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Chemical Kinetics is the branch of physical chemistry that is related to the understanding of rates of chemical reactions. It is contrary to thermodynamics, which deals with the direction in which a process occurs.
- Chemical Kinetics is used to determine the rate of chemical reaction.
- The process also helps in altering the rate of reaction.
- Ionic reactions occur at a very fast rate.
- It is also known as reaction kinetics.
- The term was first introduced in 1850 by German chemist Ludwig Wilhelmy.
- Chemical kinetics helps study about the mechanisms of the reaction.
- It is used in the fields of cosmology, biology, geology and engineering.
- The process is entirely based on the physical process of reactions.
- Concentration, temperature, pressure and catalyst are factors influencing the rate of reaction.
Key Terms: Chemical Kinetics, Kinetics, Transition State, Complex Reaction, Reaction Kinectics, Rate of Reaction, Parallel Reactions, Zero Order Reactions, First Order Reaction, Half-life of a Reaction
What is Chemical Kinectics?
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Chemical Kinetics is a process that deals with the speed at which chemical reactions take place. It will carry out experimental conditions to determine the speed of the chemical reaction.
- Chemical Kinetics tells about the transition state of different elements.
- It helps in the construction of a mathematical model which describes various chemical reactions.
- The reaction is used to transform one chemical substance into another.
- Curve-fitting and direct computations are two chemical kinetics methods.
- Free energy change will determine whether the chemical change will take place.
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Chemical Reactions
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A chemical reaction is a process in which one or more substances (reactants) are converted to one or more different substances (products). Substances are either chemical elements or compounds.
- A chemical reaction rearranges constituent atoms of reactants to create different substances as products.
- It rearranges the atoms of reactants.
- In these reactions, bonds are broken inside the molecules.
- The new compounds formed after chemical reactions are called products.
- It depends upon the physical changes like precipitation, heat production and colour change.
Examples of Chemical Reactions
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Rate of a Chemical Reaction
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The rate of a chemical reaction can be expressed as follows:
- The rate of a chemical reaction decreases in the concentration of the reactant.
- It increases with an increase in the concentration of the product.
- The SI unit used to measure the concentration of a chemical is molar, which is also written as mol/L.
- Molar indicates the number of moles in 1 litre of the chemical.
The process can be explained as follows:
- If R → P is a chemical reaction where R= Reactant and P= Product.
- Let's assume [R]1 and [R]2 are concentrations of reactants (R) at times T1 and T2, respectively.
- [P]1 and [P]2 are concentrations of the product (P) at time T1 and T2, respectively.
Rate of disappearance of reactant [R] =(-1) X [R]2 - [R]1 / T2 - T1
- Since it is calculating the disappearance of reactant it is multiplied with (-1).
Rate of appearance of product [P] = [P]2 - [P]1 / T2 - T1
- The above formulas can be used to calculate the average rate of reaction and instantaneous rate.
- The rate reaction equation is a differential equation.
Average Rate of Reaction
In the case of average rate, the difference between reactants, products and time is considered from the beginning to the end of the reaction. It uses the starting and ending times of the reaction concentration of chemicals are used.
Instantaneous Rate of Reaction
In the case of instantaneous rate, the difference between concentrations of chemicals is for a small time window during the reaction. It uses the concentration of chemicals recorded at two different times during the reaction. The time difference is very small in the case of instantaneous rate.
Rate of Reaction
Factors Affecting the Reaction Rate
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The rate of reaction depends upon the following factors which are as follows:
Concentration of Chemicals
The concentration of chemicals in reactants and products plays a significant role in the reaction. The representation of the rate of reaction in terms of concentration is termed the rate equation/rate law. The rate of expression can also be written as
Rate ∝ [A] x [B] y
- where x, y are the stoichiometric coefficients of reactant A and reactant B respectively.
- Stoichiometric Coefficient is the number mentioned in front of ions, molecules of a chemical to balance the equation on both ends.
Rate of Constant
When the concentrations of both reactants are in unity, then the rate of reaction calculated is termed as rate constant, usually denoted with the letter “K” and sometimes also termed as proportionality constant in the rate of expression. The rate of expression can further be mentioned as
Rate = K [A] x [B] y
(a) Chemical reaction rates and (b) reaction rate constants for the infinite-level system initially at a temperature of 298 K.
Order of Reaction
The sum of powers of the reactants in rate expression is termed as order of reaction. If Rate = K [A] x [B] y then order of reaction = x+y
Molecularity of Reaction
The total number of atoms, ions and molecules taking part in making an elementary reaction successful by simultaneously colliding against each other is called the molecularity of the reaction.
