- Elementary reaction is a reaction that occurs in a single step and cannot be further divided into simpler reactions.
Example:
O₃(g) → O₂(g) + O(g) - Molecularity is the number of reactant molecules taking part in an elementary reaction. It is always a whole number (1, 2, 3) and never zero or fractional.
- If one molecule is involved, the reaction is unimolecular (molecularity = 1).
Example:
C₂H₅I(g) → C₂H₄(g) + HI(g) - If two molecules are involved, the reaction is bimolecular (molecularity = 2).
Example:
2NO₂(g) → 2NO(g) + O₂(g) - Order and molecularity of elementary reactions:
For an elementary reaction, the order of reaction is equal to its molecularity.
Example:
2NO₂(g) → 2NO(g) + O₂(g)
Rate = k[NO₂]²
Order = 2 = Molecularity - In complex reactions, the reaction occurs in several elementary steps. The slowest step is called the rate determining step, and it decides the rate of the overall reaction.
Example:
NO₂Cl(g) → NO₂(g) + Cl(g) (slow)
Rate = k[NO₂Cl]
Definitions [20]
Definition: Chemical Kinetics
Chemical kinetics is the branch of chemistry which deals with the study of chemical reactions with respect to the reaction rates, the effect of various arrangements of atoms and the formation of intermediates. It also describes the conditions in which rates can be altered.
Definition: Rate of Reactions
The rate of change of concentration of either products or reactants per unit time is termed the rate of reaction.
Reactants ⟶ Products
Definition: Average Rate
It is the ratio of the total change in concentration of a reactant or product to the total time taken by the reaction.
\[r_{\mathrm{av}}=-\frac{\Delta[R]}{\Delta t}=+\frac{\Delta[P]}{\Delta t}\]
Define the rate of a reaction.
The rate of chemical reaction is defined as the change in concentration of reactant or product per unit time.
The rate of a chemical reaction may be defined as the change in concentration of any of the reactants or any of the products per unit time. Thus,
`"Rate of reaction" = "(Change in concentration of a reactant or a product)"/"Time taken for the change"`
Definition: Instantaneous Rate
The rate of a reaction at any instant of time is called the instantaneous rate of reaction. It is equivalent to the small change in concentration (dx) in a small interval of time (dt).
Instantaneous rate of reaction,
\[r_{\mathrm{inst}}=\frac{-dx}{dt}\]
Definition: Rate Law
Rate law is the expression in which the reaction rate is given in terms of molar concentration of reactants, with each term raised to some power, which may or may not be the same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.
Definition: Order of Reaction
The sum of the coefficients (or powers) of the reacting species that are involved in the rate law expression for the reaction represents the order of the reaction.
Definition: Molecularity of Reactions
The number of reacting species which must collide simultaneously in order to bring about a chemical reaction is called the molecularity of a reaction.
Define molecularity.
The molecularity of an elementary reaction refers to how many reactant molecules are involved in reactions.
Define molecularity of reaction.
The molecularity of a chemical reaction is the number of molecules or ions involved in an elementary process. The integer value represents the number of entities involved in a reaction.
Define the half-life of a first-order reaction.
The time in which concentration of reactant becomes half of its initial concentration is called half Life. It is denoted by `t_(1/2)`.
Define first-order reaction.
A chemical reaction in which the rate of reaction depends solely linearly on the concentration of one ingredient is referred to as a first-order reaction.
A first-order reaction is a reaction whose rate depends upon the first power of the concentration of reactants, i.e., the rate is directly proportional to the concentration of reactants.
Define half life of a reaction.
Half life of a reaction is defined as the time required for the reactant concentration to reach one half of its initial value.
Definition: Instantaneous rate of reaction
The rate of reaction at a particular instant of time is called instantaneous rate of reaction.
Definition: Average rate of reaction
The change in concentration of reactant or product divided by time interval is called average rate of reaction.
