Chemistry/Chapter 7: Chemical Kinetics
7.2.1 Order of Reaction and Rate Equation
Order of Reaction:
Explanation level
This topic explains the order of reaction and the rate equation. It describes how the rate of a reaction depends on the concentration of reactants and how the order is determined.
Definition
Order of Reaction: The number of molecules of reactants participating in the rate-determining step.
Rate Equation: An expression that relates the rate of a reaction to the rate constant and the concentration of reactants raised to appropriate powers.
Main Idea
Order of reaction may be defined as the number of molecules of reactants participating in the rate-determining step. Consider a general reaction between a moles of A and b moles of B to give c moles C and d moles of D.
aA + bB → cC + dD
The rate equation can be written as:
Rate ∝ [A]x [B]y
Rate = k[A]x [B]y
- k = rate constant
- x = order of reaction with respect to A
- y = order of reaction with respect to B
- x + y = overall order of reaction
Core Concept
- Rate law: Expression relating rate to concentration.
- Rate constant (k): Rate when concentration of each reactant is unity.
- Order of reaction: Sum of exponents in the rate equation.
- Order determined experimentally, not from stoichiometric coefficients.
Detailed Explanation
Rate Law
Rate of reaction is defined as the instantaneous change in concentration of a reactant or product at a given time. Experimental studies on reaction rates show that the rate of a chemical reaction is proportional to the molar concentration of reactants each raised to a power, the value of which is determined experimentally.
Thus for a general reaction:
A → Product
Rate ∝ [A]x
Rate = k[A]x
Where k is proportionality constant and is known as rate constant and the expression as rate law or rate equation. The exponent x in the rate equation is called order of reaction with respect to reactant A.
When [A] = 1 M:
Rate = k
Thus, the rate constant can be defined as the rate of reaction when the molar concentration of each reactant is unity. The rate constant gives the relationship between concentration and reaction rate. Each reaction has its own specific rate constant, independent of concentration and time. But the value of the rate constant changes with temperature.
Order of Reaction
Order of reaction may be defined as the number of molecules of reactants participating in the rate determining step.
Consider a general reaction between a moles of A and b moles of B to give c moles C and d moles of D.
aA + bB → cC + dD
The rate equation can be written as:
Rate ∝ [A]x [B]y
Rate = k[A]x [B]y
Types of Order of Reactions
a) Zero Order Reactions
A reaction for which the sum of exponents of rate equation is zero is called zero order reaction.
Rate ∝ [A]0
Rate = k
b) First Order Reactions
A reaction for which the sum of exponents of rate equation is one is called first order reaction.
Rate ∝ [A]1
Rate = k[A]
c) Second Order Reactions
A reaction for which sum of exponents of rate equation is two is called the second order reaction.
Rate ∝ [A]2 or Rate ∝ [A]1[B]1
d) Third Order Reactions
A reaction for which sum of exponents of rate equation is three is called the third order reaction.
Rate ∝ [A]3 or Rate ∝ [A]2[B]1 or Rate ∝ [A]1[B]1[C]1
e) Fractional Order Reactions
A reaction for which the sum of exponents of rate equation is in fraction is called the fractional order reaction.
f) Pseudo First Order Reaction
A bimolecular reaction for which solvent is in excess and its concentration remains constant and does not take part in rate determining step is called a Pseudo First order reaction.
Intermediate Level
Rate Law
Rate = k[A]x [B]y
- k = rate constant
- x, y = orders with respect to A and B
- x + y = overall order
Rate Constant (k)
- Rate when concentration of each reactant is unity.
- Independent of concentration and time.
- Changes with temperature.
Order of Reaction
- Number of molecules participating in rate-determining step.
- Determined experimentally.
Types of Order
| Order | Rate Equation | Example |
|---|---|---|
| Zero | Rate = k | — |
| First | Rate = k[A] | — |
| Second | Rate = k[A]² or k[A][B] | — |
| Third | Rate = k[A]³ or k[A]²[B] or k[A][B][C] | — |
| Fractional | Rate = k[A]0.5 | — |
| Pseudo First | Rate = k[A] (solvent in excess) | — |
Important Points
- 1. Rate law: Rate = k[A]x[B]y.
- 2. Rate constant (k): Rate when concentration is unity.
- 3. Order: Sum of exponents in rate equation.
- 4. Order determined experimentally.
- 5. Types: Zero, first, second, third, fractional, pseudo first.
- 6. Pseudo first order: Solvent in excess.
- 7. Rate constant changes with temperature.
Remember
- - **Rate law:** Rate = k[A]x[B]y
- - Rate constant: Rate when concentration = 1 M
- - Order: Sum of exponents
- - Types: Zero, first, second, third, fractional, pseudo first
- - Order determined: Experimentally
Common Mistakes
- 1. Thinking order is from stoichiometry: It is determined experimentally.
- 2. Confusing rate constant with rate: Rate constant is a proportionality constant.
- 3. Forgetting the temperature dependence: Rate constant changes with temperature.
- 4. Mixing up order types: Zero = Rate = k; First = Rate = k[A]; etc.
Quick Revision
- Rate law: Rate = k[A]x[B]y.
- Rate constant (k): Rate when concentration is unity.
- Order: Sum of exponents in rate equation.
- Order determined experimentally.
- Types: Zero, first, second, third, fractional, pseudo first.
- Pseudo first order: Solvent in excess.
- Rate constant changes with temperature.
60-Second Revision
- **Rate law:** Rate = k[A]x[B]y
- Rate constant: Rate when concentration = 1 M
- Order: Sum of exponents
- Types: Zero, first, second, third, fractional, pseudo first
- Order determined: Experimentally
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- Topic: 7.2.1 Order of Reaction and Rate Equation
- Chapter: 7 (Chemical Kinetics)
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7.2.1 Order of Reaction and Rate Equation
Sticky Notes · Quick Revision
Key Point 1
- **Rate law:** Rate = k[A]x[B]y
Key Point 2
- Rate constant: Rate when concentration = 1 M
Key Point 3
- Order: Sum of exponents
Key Point 4
- Types: Zero, first, second, third, fractional, pseudo first
Key Point 5
- Order determined: Experimentally
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