- This is always a whole number.
- It can not exceed the number 3 or be less than 1.
- Elementary Reaction is a chemical reaction that takes place in just 1 step.
Effect of Temperature
Most chemical reactions are observed to accelerate the reaction process with an increase in temperature. It has also been observed that a rise in temperature by 10° results in the rate constant (k) becoming double for that reaction.
Arrhenius equation
The equation shows the relation between temperature and rate constant. Arrhenius equation is given as
k = A e -Ea /RT
- Here k = rate constant
- A= Frequency factor
- Ea = Activation Energy
- R= Gas constant
- T= Temperature
- Comparing rate constant of a reaction at 2 different temperatures
Log k 2 / k 1 = Ea / 2.303R [ T 2 - T 1 / T 1T 2]
Effect of Catalyst
A substance which increases the rate of chemical reaction without itself undergoing any chemical change permanently is called catalyst.
Zero Order Reactions
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The rate of chemical reaction is proportional to the zero power to that of the reactant’s concentration.
Rate ∝ [A] 0
- Rate = K[A] 0 which gives Rate = K
- Further the zero order reaction equation can be written as
K = [R] 0 - [R] / t
- R 0 = Initial concentration of Reactant R
- T = time
- K= Reaction Constant
- R = Concentration of Reactant.
Derivation of Zero Order Reaction
Equation for zero-order reaction the R → P
- Rate = - d[R] / dt = k[R] 0
- Rate = - d[R] / dt = k x 1 {anything raised to power 0 is 1}
- d[R] = -k x dt
- R = -kt + I { Integrating both sides and I is integration constant}
- At t=0 the concentration of reactant is [R] 0,which denotes the initial concentration of reactant. Substitute the same in the previous equation.
- [R] 0 = I
- Further substituting the value of I in the above equation.
- R = -kt + [R] 0
- Hence we get the zero order reaction equation:
K = [R] 0 - [R] / t
First Order Reaction
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The rate of chemical reaction is proportionate to the first power of the concentration of the reactant [R]. The equation for first order chemical reaction after integrating the differential rate equation and substituting 1 to the power of reactant we get.
Log [R] 1 / [R] 2 = k (T 2 - T 1 ) / 2.300
Where [R] 1 & [R] 2 are concentrations of reactant at T 1 & T 2 during the chemical reaction.
Pseudo First Order Reaction
Pseudo First Order Reaction have two molecules on the reactant side but because one reactant is in excess it behaves as a first order reaction. Such reactions are termed pseudo first order reactions.
Half-life of a Reaction
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It is the time taken by concentration of reactants to reduce by half compared to its initial concentration. Half life of a zero order reaction can be written as:
T 1/2 = R 0 / 2k
- Where T 1/2 stands for time for the reaction to reach half life
- R 0 is the reactant concentration and k stands for the rate constant.
- Equation for half life of a first order reaction:
T 1/2 = 0.693 / k
Important Topics for JEE MainAs per JEE Main 2024 Session 1, the important topic included in the chapter Chemical Kinetics is First Order Reaction. Some memory based important questions asked in JEE Main 2024 Session 1 include:
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Things to Remember
- Chemical Kinetics governs the rate of reactions along with their mechanisms.
- In a chemical change, reactants and products are involved.
- The rate of reaction is positive for the product and negative for the reactant.
- It is derived from a Greek word meaning chemical movement.
- The rate decreases the reaction progress due to a decrease in the concentration(s) of the reactant(s).
- As the chemical reaction proceeds, the concentration of the reactants decreases, i.e., products are produced.
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Previous Year Questions
- Calculate the activation energy for the reaction (R=8.314JK−1mol−1)
- When a graph is plotted between log K and (1T), the slope of line obtained represents: (K = rate constant, T = temperature)
- Inversion of cane sugar by acid hydrolysis in excess of water is
- For a reaction, the half-life is independent of initial concentration. What is the order of that reaction?
- For a 1st order reaction the unit of rate constant is
- Formula for half life of a zero order reaction is:
- Catalyst increase rate of reaction while photosensitizer initiate a reaction. Catalyst decrease E but photosensitizer act as energy carrier
- The minimum energy required for molecules to enter into a chemical reaction is called
- The effect of a catalyst in a chemical reaction is to change
Sample Questions
Ques. What is the activation energy of a reaction? (3 marks)
Ans. The minimum extra energy required by the reactant or absorbed by the reactant molecule to form the activated complex is called activation energy. The SI unit is J/ Mol.
