How to Find the Order of a Reaction from the Rate Law and Concentration Changes
Chemistry · Chemical Kinetics · NEET
The order of a reaction is just the sum of the powers of the concentration terms in the rate law. For rate = k[A]^x[B]^y, the order is x + y. Memory hook: "add the powers on top of the brackets." This matters for NEET because many Chemical Kinetics questions give you a rate law or concentration-change data and ask only for the order.
To find the order, add the powers written above the concentration brackets in the rate law. In rate = k[A]^x[B]^y, the overall order is simply x + y. The rate constant k and the balanced-equation coefficients play no part.
Your doubts, answered
How do I get the order straight from a rate law like rate = k[A]^2[B]?
Just add the powers written on the concentration brackets. In rate = k[A]^2[B], the power on A is 2 and the power on B is 1 (a hidden 1). Overall order = 2 + 1 = 3. The number k and the units do not matter for the order. Only the powers count.
What if the powers are fractions or negative, like [A]^(-1/2)[B]^(3/2)?
You still add them. Order = -1/2 + 3/2 = 1. Order can be zero, a fraction, or even negative. It does not have to be a whole number, because it is found by experiment, not by the balanced equation. NEET 2023 used exactly this idea.
How do I find the order when the question says 'concentration is doubled/tripled'?
See how many times the rate changes. If doubling a reactant makes the rate 2 times bigger, the power (order in that reactant) is 1. If the rate becomes 4 times (2^2), the power is 2. If the rate does not change, the power is 0. In general, if concentration goes up n times and rate goes up n^p times, then p is the order in that reactant.
Do I use the balanced equation coefficients to find the order?
No. This is the most common mistake. The order comes only from the experimental rate law, not from the coefficients in the balanced equation. For example 2A gives products may still be first order. Never copy stoichiometric coefficients as the order.
How do I find order from a reaction mechanism with a slow step?
Write the rate law using only the slow (rate-determining) step. Then replace any intermediate using the fast equilibrium step. Finally add the powers. This is how the NEET 2017 mechanism question gave an order of 1.5.
What is the difference between 'order in a reactant' and 'overall order'?
Order in a reactant is the power on that one reactant (like the 2 on [A]). Overall order is the sum of all the powers. So for rate = k[A]^2[B], order in A is 2, order in B is 1, and overall order is 3.
⚠️ The NEET trap ✗ Reading the order off the balanced equation. For 2A + B gives products, students write order = 2 + 1 = 3 by copying the coefficients. ✓ Order comes only from the experimental rate law. If the measured rate law is rate = k[A][B], the order is 1 + 1 = 2, no matter what the coefficients are. Add the powers in the rate law, never the coefficients. 🧠 Powers in the rate law tell the order; coefficients in the equation do NOT.
Real NEET questions
NEET 2023 Phase 1
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 rate = k[A]^2[B]. Triple [A] while [B] stays the same. New rate = k(3[A])^2[B] = 9 k[A]^2[B] = 9 times the old rate. So the rate increases by a factor of nine. The key point: the power on A is 2, so multiplying [A] by 3 multiplies the rate by 3^2 = 9.
NEET 2023 Phase 2
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 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. This shows order can include fractions and negative powers.
NEET 2017
Mechanism of a hypothetical reaction X2 + Y2 gives 2XY is given below: (i) X2 in equilibrium with X + X (fast); (ii) X + Y2 gives XY + Y (slow); (iii) X + Y gives XY (fast). The overall order of the reaction will be
A · 1
B · 2
C · 0
D · 1.5 ✓
Solution: The rate is set by the slow step (ii): rate = k[X][Y2]. But X is an intermediate, so replace it using the fast equilibrium step (i): X2 in equilibrium with 2X gives Keq = [X]^2/[X2], so [X] = Keq^(1/2)[X2]^(1/2). Put this back: rate = k' [X2]^(1/2)[Y2]. Add the powers: 1/2 + 1 = 3/2 = 1.5. So the overall order is 1.5.
Solved Chemical Kinetics NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Order is found by experiment, so it can be zero, a fraction (like 0.5 or 1.5), or even negative. NEET has asked questions where the order is 1.5, so do not assume it must be a whole number.
Can I find the order just from the balanced chemical equation?
No. You can only find the order from the experimental rate law. The balanced equation coefficients are not the order, unless the reaction is a simple elementary reaction, which the question will tell you.
What is the fastest way to find the overall order?
Write down the rate law and add all the powers on the concentration terms. For rate = k[A]^x[B]^y, overall order = x + y. Ignore k and the units.
If doubling a reactant does not change the rate, what is the order in that reactant?
It is zero. If the rate stays the same when you change a reactant's concentration, that reactant has power 0, so it does not appear in the effective rate law.
How is order different from molecularity?
Order is found from experiment and can be zero or a fraction. Molecularity is the number of particles in one step of the mechanism and is always a whole number of 1, 2, or 3. See the next topic for a full comparison.
