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Activation energy is the minimum energy required by molecules/atoms so that they can undergo a reaction or perform a task. It has many real-life uses. For example- starting a car engine. Turning the key causes a spark that triggers the combustion of gasoline in the engine. Gas combustion would not occur without a source of energy to initiate the reaction. This source of energy is the activation energy.
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Key Terms: Rate of Reaction, Chemical Reactions, temperature, reactant, catalyst, Gas combustion, energy, activation energy
Activation Energy: Definition
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Activation energy is defined as the least amount of energy that is required for atoms or molecules to perform a chemical reaction. It can be simply described as extra energy given to something to get some work done.
The standard units of activation energy are LCal/Mol, KJ/mol, J/ mol. It is denoted by Ea.
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Formula of Activation Energy
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The formula used to determine the value of activation energy, Ea is-
K=Ae-Ea/RT
Where,
K indicates the constant rate
A indicates Arrhenius Constant
Ea indicates Activation energy
R determines Gas constant = 8.34 J/K/mol
K=Ae-Ea/RT
Taking log on both sides
log K = log A - (Ea/RT)log e
2.303 log K = 2.303 log A - Ea/RT
log K = log A - Ea/ 2.303 RT
Concept of Activation Energy
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It is next to impossible to perform a chemical experiment unless we provide some energy in any form to the reacting system. Therefore an amount of extra energy is provided to the reactant to bring the energy of reactant equals to the threshold energy.
Let us consider two compounds M and N perform a reaction to form a product O.
M+N → O
As we have already studied that reactants have a certain amount of energy, Er. Now, consider that some amount of energy is provided to the reactant. When they gain energy the molecules present in M and N will collide and will stick together to form a bond MN.
To cross the barrier, extra energy is required and the energy provided is the activation energy.
Therefore the excess energy (the energy which is supplied above the average energy of the reactant), supplied is known as activation energy.
Let’s assume that the reactants i.e. M and N are having an energy of 50J, and to cross the transition state, it will require 80J of energy and this energy will be threshold energy(ET). This implies that 30J of extra energy will require to cross the barrier, so the extra energy required is known as activation energy (Ea). After that, product C will be formed.
Factors affecting Activation Energy
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Activation energy can be affected by two of the factors:-
- Nature of Reactant
- Catalyst
1. Nature of Reactant
Activation energy can be affected by the nature of the reactant- ionic reactant and covalent reactant. In the case of a covalent reactant, the value of activation energy will be high as some energy will be required to break the older bonds. In the case of ionic reactants, the value of activation energy will be lesser than the value of activation energy as there will be an attraction between the reacting species.
2. Catalyst
The catalyst acts as an important factor in affecting activation energy. There are two types of catalysts- negative catalysts and positive catalysts. In the case of a negative catalyst, the value of activation energy will be high whereas, in the case of a positive catalyst, the value of activation energy will be low.
Catalyst
Examples of Activation Energy
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The common type of reaction that uses activation energy involves many kinds of fire or combustion. There are basically two types of reaction- Exothermic reaction and Endothermic reaction
Exothermic reaction
An exothermic reaction is a type of reaction where energy will be released in the form of light or heat. The energy released will be negative for the change in enthalpy.
The activation energy (Ea) for the forward reaction will be less than the activation energy for the backward reaction.
Let,
Activation energy for Forward reaction (Ea)f
Activation energy for Backward reaction (Ea)b
(Ea)f < (Ea)b
(ROR)f > (ROR)b
ΔH = (Ea)f – (Ea)b
ΔH = -ve value
(Ea)f = ΔH Only one can be possible for Exothermic Reaction
(Ea)f > ΔH
(Ea)f < ΔH
Exothermic Reaction
Endothermic reaction
When a chemical bond is broken by a release of energy, the energy can be supplied from various sources like heat, sunlight or light. In the endothermic reaction, chemical bonds are formed by absorbing the heat from its surroundings and the energy released will be positive.
The activation energy for the forward reaction will be greater than the energy for the backward reaction
Let,
Activation energy for Forward reaction (Ea)f
Activation energy for Backward reaction (Ea)b
(Ea)f ? (Ea)b
(ROR)f < (ROR)b
ΔH = (Ea)b – (Ea)f
ΔH = +ve value
(Ea)f > ΔH (Always)
This value of (Ea)f is universal.
Endothermic reaction
Activation Energy Catalysts
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A catalyst is a type of substance that plays a vital role in either increasing or decreasing the rate of a chemical reaction. A catalyst tends to lower the activation energy. They are basically of two types- Negative catalyst and Positive catalyst.
Negative catalyst
It is a type of catalyst that helps in decreasing or slowing down the rate of the chemical reaction.
