“Order from experiment, molecularity from the step”
Telling them apart
Order is measured and can be zero or fractional. Molecularity is counted from a single step and is always a whole number.
Chemistry, Class 11 and 12 · Gujarat Board, CBSE, GUJCET, JEE and NEET
Chemical Kinetics formulas
Kinetics asks how fast, not how far. Order, rate constant and half-life are the whole chapter, and the first order results are asked so often that they are worth memorising as a block.
18 formulas, 3 memory hooks, and the 5 mistakes that cost the most marks.
Rate and order
| What | Formula | Symbols and notes |
|---|---|---|
| Rate of reaction | rate = −Δ[R]/Δt = +Δ[P]/Δt | divided by the stoichiometric coefficient |
| Rate law | rate = k[A]x[B]y | x and y come from experiment, not from the equation |
| Order | order = x + y | can be zero, fractional or negative |
| Molecularity | the number of species colliding in one step | always a whole number, never zero |
| Units of k, zero order | mol L−1 s−1 | the rate itself |
| Units of k, first order | s−1 | independent of concentration units |
| Units of k, second order | L mol−1 s−1 | general form: (mol L−1)1−n s−1 |
Integrated rate laws
| What | Formula | Symbols and notes |
|---|---|---|
| Zero order | [R] = [R]₀ − kt | a straight line of concentration against time |
| Zero order half-life | t1/2 = [R]₀/2k | proportional to the starting concentration |
| First order | k = (2.303/t)log([R]₀/[R]) | the most used equation in the chapter |
| First order half-life | t1/2 = 0.693/k | independent of the starting concentration |
| First order graph | log[R] against t is a straight line | slope = −k/2.303 |
| After n half-lives | [R] = [R]₀/2n | the quick route for whole numbers of half-lives |
Temperature and catalysts
| What | Formula | Symbols and notes |
|---|---|---|
| Arrhenius equation | k = Ae−Ea/RT | A is the frequency factor |
| Logarithmic form | log k = log A − Ea/2.303RT | a straight line of log k against 1/T |
| Two temperatures | log(k₂/k₁) = (Ea/2.303R)(1/T₁ − 1/T₂) | the standard numerical |
| Rule of thumb | rate roughly doubles for every 10 K rise | near room temperature |
| Catalyst | lowers Ea, speeds both directions equally | it does not change ΔH or the position of equilibrium |
Taught by Darshit Bhatt, founder and lead faculty at Darshit Sir Institute. Instrumentation and Control engineer, with a master's degree and a chartered degree, teaching in Paldi, Ahmedabad since 2006.
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Memory hooks
How to remember this chapter.
“First order forgets where it started”
Half-life
For first order, t1/2 = 0.693/k, whatever the initial concentration. For zero order the half-life depends on it.
“A catalyst lowers the hill”
What a catalyst does
It provides a path with lower activation energy. Both forward and reverse rates rise by the same factor, so the equilibrium position does not move.
Where marks go
The mistakes that cost the most.
- Reading the order off the balanced equation. Order comes from experiment.
- Using the first order half-life formula for a zero order reaction.
- Forgetting the 2.303 when moving between natural and base ten logarithms.
- Thinking a catalyst shifts the equilibrium. It only makes both directions faster.
- Using °C in the Arrhenius equation, which needs kelvin.
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