Rate of Reaction
The rate of a reaction tells us to what speed the reaction occurs. Let us consider a simple reaction-
R → P
The concentration of R decreases and that of P increases with time. The rate of a reaction is defined as the change in concentration of any of reactant or product with time. As we know that during the progress of a reaction the concentration of R keeps on falling with time. The rate of reaction at any given instant is given by the expression
Rate of reaction = − d[R]/dT
Since the rate of reaction is not uniform throughout the reaction, hence, the rate of disappearance of reactants or rate of appearance of product in small interval of time is given as-
Rate of reaction = − d[R]/dT = + d[P]/dT
Example: 3H2 + N2 = 2NH3
for this reaction, the rate of reaction may be written as:
− 1/3 d[H2]/dT = − d[N2]/dT = + 1/2 d[NH3]/dT
Molecularity and Order of Reaction
While both terms describe the stoichiometry and kinetics of a reaction, they represent very different concepts in physical chemistry.
1. Molecularity:
The number of reactant molecules which take part in the formation of activated complex in the slowest step of a chemical reaction is caleed Molecularity of that reaction. Molecularity is a whole number but can never be zero or fraction. Molecularity more than three is generally rare.
Reaction may be uni, bi or ter-molecular depending upon whether one, two or three reactant molecules are involved in the slowest step of a chemical reaction.
Molecularity of a reaction is a theoretical value. So, it is not the real quantity like order of reaction. For a complex reaction, the molecularity of reaction is expressed for each step and hence overall value is meaningless.
2 O3 = 3 O2
O3 → O2 + O (Fast) (Unimolecular)
O3 + O = 2 O2 (Slow) (Bimolecular)
2. Order of Reaction:
The sum of powers of the concentration term in the rate equation of a chemical reaction is called Order of reaction.
Order of reaction obtained from the rate law equation which is obtained experimently.
Let us consider a general chemical reaction-
nA + mB = Product
then, the rate of reaction = k[A]x [B]y
The value of x and y may or may not be equal to n and m respectively because the value of x and y are determined experimently.
If the reaction takes place in more than one steps, then, the slowest step is rate determining step.
The value of order of reaction may be positive, negative, zero, fraction or integer. The order of reaction can never be more than or equal to the molecularity of the reaction.
Comparison Table
| Feature | Molecularity | Order of Reaction |
|---|---|---|
| Definition | Number of molecules taking part in an elementary step. | Sum of exponents in the experimental rate law. |
| Nature | Theoretical concept. | Experimental property. |
| Values | Always a whole number (1, 2, or 3). Cannot be zero or fractional. | Can be zero, fractional, or an integer. |
| Complex Reactions | Only defined for elementary (single-step) reactions. | Applies to the overall reaction, regardless of steps. |
| Mechanism | Derived from the balanced equation of an elementary step. | Determined by the Slowest Step (Rate Determining Step). |
Concentration Dependence of Reaction Rates
For a general chemical reaction, the rate at which it proceeds is directly proportional to the concentration of the reactants, raised to some power. This relationship is expressed through the Rate Law.
Rate Law Expression:
It is an experimentally determined expression which relates the rate of reaction with concentration of reactants.
For a hypothetical reaction,
A + B → Products
Rate ∝ [A]m [B]n
or Rate = k[A]m [B]n
- [A] and [B]: Molar concentrations of reactants.
- k: The rate constant (specific to a reaction at a given temperature).
- m and n: The orders of the reaction with respect to A and B.
If [A] = [B] = 1 mol L–1
then, Rate = k
Thus, rate constant may be defined as the rate of reaction when the concentration of each reactant in the reaction is unity.
Factors Influencing the Rate of Reaction
The rate of a chemical reaction is the speed at which reactants are converted into products. This speed is governed by several physical and chemical variables.
1. Concentration of Reactants
Increasing the concentration of reactants typically increases the reaction rate. According to the Collision Theory, a higher concentration means more particles per unit volume, leading to a higher frequency of effective collisions.
2. Temperature
Higher temperatures significantly increase reaction rates. This happens because:
- Particles gain kinetic energy and move faster (more collisions).
- A larger fraction of molecules possess the required Activation Energy (Ea) to overcome the energy barrier.
The relationship is often described by the Arrhenius Equation:
3. Pressure
For reactions involving gases, increasing pressure effectively increases the concentration. By compressing the gas into a smaller volume, the particles are crowded together, increasing the collision rate.
4. Solvent
The nature of the solvent affects the rate, especially in ionic or polar reactions. Factors include:
- Viscosity: Highly viscous solvents can slow down the diffusion of reactants.
- Polarity: A polar solvent may stabilize a transition state or intermediate, lowering the activation energy.
5. Light (Photochemical Reactions)
Certain reactions, such as the photosynthesis or the reaction between H2 and Cl2, are influenced by light. Photons provide the specific energy required to break chemical bonds, initiating the reaction. These are known as photochemical reactions.
6. Catalysts
A catalyst is a substance that increases the rate of a reaction without being consumed. It works by providing an alternative reaction pathway with a lower activation energy.
Homocatalysis (Homogeneous Catalysis)
In homocatalysis, the catalyst is in the same phase (gas or liquid) as the reactants.
Example: The oxidation of SO2 to SO3 using NO(g) as a catalyst.
Heterocatalysis (Heterogeneous Catalysis)
In heterocatalysis, the catalyst is in a different phase than the reactants (usually a solid catalyst with gaseous or liquid reactants).
Example: The Haber Process, where solid Iron (Fe) catalyzes the reaction between Nitrogen and Hydrogen gases.
7. Inhibitors
An inhibitor is a substance that decreases the rate of a chemical reaction or prevents it from occurring altogether. Unlike catalysts, which lower activation energy, inhibitors often work by:
- Reacting with a reactant to form a stable complex.
- Blocking active sites on an enzyme (competitive inhibition).
- Breaking chain reactions (common in polymer chemistry and food preservation).
Example: Antioxidants added to food act as inhibitors to slow down the oxidation of fats.
8. Catalytic Poisons
A catalytic poison (or simply a poison) is a substance that reduces the effectiveness of a catalyst. It "deactivates" the catalyst by binding strongly to its active surface, preventing reactant molecules from adsorbing.
- Poisons are usually impurities present in the reaction mixture.
- The effect is often irreversible, requiring the catalyst to be replaced.
Example: Arsenic or Sulfur compounds can "poison" the platinum catalyst used in industrial processes.
9. Catalytic Promoters
A promoter is a substance that is not a catalyst itself but increases the activity of a catalyst when added in small amounts. They work by:
- Increasing the surface area of the catalyst.
- Modifying the electronic structure of the catalyst to make active sites more reactive.
Example: In the Haber Process for the synthesis of ammonia, Molybdenum (Mo) or Potassium Oxide (K2O) is added as a promoter to increase the efficiency of the Iron (Fe) catalyst.
Related Topics
Methods of Determination of Order of Reaction
Rate Constant of Zero Order Reaction
Rate Constant of First Order Reaction
Rate Constant of Second Order Reaction