Rate Law, Order and Molecularity
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.
In one line: Order comes from data; molecularity comes from a single step.
Practice This ConceptUnderstand it
- 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.
Example
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.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
In an experiment, when [A] is doubled at constant [B], rate becomes four times; order in A = 2.
- 1When [B] is doubled at constant [A], rate doubles; order in B = 1.
- 2Rate law = k[A]2[B], overall order = 3.
Method you can reuse
- 1Compare two experiments where only one reactant concentration changes.
- 2Write rate ratio = concentration ratio raised to power x.
- 3Solve for x.
- 4Repeat for each reactant, then add powers for overall order and derive the unit of k from rate = k concentration powers.
Visual tip
A data table comparing concentration change and rate change is more useful than a structural diagram for this concept.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
For rate = k[A]1[B]0, what is the overall order and what happens to rate if [B] is doubled?
Think first, then show answerHide answer
Common wrong answer
Calling molecularity zero or fractional; molecularity is never zero or fractional because it counts species in an elementary step.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
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
Add for 2 marks
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.
- 3
Add for 3 marks
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.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
Help improve this page
Found something confusing, incorrect, or missing?