Class 12 Chemistry Part 1 (reprint 2026-27), after Solutions and Electrochemistry. It covers the rate of reaction, rate law, order and molecularity, zero and first order integrated laws, the Arrhenius equation, catalysts and collision theory, matching NEET Unit 8.
