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Rate Law, Order and Molecularity

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Student-friendly explanation

How to write this in exams

1
Start with the exact idea
Rate law expresses reaction rate in terms of molar concentrations of reactants raised to experimentally determined powers. Order is the sum of these powers. Molecularity is the number of reacting species taking part in a single elementary step.
2
Then show how to use it
Compare two experiments where only one reactant concentration changes. Write rate ratio = concentration ratio raised to power x. Solve for x. Repeat for each reactant, then add powers for overall order and derive the unit of k from rate = k concentration powers.
3
Add one concrete example
If rate = k[NO]^2[O2], order with respect to NO is 2, order with respect to O₂ is 1, and overall order is 3. This does not automatically mean that three molecules collide in one step unless the step is elementary.
4
Avoid this incomplete answer
Calling molecularity zero or fractional; molecularity is never zero or fractional because it counts species in an elementary step.

Definition

Example

If rate = k[NO]^2[O2], order with respect to NO is 2, order with respect to O₂ is 1, and overall order is 3. This does not automatically mean that three molecules collide in one step unless the step is elementary.

Rule to remember

Rate law: rate = k[A]^a[B]^b. Overall order = a + b. Unit of k depends on order: for nth order, unit of k = (mol L^-1)^(1-n) s^-1. Molecularity can be unimolecular, bimolecular, or termolecular for elementary steps.

Memory hook

Order comes from data; molecularity comes from a single step.

Examples and method

Worked example

In an experiment, when [A] is doubled at constant [B], rate becomes four times; order in A = 2. When [B] is doubled at constant [A], rate doubles; order in B = 1. Rate law = k[A]^2[B], overall order = 3.

Method to apply

Compare two experiments where only one reactant concentration changes. Write rate ratio = concentration ratio raised to power x. Solve for x. Repeat for each reactant, then add powers for overall order and derive the unit of k from rate = k concentration powers.

Diagram support

A data table comparing concentration change and rate change is more useful than a structural diagram for this concept.

How CBSE asks it

CBSE questions commonly give initial rate data and ask for order, rate law, or unit of k. Assertion-reason questions often contrast order and molecularity.

Avoid common mistakes

Common confusion

Students often take powers directly from the balanced equation for every reaction, but rate law powers are experimental for overall reactions.

Common wrong answer

Calling molecularity zero or fractional; molecularity is never zero or fractional because it counts species in an elementary step.

Exam tip

Use experimental data to find order. Use the mechanism or elementary-step statement only when asked about molecularity.

Quick check

For rate = k[A]^1[B]^0, what is the overall order and what happens to rate if [B] is doubled?

Overall order = 1 + 0 = 1. Doubling [B] has no effect on rate because the reaction is zero order with respect to B.

Answer writing and exam use

1-mark answer

2-mark answer

Rate law expresses reaction rate in terms of molar concentrations of reactants raised to experimentally determined powers. Order is the sum of these powers. Molecularity is the number of reacting species taking part in a single elementary step. Rate law: rate = k[A]^a[B]^b. Overall order = a + b. Unit of k depends on order: for nth order, unit of k = (mol L^-1)^(1-n) s^-1. Molecularity can be unimolecular, bimolecular, or termolecular for elementary steps. If rate = k[NO]^2[O2], order with respect to NO is 2, order with respect to O₂ is 1, and overall order is 3. This does not automatically mean that three molecules collide in one step unless the step is elementary.

3-mark answer

For rate = k[A]^a[B]^b, the exponents a and b are determined from experimental rate data and need not match the balanced chemical equation. The overall order is a + b. Molecularity applies only to an elementary step and is always a whole number. A complex reaction may have a rate law controlled by its slowest step. Rate law: rate = k[A]^a[B]^b. Overall order = a + b. Unit of k depends on order: for nth order, unit of k = (mol L^-1)^(1-n) s^-1. Molecularity can be unimolecular, bimolecular, or termolecular for elementary steps. In an experiment, when [A] is doubled at constant [B], rate becomes four times; order in A = 2. When [B] is doubled at constant [A], rate doubles; order in B = 1. Rate law = k[A]^2[B], overall order = 3. CBSE questions commonly give initial rate data and ask for order, rate law, or unit of k. Assertion-reason questions often contrast order and molecularity. Calling molecularity zero or fractional; molecularity is never zero or fractional because it counts species in an elementary step.

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