Definition: Order of reaction
The sum of powers of concentration terms in the rate law is called order of reaction.
Definition: Chemical kinetics
The branch of chemistry which deals with the rate of chemical reactions and the factors affecting them is called chemical kinetics.
Definition: Rate of reaction
The rate which describes how rapidly reactants are consumed or products are formed is called rate of reaction.
Definition: Zero order reaction
The reaction whose rate is independent of concentration of reactant is called zero order reaction.
Definition: Pseudo-first order reaction
The reaction which is expected to be of higher order but follows first order kinetics due to large excess of one reactant is called pseudo-first order reaction.
Formulae [1]
Formula: Rate Law Expression
\[\mathrm{Rate}=k[A]^a[B]^b\]
Where:
k = Rate constant
[A], [B] = Concentrations of reactants
a = Order with respect to A
b = Order with respect to B
Overall Order of Reaction:
Order = a + b
Theorems and Laws [2]
Collision Theory
According to this theory, the reactant molecule is assumed to be a hard sphere, and the reaction is postulated to occur when molecules collide with each other. It is related to the rate as,
\[\mathrm{rate}=PZ_{AB}e^{-E_{a}/RT}\]
where,
Zₐᵦ = collision frequency of reactants A and B.
P = steric factor.
Theory: Collision Theory of Bimolecular Reactions
- Chemical reactions occur due to collisions between reactant molecules.
- Only those collisions are effective in which molecules possess minimum energy called activation energy (Ea).
- Molecules must have proper orientation during collision for reaction to occur.
- During collision, a temporary unstable species called activated complex is formed:
A + B–C → A···B···C → A–B + C - The energy barrier that must be overcome is called activation energy (Ea), and the fraction of molecules having sufficient energy is: f = e−Ea/RT
Key Points
Key Points: Types of Reaction
| Type of Reaction | Description | Example |
|---|---|---|
| Elementary reaction | Occurs in a single step | O₃ → O₂ + O |
| Unimolecular reaction | One reactant involved | C₂H₅I → C₂H₄ + HI |
| Bimolecular reaction | Two reactants involved | O₂ + O → O₃ |
| Complex reaction | Occurs in multiple steps | NO₂Cl → NO₂ + Cl \[\frac{\mathrm{NO}_2\mathrm{Cl}+\mathrm{Cl}\longrightarrow\mathrm{NO}_2+\mathrm{Cl}_2}{2\mathrm{NO}_2\mathrm{Cl}\longrightarrow2\mathrm{NO}_2+\mathrm{Cl}_2}\] |
Difference between Order and Molecularity:
| Order | Molecularity |
|---|---|
| Determined experimentally | Theoretical concept |
| Sum of powers in the rate law | Number of reacting molecules |
| Can be 0, a fraction or an integer | Always Integer |
| Not based on a balanced equation | Based on a balanced chemical equation |
Key Points: Integrated Rate Equations
| Concept | Zero Order Reaction | First Order Reaction |
|---|---|---|
| Rate law | Rate = k | Rate = k[A] |
| Differential form | \[-\frac{\mathrm{d[A]}}{[\mathrm{dt]}}=\mathrm{k}[\mathrm{A}]^{0}=\mathrm{k}\] | \[-\frac{\mathrm{d[A]}}{[\mathrm{dt]}}=\mathrm{k[A]}\] |
| Integrated form | \[\mathrm{k}=\frac{\left[\mathrm{A}\right]_{0}-\left[\mathrm{A}\right]_{t}}{\mathrm{t}}\] | \[\mathrm{k=\frac{2.303}{t}\log_{10}\frac{\left[A\right]_{0}}{\left[A\right]_{t}}}\] |
| Unit of k | mol L⁻¹ s⁻¹ | s⁻¹ |
| Half-life (t₁/₂) | \[\mathrm{t}_{1/2}=\frac{[\mathrm{A}]_0}{2\mathrm{k}}\] | t₁/₂ = 0.693 / k |
| Dependence | Independent of concentration | Depends on concentration |
Key Points: Temperature Dependence of Reaction Rates
- The rate constant (k) depends on temperature
- \[k=Ae^{-E_{a}/RT}\]
A = frequency factor
Ea = activation energy
T = temperature - \[e^{-E_a/RT}\] represents fraction of molecules having energy ≥ Ea
- Increase in temperature → fraction of molecules with energy ≥ Ea increases → rate increases
Key Points: Molecularity of Elementary Reactions
Important Questions [29]
- The rate of reaction for certain reaction is expressed as: 13d[A]dt=-12d[B]dt=-d[C]dt The reaction is ______.
