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Elementary reactions are those reactions that occur at one go i.e they have a single step. Elementary reactions are steps in an overall reaction i.e reactions in real life are made up of many elementary reactions. These are building blocks of complex reactions occurring all around us. An elementary reaction is one in which the rate law can be written from the coefficients of the balanced chemical equation and agrees with empirical data. These reactions have no intermediate steps and transition in a single step. When the order of a reaction is the same as the coefficient of the reaction, it is known as an elementary reaction.
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Key Terms: Elementary Reaction, Rate law, Bimolecular, Complex reactions, Termolecular reactions, Element, Chemical equation, Radioactive decay
Types of Elementary Reactions
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There are three types of Elementary Reactions”
1. Unimolecular
Those reactions which need only 1 molecule to collide with itself and produce one or more products. These reactions are generally of 1st order.
Example: Radioactive decay
A → B where Rate = k [A]
2. Bimolecular
Those reactions which come into effect on the collision of 2 molecules producing two or more products. These reactions are of 2nd order.
Example: Organic Reactions
2A → B where Rate = k [A]2
3. Termolecular
Those reactions which are carried out with the collision of 3 molecules at the same time. These reactions are very rare as all the reacting molecules must have sufficient energy and proper orientation to react simultaneously. These reactions are of 3rd order.
Example: CH3 + O2 + M → CH3 O2 + M
A + A + A → B where Rate = k [A]3
A + A + B → C where Rate = k [A]2 [B]
A + B + C → D where Rate = k [A] [B] [C]
k is the reaction rate/equilibrium constant in the above reactions.
Also Read:
| Related Articles | ||
|---|---|---|
| Collision Theory of Chemical Reactions | Integrated Rate Equations | Green Chemistry |
| Rate of a Chemical Reaction | First order reactions | SN1 Reaction |
Rate Law
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It is the expression in which reaction rate is given in terms of the molar concentration of reactants with each term raised to some power. It may or may not be similar to the stoichiometric coefficient of the reacting species in a balanced chemical equation. For example: 2NO(g) + O2(g) → 2NO2 (g)
Things to Remember
- Any reaction can take place when the particles of the reactants collide i.e the right environment must be provided for the molecules to collide with each other to produce products.
- Higher the concentration of the reactant more will be the rate of the reaction as more molecules of the reactant are available for collisions to take place. For instance, concentrated sulphuric acid will be more reactive than diluted sulphuric acid.
- Elementary reactions are single-step reactions. Many elementary reactions take place to form complex reactions.
- The rate order of a reaction is the coefficient of the reactants and it is calculated by multiplying with the rate constant k and mentioning the power of the reactants matching the coefficient on the reactant side.
- Termolecular reactions are of three different types where molecules can be of the same or different reactants. These reactions are very rare.
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Sample Questions
Ques. How many intermediates are produced in an elementary reaction? (1 mark)
Ans. An elementary reaction produces 0 intermediates and occurs in a single step.
Ques. Calculate the overall order of a reaction which has the following rate expression
a)Rate = k [A]3/2 [B]5/2
b)Rate = k [A]7/2 [B] – 2
c)Rate = k [A]2 [B] – 1 (3 marks)
Ans. Rate = k [ A ]p [ B ]q
Order = x + y
- 3/2 + 5/2 = 8/2 = 4 order
- 7/2 - 2 = 3/2 = 1.5 order
- 2 - 1 = 1 = 1st order
Ques. Is zero-order reaction possible? If yes, what is a zero-order reaction? (2 marks)
Ans. Yes, a zero-order reaction is possible.
A chemical reaction where the reaction takes place irrespective of the concentration of the reactants.
Ques. What is the molecularity of a reaction? What are different types of molecularity in chemical reactions? Show with chemical reactions. (4 marks)
Ans. Multiple reactant species (atoms, ions, or molecules) that take part in an elementary reaction must collide simultaneously to cause a chemical reaction. The number of these reactant species is called the molecularity of a reaction. The different types of molecularity are:
- Unimolecular - Ammonium nitrite decomposition
NH4NO2 → N2 + 2H2O
- Bimolecular - Hydrogen Iodide dissociation
2HI → H2 + I2
- Trimolecular / termolecular - Nitric Oxide oxidation
2NO + O2 → 2NO2
Ques. Define the rate of a reaction? [Delhi 2010]
Ans. The change in the concentration of any one of the reactants or products per unit time is called the rate of a reaction.
Ques. A reaction is of second order with respect to a reactant. How is its rate affected if the concentration of the reactant is (i) doubled (ii) reduced to half? [All India 2012]
Ans. Given rate = k [R]2
(i) If R is 2R → rate = k [2R]2 = 4 k [R]2
Therefore, the rate will become 4 times the initial rate.
(ii) If R is 1/2R → rate = k [1/2R]2 = 1/4 k [R]2
Therefore, the rate will become 1/4 times the initial rate.
Ques. Write two differences between ‘order of reaction’ and ‘molecularity of reaction’. [Delhi 2014]
Ans.
| Order of reaction | Molecularity of reaction |
|---|---|
| It can be fractional as well as zero. | it is always a whole number. |
| It is the sum of the concentration terms on which the rate of reaction actually depends. | It is the number of either atoms, ions, or molecules that must collide with each other simultaneously in order to result in a chemical reaction. |
Ques. For the first order thermal decomposition reaction, the following data were obtained: C2H5Cl(g) → C2H4(g) + HCl(g)
Calculate the rate constant (Given: log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021) (All India 2016)
Ans. Initial Pressure P0 = 0.30 atm
P0 = 0.50 atm
t = 300 seconds
Rate constant, k = (2.303 / t) x log ( P0 / 2P0- Pt )
= (2.303/300) x log ( 0.30 / 2 x 0.30 - 0.50 )
= 0.00767 x log ( 0.30 / 0.10 )
0.00767 x log 3
= 0.00767 x 0.4771
Therefore, the rate constant k is 0.0036 s-1 or 3.66 x 10-3 s-1
Ques. The following data were obtained during the first order thermal decomposition of SO2Cl2 at a constant volume :
Calculate the rate constant k (Given: log 2 = 0.3010, log 4 = 0.6021) [Comptt. Delhi 2016]
Ans. SO2Cl2 (g) → SO2 (g) + Cl2(g)
Using formula, Rate constant, k = (2.303 / 100) x log ( P0 / 2P0- Pt )
When t = 100 s
= (2.303/100) x log ( 0.4 / 2 x 0.4 - 0.7 )
= (2.303/100) x log ( 0.4 / 0.1 )
= (2.303/100) x log 4
= 0.02303 x 0.6021
Therefore, the rate constant k is 0.01386 s-1 or 1.386 x 10-2 s-1
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