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Have you ever wondered how chemical reactions occur and how molecules react to form products? Here is the answer to all your questions. Collision Theory states that for a chemical reaction to occur, the reacting particles must collide with one another that means- Molecules must collide before they can react. For successful collision, molecules must collide with sufficient energy. Molecules must collide in the proper orientation for successful collision. Only a certain fraction of the total number of collisions forms products. The rate of the reaction depends on the frequency of collisions.
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Key Terms: Rate of Reaction, Chemical Reactions, temperature, reactant, elements, collision, molecules
What is Activation Energy?
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Activation energy can be defined as the minimum amount of energy required to form a product during a chemical reaction.
It is generally denoted by Ea
The activation energy of reactants plays an important role to start a chemical reaction, Bonds between reactants must be broken, which requires energy. The energy which is required to break those chemical bonds in reactants is called activation energy.
To start the reaction at room temperature without heating, Activation energy is low at that time.
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What is collision frequency?
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Collision frequency can be defined as the number of collisions per second per unit volume of the reacting mixture.
The collision frequency is generally denoted by Z.
Consider this simple bimolecular step:
A + B →Product
As per the collision theory, the rate of the above reaction can be defined as:
Rate = ZABe -Ea/RT…….(1)
Where:
- ZAB = collision frequency of reactants A and B
- Ea = Activation Energy
- R = Universal Gas Constant
- T = Temperature in absolute scale
Collision frequency
Arrhenius Equation
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According to Arrhenius equation, All the molecules that have energy greater than or equal to activation energy will collide to form the product. But this was not true for all the chemical reactions.
Molecules having sufficient activation energy did not collide to form the product. Only a few of them collide effectively.
Scientists found that the activation energy of molecules is not the only factor that helps the reaction to form the product.
They concluded that only those molecules will form the product that has the threshold energy and proper orientation during the collision.
They introduced P to define effective collision.
Rate = PZABe-Ea/RT………..(2)
Temperature Effect on Chemical Reaction
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Thermal Energy is directly proportional to the motion of molecules. As the temperature rises, it moves faster and collides with more energy, and that leads to bond cleavages. Mostly thermal energy is supplied in the form of thermal energy.
After completion of reaction or product formation, the activation energy is returned in the form of vibrational energy which is released as heat.
Factors affect the rate of a reaction
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Dependence on concentration
The rate of chemical reaction may depend on the concentration of reactants or products at a given temperature.
What is rate law?
Rate of law can be defined as the representation of the rate of reaction in terms of the concentration of the recanting species in a balanced chemical equation.
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Rate Expression and Constant
The Rate of Reaction generally increases due to an increase in the concentration of reactants. Hence, the rate of reaction depends on the concentration of reactants.
Let’s consider this general equation:
aA + bB → cC + dD
Here, a, b, c, d are stoichiometric coefficients of both reactants and products which involve a chemical reaction.
Rate expression of this reaction will be:
Rate ∝ [A]x [B]y
Or
Rate = k [A]x [B]y
Differential rate equation will be:
d R / dt K Ax B y
where k is known as the proportionality constant
- x and y are exponents
- x and y may or may not be equal to the stoichiometric coefficients of the reactants.
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Order of a reaction
In below rate equation:
Rate = k [A]x [B]y
The overall order of this reaction will be the sum of exponents(x + y) of reacting species.
- Order of reaction can be 1, 2, 3, or integer value.
- Order of reaction can be in fraction also.
- Reaction having 0 order is independent of reactants.
Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction.
Things to Remember
- Collision theory is used to predict the rates of chemical reactions, particularly for gases.
- Collision theory is based on the assumption that for a reaction to occur it is necessary for the reacting species (atoms or molecules) to come together/collide with each other.
- The rate of the reaction in collision theory depends on the frequency of collisions. The theory also tells us that reacting particles often collide without reacting.
- Reactions occur when two reactant molecules effectively collide, each having minimum energy and correct orientation.
- Reactant concentration, the physical state of the reactants, and surface area, temperature, and presence of a catalyst are the four main factors that affect reaction rate.