Negative catalyst
Positive catalyst
It is a type of catalyst that increases the rate of reaction and helps in carrying the chemical reaction very fast.
Negative catalyst
Activated Complex
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When the molecules of reactant combine with each other at the highest energy level, an intermediate compound is formed which will remain in equilibrium with the main reactant. If an intermediate complex has equal or higher energy than the threshold limit, then it will be converted into a product.
Things to Remember
- Activation energy is defined as the least amount of energy required to perform a chemical reaction. It is denoted by Ea.
- The standard units of activation energy are LCal/Mol, KJ/mol, J/ mol.
- The activation energy depends on the nature of the reactant and catalyst.
- A catalyst is responsible for increasing or decreasing the rate of a chemical reaction.
- An activated complex is an unstable arrangement of atoms that exists momentarily at the peak of the activation energy barrier.
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Sample Questions
Ques: Define the activation energy of a reaction. (All India 2011, 1 Mark)
Ans: The least amount of energy that is absorbed by the reactant molecules to form the activated complex is called activation energy. The catalyst decreases the activation energy of the reaction.
Ques: Define the following :
(i) Order of a reaction
(ii) Activation energy of a reaction (All India 2009, 2 Marks)
Ans: (i) Order of a reaction: It is the sum of powers of molar concentrations of reacting atoms/compounds in the rate equation of the reaction. It can be calculated experimentally. The order of a reaction is a whole number, zero, fractional, positive or negative. It is meant for the reaction and not for its individual steps.
(ii) Activation energy of a reaction: The minimum amount of extra energy required by the reacting molecules to form the activated complex is called activation energy.
Ques: How does a change in temperature affect the rate, of a reaction? How can this effect on the rate constant of a reaction be represented quantitatively? (Comptt. All India 2014, 2 Marks)
Ans: The rate constant of a reaction increases when the temperature increase. It becomes nearly double for every 10° increase in temperature.
The effect can be represented quantitatively by Arrhenius equation K = Ae-Ea/RT
Where, Ea = Activation energy of the reaction; A = Frequency factor
Ques: (a) Explain why H2 and O2 do not react at room temperature.
(b) Write the rate equation for the reaction A2 + 3B2 → 2C, if the overall order of the reaction is zero. (Comptt. All India 2017, 2 Marks)
Ans: (i) H2 and O2 do not react at room temperature because they do not have a sufficient amount of activation energy to overcome the exceptionally high activation energy barrier.
(ii) A2 + 3B2 → 2C
Rate = (dx/dt)
Rate = K[A]° [B]° = K (rate constant)
Ques: For a decomposition reaction the values of rate constant k at two different temperatures are given below :
k1 = 2.15 × 10-8 L mol-1s-1 at 650 K
k2 = 2.39 × 10-7 L mol-1s-1 at 700 K
Calculate the value of activation energy for this reaction.
(R = 8.314 JK-1mol-1) (All India 2009, 2 Marks)
Ans: Given that:
k1 = 2.15 × 10-8Lmol-1s-1, T1 = 650 K
k2 = 2.39 × 10-7 Lmol-1s-1, T2 = 700 K
R = 8.314 J K-1mol-1 Ea =?
Ques: The rate of a reaction becomes four times when the temperature changes from 293 K to 313 K. Calculate the energy of activation (Ea) of the reaction assuming that it does not change with temperature. [R = 8.314 JK-1mol-1, log 4 = 0.6021] (All India 2013, 3 Marks)
Ans: ![The rate of a reaction becomes four times when the temperature changes from 293 K to 313 K. Calculate the energy of activation (Ea) of the reaction assuming that it does not change with temperature. [R = 8.314 JK-1mol-1, log 4 = 0.6021]](https://images.collegedunia.com/public/image/640fccb10beb52b90170f7b9a5823e63.png?tr=w-367,h-305,c-force?tr=w-362,h-301,c-force)
Ques: The rate constant of a reaction at 500 K and 700 K are 0.02 s-1 and 0.07 s-1 respectively. Calculate the value of activation energy, Ea (R = 8.314 JK-1mol-1) (Comptt. Delhi 2015, 3 Marks)
Ans: Given that
k2 = 0.07 s-1
k1 = 0.02 s-1
T1 = 500 K
T2 = 700 K
Ea = ?
Ques: The rate of a reaction increases four times when the temperature changes from 300 K to 320 K. Calculate the energy of activation of the reaction, assuming that it does not change with temperature. (R = 8.314 JK-1mol-1) (All India 2016, 3 Marks)
Ans: Given that
T1 = 300K
T2 = 320K
Consider K1 = K
K2 = 4 K
R = 8.314
Ea = ?
Substituting these values in the formulae of activation Energy,
Energy of Activation Ea = 55327.46 = 55.3 KJmol-1
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