Rate Law & Rate Constant
Understanding the dependency of rate on concentration.
By chemca Team • Updated Jan 2026
The Rate Law is an experimentally determined expression that relates the rate of a reaction to the concentration of reactants raised to some power. This power is called the order of the reaction.
1. Rate Law Expression
The Equation
For a general reaction: $aA + bB \rightarrow \text{Products}$
$$ \text{Rate} \propto [A]^x [B]^y $$
$$ \text{Rate} = k [A]^x [B]^y $$
- $k$: Rate Constant (or Specific Reaction Rate).
- $x$ and $y$: Orders of reaction with respect to A and B.
- Note: $x$ and $y$ may or may not be equal to stoichiometric coefficients $a$ and $b$. They are determined experimentally.
2. Order of Reaction
Sum of Powers
The sum of powers of the concentration terms in the rate law expression is called the overall order of the reaction.
$$ \text{Order} (n) = x + y $$
Characteristics of Order:
- Can be 0, 1, 2, 3 (integer) or even fractional (e.g., 1.5, 0.5).
- Can be zero (Zero Order Reaction).
- It is an experimental quantity.
- Applicable to both elementary and complex reactions.
3. Molecularity
Colliding Particles
The number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction that must collide simultaneously to bring about a chemical reaction.
| Property | Order | Molecularity |
|---|---|---|
| Definition | Sum of powers in rate law. | Number of colliding molecules. |
| Values | Integer, Fraction, or Zero. | Always a whole number (1, 2, 3). Never zero or fraction. |
| Determination | Experimental. | Theoretical (from mechanism). |
| Scope | Elementary & Complex reactions. | Only Elementary reactions. |
4. Units of Rate Constant ($k$)
General Formula
From the rate law: $k = \frac{\text{Rate}}{[\text{Conc}]^n}$
$$ \text{Unit of } k = \left( \frac{mol}{L} \right)^{1-n} s^{-1} $$
Specific Units:
- Zero Order (n=0): $mol \, L^{-1} \, s^{-1}$ (Same as Rate)
- First Order (n=1): $s^{-1}$ (Time inverse)
- Second Order (n=2): $L \, mol^{-1} \, s^{-1}$
Tip: You can identify the order of a reaction just by looking at the units of the rate constant.
Knowledge Check
Test your understanding of Rate Laws
No comments:
Post a Comment