Factors Influencing Rate of a Reaction
Quick Notes
- Rate of reaction depends on factors like concentration, temperature, pressure, catalyst, and surface area.
- Rate law expresses how rate depends on the concentration of reactants.
- Rate constant (k) is a proportionality factor in the rate law.
- Order of reaction is the sum of powers of concentrations in rate law; can be zero, fractional, or integer.
- Molecularity is the number of molecules involved in an elementary step – always a whole number and theoretical.
Full Notes
Chemical reactions occur at different speeds. Some are instantaneous (like acid-base reactions), while others are slow (like rusting).
Among several factors, concentration plays a key role in influencing how quickly a reaction proceeds.
Dependence of Rate on Concentration
Key Concept:
The rate of a reaction generally increases with an increase in the concentration of reactants. This is because higher concentration means more particles per unit volume, leading to more collisions.
Example: For the reaction R → P
The rate is observed to be directly proportional to the concentration of R:
Rate Expression and Rate Constant
A rate law is a mathematical expression that relates the reaction rate to the concentrations of reactants.
It is determined experimentally, not from the balanced chemical equation.
Example: For the reaction A + B → C, the rate law might be:
![NCERT 12 Chemistry illustration of a general rate law rate = k[A]^x[B]^y and definition of overall reaction order x+y.](images/orderslaw.png)
The overall order of the reaction is x + y
This can also be expressed as a differential rate equation, with the instantaneous rate of a reaction (– d[R]/dt) being substituted for rate.
![NCERT 12 Chemistry equation showing differential rate form −d[R]/dt = k[A]^x[B]^y.](images/differentiallaw.png)
- The exponents x and y are not necessarily the stoichiometric coefficients.
- Rate constant k is specific for a reaction at a given temperature.
Units of Rate Constant
Units of the rate constant depend on the overall order of reaction.
| Order | Units of k |
|---|---|
| 0 | mol L−1 s−1 |
| 1 | s−1 |
| 2 | L mol−1 s−1 |
| 3 | L2 mol−2 s−1 |
Order of a Reaction
The order of reaction with respect to a reactant tells us how the rate changes as the concentration of that reactant changes.
It is the power (exponent) of the reactant concentration in the rate equation:
- rate = k[A]1 → first order in A.
- rate = k[A]1[B]2 → first order in A, second order in B, overall order = 3.
Zero Order Reaction:
Rate is independent of concentration:
Rate = k
Molecularity of a Reaction
Many chemical reactions occur through a sequence of elementary steps:
- Each step involves the breaking or forming of a small number of bonds
- The overall reaction is the sum of all elementary steps that occur
- How the steps link together is referred to as the ‘reaction mechanism’
Molecularity describes how many reactant species are involved in an elementary step of a reaction mechanism.
It applies only to individual steps, not to the overall reaction.
Unimolecular Reaction
A → B
One molecule decomposes
Rate = k[A]
First order
Bimolecular Reaction
A + B → C
Two molecules collide
Rate = k[A][B]
Second order (1st order in A, 1st in B)
Termolecular Reaction (rare)
A + B + C → D
Three particles collide simultaneously
Rate = k[A][B][C]
Third order overall
Why Are Termolecular Reactions Rare?
To have three particles collide at the same time in the correct orientation and with the required energy (see Collision Model) is highly unlikely. That’s why:
- Most reactions proceed through a series of bimolecular steps.
- Reaction mechanisms break complex reactions into a sequence of simpler elementary reactions.
Unimolecular steps usually involve just one reacting species, but they can still be triggered by collisions with non-reactive particles, like solvent molecules. These collisions may supply energy to break bonds, but because the solvent doesn’t change chemically, it isn’t considered a reactant — and its concentration doesn’t affect the rate.
Example 2NO + O2 → 2NO2
It may be found experimentally that: rate = k[NO]2[O2]
Here, order = 3
If this is an elementary step, molecularity = 3
Rate-Determining Step (RDS)
The slowest elementary step in a mechanism limits the overall reaction rate. It determines the rate equation (see).
A species must appear in the rate equation only if it is part of (or influences) the RDS.
Notes: Difference Between Order and Molecularity
- Order of a reaction is determined experimentally.
- It can be zero, fractional, or whole number.
- Molecularity is always a positive integer, never zero or fractional.
- Order applies to both elementary and complex reactions.
- Molecularity applies only to elementary reactions.
- It has no meaning for complex reactions.
- In complex reactions, the order is determined by the slowest step (rate-determining step).
- The molecularity of that slowest step is the same as the overall order.
Summary
- Rate laws relate reaction rate to reactant concentrations and define the rate constant and order.
- Order is experimental and can be zero, fractional, or integer while molecularity is an integer for an elementary step.
- The slowest step controls the overall rate and therefore the observed rate law.
- Units of the rate constant depend on the overall order of the reaction.