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Sample Questions
Ques. Calculate the overall order of a reaction which has the rate expression
(a) Rate = k [A] 1/2 [B] 3/2
(b) Rate = k [A] 3/2 [B]–1(2 marks)
Solution:
(a) Rate = k [A]x [B]y
Order = x + y
So order = 1/2 + 3/2 = 2,
This above reaction is of second order.
(b) Order = 3/2 + (–1) = 1/2,
The above reaction is of half order.
Ques. Identify the reaction order from each of the following rate constants
k = 2.3 × 10–5 L mol–1 s–1
k = 3 × 10–4 s–1(4 marks)
Solution:
(i) Unit L mol–1 s–1 represents second order reaction, therefor
k = 2.3 × 10–5 L mol–1 s–1 is a second order reaction.
(ii) The unit of a first order rate constant is s–1 therefore
k = 3 × 10–4 s–1 represents a first order reaction.
- Molecularity of a Reaction
The number of reacting species taking part in an elementary reaction, which must collide simultaneously to bring about a chemical reaction, is called the molecularity of a reaction.
The Molecularity of a reaction cannot be zero or a non-integer value.
- Unimolecular Reaction
When one species is involved in a chemical reaction it is called a unimolecular reaction.
For e.g.: Ammonium nitrate.
NH4NO2 → N2 + 2H2O
Ammonium nitrate
- Bimolecular reaction
When two species collide simultaneously to bring out a chemical reaction is called a bimolecular reaction.
For e.g.: Dissociation of hydrogen iodide
2HI → H2 + I2
- Trimolecular reaction
The trimolecular reaction involves the collision of three reacting species simultaneously. The reactions which have reacting species are very rare and that also proceed very slowly.
For e.g.: 2NO + O2 → 2NO2
- Complex reaction
The reaction involves more than 3 reacting species called complex reactions and must proceed in more than one step.
KClO3 + 6FeSO4 + 3H2SO4 → KCl + 3Fe2(SO4)3 + 3H2O
Ques. Mention any two important criterias in order for a reaction to occur. Who gave collision theory? What do we call the collisions which lead to the formation of products?(2 marks)
Ans. The important criterias include-
i) The molecules must collide.
ii) The molecules must have correct orientation, that is, they must collide from the correct side.
The Collision Theory was given by Max. Trautz and W. Lewis v
Collisions which lead to the formation of products are called Effective collisions.
Ques. The number of collisions depend upon:
a)Pressure
b)Concentration
c)Temperature
d)All the above (1 mark)
Ans. The number of collisions depend upon all of the above factors.
Ques. Match the statements given in Column I and Column II. (2 marks)
Ans. (i —» c), (ii -> a), (iii -> d), (iv -> f), (v -» b), (vi-> e)
(i) Catalyst alters the rate of reaction by lowering activation energy.
(ii) Molecularity cannot be fraction or zero. If molecularity is zero, reaction is not possible.
(iii) Second half-life of first order reaction is same as first because half-life
Time is temperature independent. ‘
(iv) e-Ea/RT refers to the fraction of molecules with kinetic energy equal to greater than activation energy.
(v) Energetically favorable reactions are sometimes slow due to improper orientation of molecules causing some ineffective collision of molecules.
(vi) Area under the Maxwell-Boltzmann curve is constant because the total probability of a molecule taking part in a chemical reaction is equal to one.
Ques. According to the Collision theory, the rate of the reaction is:
a)Directly proportional to the rate of collisions. b)Inversely proportional to the rate of collisions.
c)Does not depend upon the rate of collisions.
d)Is always less than the rate of the reaction.(1 mark)
Ans. In the collision theory, the rate of reaction is a)Directly proportional to the rate of collisions
Ques. The minimum energy a molecule should possess in order to enter into a fruitful collision is known as: a)Reaction energy b)Collision energy c)Activation energy d)Threshold energy(1 mark)
Ans. d)Threshold energy is the minimum energy a molecule should possess in order to enter into a fruitful collision.
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