Units of Rate Constant for Zero, First, Second and Third Order Reactions
Chemistry · Chemical Kinetics · NEET
The unit of the rate constant k changes with the order of the reaction. Use one master formula: unit of k = (mol L⁻¹)^(1−n) s⁻¹, where n is the order. So zero order = mol L⁻¹ s⁻¹, first order = s⁻¹, second order = mol⁻¹ L s⁻¹, third order = mol⁻² L² s⁻¹. Memory hook: as order goes up by 1, one more "per mol L⁻¹" (per concentration) gets added to the unit.
One master formula gives every unit: put the order n into (mol L⁻¹)^(1−n) s⁻¹. As order rises by 1, one extra "per mol L⁻¹" is added, so L climbs to the top and its power grows.
Your doubts, answered
Why does the unit of the rate constant change with order but the rate always stays mol L⁻¹ s⁻¹?
Rate is a speed of concentration change, so its unit is always mol L⁻¹ s⁻¹ (concentration per second), no matter the order. But rate = k × [concentration]^n. The [concentration]^n part has a unit of (mol L⁻¹)^n. So k must carry whatever unit is left over to make the two sides match. That leftover changes as n changes, which is exactly why the unit of k changes with order.
What is the one formula to get the unit of k for ANY order?
Unit of k = (mol L⁻¹)^(1−n) s⁻¹, where n is the order. Just put the order value for n. n=0 gives (mol L⁻¹)^1 s⁻¹ = mol L⁻¹ s⁻¹. n=1 gives (mol L⁻¹)^0 s⁻¹ = s⁻¹. n=2 gives (mol L⁻¹)^−1 s⁻¹ = mol⁻¹ L s⁻¹. This single formula saves you from memorising four separate units for NEET.
Why is the unit of k for a first order reaction just s⁻¹ (no mol or L)?
For first order, rate = k[A]. Rate is mol L⁻¹ s⁻¹ and [A] is mol L⁻¹. When you divide, mol L⁻¹ cancels with mol L⁻¹, leaving only s⁻¹. So the concentration units fully cancel and k has time⁻¹ units only. This is a favourite NEET trap: k = s⁻¹ always means first order.
How do I quickly derive the unit instead of memorising?
Write k = rate ÷ [conc]^n. Put rate = mol L⁻¹ s⁻¹ and [conc]^n = (mol L⁻¹)^n. Then k = (mol L⁻¹ s⁻¹) ÷ (mol L⁻¹)^n = (mol L⁻¹)^(1−n) s⁻¹. Deriving takes 10 seconds and never fails, even for fractional orders like n = 1/2.
What is the unit of rate constant for second and third order?
Second order (n=2): (mol L⁻¹)^(1−2) s⁻¹ = (mol L⁻¹)^−1 s⁻¹ = mol⁻¹ L s⁻¹. Third order (n=3): (mol L⁻¹)^(1−3) s⁻¹ = (mol L⁻¹)^−2 s⁻¹ = mol⁻² L² s⁻¹. Note L moves to the top and its power grows as order grows.
If time is given in minutes instead of seconds, does the unit change?
Yes. Replace s with min. For example, a zero order k could be mol L⁻¹ min⁻¹, and a first order k could be min⁻¹. NEET questions sometimes use min⁻¹ or year⁻¹ (in radioactive decay). The order part (mol/L powers) stays the same; only the time unit swaps.
⚠️ The NEET trap ✗ Zero order k has unit s⁻¹ and first order k has unit mol L⁻¹ s⁻¹ (students swap the two). ✓ Zero order k = mol L⁻¹ s⁻¹ (same as rate itself, because [conc]^0 = 1). First order k = s⁻¹ (concentration units fully cancel). NEET 2026 tested exactly this matching. 🧠 Order 0 = k looks like RATE (mol L⁻¹ s⁻¹). Order 1 = k looks like a clock (s⁻¹).
Real NEET questions
NEET 2026
Match List-I (Order of reaction) with List-II (Unit of rate constant):
List-I: (A) Zero order (B) First order (C) Second order (D) Third order
List-II: (I) mol⁻¹ L s⁻¹ (II) mol⁻² L² s⁻¹ (III) s⁻¹ (IV) mol L⁻¹ s⁻¹
A · (A)-(IV), (B)-(II), (C)-(I), (D)-(III)
B · (A)-(IV), (B)-(III), (C)-(I), (D)-(II) ✓
C · (A)-(IV), (B)-(III), (C)-(II), (D)-(I)
D · (A)-(I), (B)-(II), (C)-(III), (D)-(IV)
Solution: Use the master formula: unit of k = (mol L⁻¹)^(1−n) s⁻¹ = mol^(1−n) L^(n−1) s⁻¹. Zero order (n=0): mol L⁻¹ s⁻¹ → (IV). First order (n=1): s⁻¹ → (III). Second order (n=2): mol⁻¹ L s⁻¹ → (I). Third order (n=3): mol⁻² L² s⁻¹ → (II). So (A)-(IV), (B)-(III), (C)-(I), (D)-(II), which is option B.
NEET 2018
When the initial concentration of the reactant is doubled, the half-life period of a zero order reaction:
A · Is tripled
B · Is doubled ✓
C · Is halved
D · Remains unchanged
Solution: For a zero order reaction, t½ = [A]₀ ÷ (2k). Notice the unit of k here is mol L⁻¹ s⁻¹, so [A]₀ ÷ k gives a time unit — the formula is dimensionally correct only because zero order k carries mol L⁻¹ s⁻¹. Since t½ is directly proportional to [A]₀, doubling [A]₀ doubles the half-life. Answer: B.
Solved Chemical Kinetics NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is the unit of rate constant same as the unit of rate?
Only for a zero order reaction. There, rate = k, so both are mol L⁻¹ s⁻¹. For every other order the units differ, because k = rate ÷ [conc]^n.
What is the general formula for the unit of rate constant?
Unit of k = (mol L⁻¹)^(1−n) s⁻¹, where n is the order of the reaction. This works for whole-number and fractional orders.
What is the unit of k for a pseudo first order reaction?
It is s⁻¹ (or min⁻¹), the same as a normal first order reaction, because the effective order is 1.
Why is this important for NEET?
NEET often gives you only the unit of k and asks you to identify the order, or gives the order and asks for the unit (like NEET 2026). Knowing one formula lets you answer both types in seconds without memorising a table.
Does temperature change the unit of the rate constant?
No. Temperature changes the value of k (via the Arrhenius equation) but not its unit. The unit depends only on the order of the reaction.
