- 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 [13]
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"`
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: 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: Average rate of reaction
The change in concentration of reactant or product divided by time interval is called average rate of reaction.
Definition: Instantaneous rate of reaction
The rate of reaction at a particular instant of time is called instantaneous rate of reaction.
Definition: Order of reaction
The sum of powers of concentration terms in the rate law is called order 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.
Theorems and Laws [1]
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: Molecularity of Elementary Reactions
Important Questions [28]
- 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
- Define molecularity of reaction.
- Identify the molecularity of following elementary reaction: NO(g) + O3(g) → NO3(g) + O(g)
- Distinguish between Order and 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.
- 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?
- Obtain the expression for half-life and rate constant of the first-order reaction.
- Explain pseudo first order reaction with a suitable example.
- 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?
- Show that the time required for 99.9% completion of a first-order reaction is three times the time required for 90% completion.
- Derive integrated rate law for a zero-order reaction A⟶Product.
- Derive integrated rate law for zero order reaction.
- 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]
- 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
