The rate law is an equation that tells you how fast a reaction goes based on the concentration of the reactants, like Rate = k[A]^x[B]^y. The letter k is the rate constant, a fixed number for that reaction at a fixed temperature. The most important NEET point: the powers x and y are found by doing experiments, NOT by copying the numbers in front of the balanced equation. Memory hook: "Powers come from the lab bench, not the equation."
A rate law has two parts to watch: k (the fixed rate constant that only changes with temperature or a catalyst) and the powers on each concentration, which must be found by experiment, not copied from the balanced equation.
Your doubts, answered
Are the powers in the rate law the same as the coefficients in the balanced equation?
Usually NO. For a general reaction aA + bB gives products, the rate law is Rate = k[A]^x[B]^y. The powers x and y may or may not equal a and b. NCERT states this clearly: the rate law must be found by experiment, not by reading the balanced equation. Example: for CH3COOC2H5 + H2O, the rate = k[ester]^1[H2O]^0, so water has power 0 even though it is a reactant. Only for simple elementary reactions do the powers happen to match the coefficients.
What is the rate constant k and what does it actually mean?
k is the proportionality constant in Rate = k[A]^x[B]^y. It is a fixed number that connects the concentrations to the rate. A large k means a fast reaction; a small k means a slow one. When all concentrations are 1 mol/L, the rate equals k, so k is like the rate at unit concentration. It is also called the specific reaction rate.
What is the difference between rate and rate constant?
Rate changes as the reaction goes on, because concentrations keep dropping, so rate usually decreases with time. The rate constant k does NOT change during the reaction. k stays the same no matter how much reactant is left. k only changes if you change the temperature or add a catalyst. So: rate depends on concentration, k does not.
Does k depend on concentration or temperature?
k does NOT depend on concentration. Changing how much reactant you add does not change k. k DOES depend on temperature (it rises when temperature rises) and on whether a catalyst is present. This is a very common NEET trap, so remember: k is fixed for a given reaction at a given temperature.
How do I find the powers x and y from experimental data?
You change one reactant at a time and watch the rate. If doubling [A] doubles the rate, the power of A is 1. If doubling [A] makes the rate 4 times bigger, the power is 2 (because 2^2 = 4). If doubling [A] does not change the rate, the power is 0. You do this for each reactant separately, keeping the others constant.
⚠️ The NEET trap ✗ For 2NO + O2 gives 2NO2, students write Rate = k[NO]^2[O2] by copying coefficients, then assume this coefficient-copying rule works for every reaction. ✓ The rate law must be found by experiment. It only matches the coefficients for some reactions (like this NO example, which is elementary). For CH3COOC2H5 + H2O the rate is k[ester][H2O]^0, which does NOT match the coefficients. 🧠 Coefficients guess, experiments confirm. Never write a rate law just by staring at the balanced equation.
Real NEET questions
NEET 2023
For a certain reaction, the rate = k[A]^2[B]. When the initial concentration of A is tripled keeping the concentration of B constant, the initial rate would
A · Decrease by a factor of nine
B · Increase by a factor of six
C · Increase by a factor of nine ✓
D · Increase by a factor of three
Solution: The rate law is R = k[A]^2[B]. Triple [A] while keeping [B] the same: R' = k(3[A])^2[B] = 9 k[A]^2[B] = 9R. So the rate becomes 9 times bigger. Note the power of A is 2, so a factor of 3 on A gives 3^2 = 9 on the rate. Answer: C.
NEET 2023
The correct option for the rate law that corresponds to an overall first order reaction is
A · Rate = k[A]^(1/2)[B]^2
B · Rate = k[A]^(-1/2)[B]^(3/2) ✓
C · Rate = k[A]^0[B]^2
D · Rate = k[A][B]
Solution: Overall order = sum of all the powers in the rate law. Check each: A) 1/2 + 2 = 5/2. B) -1/2 + 3/2 = 1. C) 0 + 2 = 2. D) 1 + 1 = 2. Only option B adds up to 1, so it is overall first order. Answer: B. This shows powers in a rate law can even be fractions or negative, because they come from experiment.
NEET 2016
The decomposition of PH3 on tungsten at low pressure is a first-order reaction. It is because the
A · rate is proportional to the surface coverage ✓
B · rate is inversely proportional to the surface coverage
C · rate is independent of the surface coverage
D · rate of decomposition is very slow
Solution: On a tungsten surface, PH3 first sticks (adsorbs) to the surface. At low pressure the amount stuck (surface coverage) is small and is directly proportional to [PH3]. Since the reaction happens on the stuck layer, Rate is proportional to surface coverage, which is proportional to [PH3]^1. So the rate law gives first order. Answer: A. At high pressure the surface fills up, coverage becomes constant, and the reaction turns zero order.
Solved Chemical Kinetics NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is an equation that says how fast a reaction goes based on the concentrations of the reactants, written as Rate = k[A]^x[B]^y. It is also called the rate equation or rate expression.
What is the rate constant k?
k is the fixed proportionality number in the rate law. It stays constant while the reaction runs. It only changes with temperature or a catalyst, not with concentration.
Why must the rate law be found by experiment?
Because the powers of the concentration terms do not always match the coefficients in the balanced equation. Only experiments tell you the true powers, so NCERT says the rate law cannot be predicted just by looking at the equation.
Does the rate constant change during a reaction?
No. During a reaction the concentrations fall so the rate falls, but k stays exactly the same at a fixed temperature.
Can the powers in a rate law be fractions or negative?
Yes. As the NEET 2023 PYQ shows, a valid rate law can have powers like 1/2, 3/2, or even negative values, because the powers come from experimental data.