- In a first-order reaction A → B, 60% of a given sample of a compound decomposes in 45 mins. What is the half-life of reaction? Also, write the rate law equation for the above first-order reaction.
- For the reaction A + B → P. If [B] is doubled at constant [A], the rate of reaction doubled. If [A] is triple and [B] is doubled, the rate of reaction increases by a factor of 6. Calculate the rate
- Identify the molecularity of following elementary reaction: NO(g) + O3(g) → NO3(g) + O(g)
- Distinguish between Order and Molecularity of reaction.
- Define molecularity of reaction.
- A reaction takes place in two steps: (i) NOA(g)+ClA2A(g)⟶NOClA2A(g) (ii) NOClA2A(g)+NOA(g)⟶2NOClA(g) (a) Write the overall reaction. (b) Identify the reaction intermediate.
- Derive an expression for the relation between half-life and rate constant for first-order reaction.
- Obtain the expression for half-life and rate constant of the first-order reaction.
- Define the half-life of a first-order reaction.
- Which of the following correctly represents integrated rate law equation for a first order reaction in a gas phase?
- Explain pseudo first order reaction with a suitable example.
- Show that the time required for 99.9% completion of a first-order reaction is three times the time required for 90% completion.
- Sucrose decomposes in acid solution into glucose and fructose according to the first order rate law with 𝑡12 = 3 hours. What fraction of the sample of sucrose remains after 8 hours?
- What is zero order reaction?
- Derive integrated rate law for zero order reaction.
- Derive integrated rate law for a zero-order reaction A⟶Product.
- If unit of rate constant is mol dm−3s−1, the order of reaction would be ______.
- The Integrated Rate Equation for First Order Reaction is a → Products
- Derive the Relation Between Half Life and Rate Constant for a First Order Reaction
- The Rate Constant for a First Order Reaction is 100 S–1. the Time Required for Completion of 50% of Reaction is
- Sucrose decomposes in acid solution to give glucose and fructose according to the first order rate law. The half life of the reaction is 3 hours. Calculate fraction of sucrose which will remain after 8 hours
- The Unit of Rate Constant for Zero Order Reaction is
- Which Among the Following Reactions is an Example of a Zero Order Reaction?
- The Half-life Period of Zero Order Reaction A → Product is Given by
- The Half Life Period of a First Order Reaction is . Calculate the Rate Constant
- In a First-order Reaction a → Product, 80% of the Given Sample of Compound Decomposes in 40 Min. What is the Half-life Period of the Reaction?
- Explain pseudo-first-order reaction with an example.
- How will you determine activation energy from rate constants at two different temperatures?
Concepts [14]
- Concept of Chemical Kinetics
- Rate of Reactions
- Rate of Reaction and Reactant Concentration
- Molecularity of Elementary Reactions
- Integrated Rate Equations
- First Order Reactions
- Zero Order Reactions
- Half Life Period of a Reaction
- Pseudo First Order Reaction
- Integrated Rate Law for Gas Phase f Reactions
- Collision Theory of Bimolecular Reactions
- Temperature Dependence of Reaction Rates
- Effect of a Catalyst on the Rate of Reaction
- Overview of Chemical Kinetics