- The energy is required to activate the molecules.
- The atoms will undergo a chemical reaction transformation.
- Nature of reactants and catalyst affect the rate of activation energy.
- Exothermic reaction is an example of activation energy.
Ques. Differentiate between rate of reaction and rate of constant? (3 marks)
Ans. The difference between the rate of expression and rate of constant are as follows:
| Rate of Reaction | Rate of Constant |
|---|---|
| Rate of reaction is a process of changing the concentration of a reactant or product in unit time. | Rate of constant is a process where concentration of reactant is equivalent to unity in a chemical reaction. |
| It depends upon the concentration of reactants, catalyst and temperature. | It depends upon the concentration of reactants and temperature. |
| Its SI unit is mol L-1 s-1 | Its SI unit is (mol L-1)1-n s-1 |
Ques. A chemical reaction is found to be in first order in X and second order in Y.
(A) Mention it’s differential rate equation.
(B) What would be the effect on rate if concentration of Y is tripled?
(C) What would be the effect on rate if concentration of both X and Y is increased by 2 times? (4 Marks)
Ans. (A) Differential rate equation :
dx / dt = R 1 = K [X] 1 [Y] 2
(B) Concentration of Y is increased 3 times:
Rate, R 2 = K [X] 1 [3Y] 2
On solving R 2 = 9*R 1
Hence the rate of reaction increases by 9 times on tripling Y concentration
(C) When concentration of both X and Y are increased 2 times
R 3 = K [2X] 1 [2Y] 2
On solving R 3 = 8*R 1
Hence the rate of reaction increases by 8 times on doubling the concentration of X and Y
Ques. Define ‘rate of a reaction’? (2 Marks)
Ans. Rate of a reaction: Either, The change in the concentration of any one of the reactants or products per unit time is called rate of a reaction. Or, The rate of a chemical reaction is the change in the molar concentration of the species taking part in a reaction per unit time. The change in concentration of reactant or product per unit time is called rate of reaction.
Ques. Define the following:
(A) Elementary step in a reaction
(B) Rate of a reaction
(C) Pseudo First Order Reaction? (3 Marks)
Ans. (A) Elementary step in a reaction: Those reactions which take place in one step are called elementary reactions.
Example : Reaction between H2, and I2 to form 2HI
H2 + I2 → 2HI
(B) Rate of a reaction: The change in the concentration of any one of the reactants or products per unit time is called rate of reaction.
(C) Pseudo First Order Reaction: It have two molecules on the reactant side where one reactant is in excess and it behaves as a first order reaction. Such reactions are termed pseudo first order reactions.
Ques. What do you understand by the rate law and rate constant of a reaction? Identify the order of a reaction if the units of its rate constant are : (A) L-1 mol s-1 (B) L mol-1 s-1? (3 Marks)
Ans. The rate of reaction is found to depend on α concentration of term of reactant A and β concentration term of reactant B
Then Rate of reaction ∝ [A]α [B]β
or Rate = K [A]α [B]β
This expression is called Rate law.
‘K’ in this expression is called Rate constant. Rate constant’s unit:
(A) Unit = L-1 mol s-1 → Zero order reaction
(B) Unit = L mol-1 s-1 → Second order reaction.
Ques. Define the following terms:
(A) Zero order reaction.
(B) Half life period of reaction (t1/2)? (2 Marks)
Ans. (A) Zero Order Reaction: The rate of chemical reaction is proportional to the zero power to that of the reactant’s concentration. Rate ∝ [A] 0 where Rate = K[A] 0 which gives Rate = K
(B) Half Life Period of Reaction: The time taken for half of the reaction to complete is called half life period.
Ques. The initial concentration of N2O5 in the following first order reaction N2O5(g) → 2 NO2(g) + 1/2O2 (g) was 1.24 × 10–2 mol L–1 at 318 K. The concentration of N2O5 after 50 minutes was 0.20 × 10–2 mol L–1. Calculate the rate constant of the reaction at 318 K? (3 marks)
Ans. For a first order reaction it is given as:
- Log [R]1/[R]2 = k(t2-t1)/2.303
- k = 2.303/ (t2-t1) x Log [R]1/[R]2
- k = 2.303/ (50 – 0) x Log 1.24 × 10–2 /0.20 × 10–2
- k = 2.303 / 50 x log 6.2
- k = 0.0364
Ques. Identify the reaction order from each of the following rate constants. (A) k = 4.3 × 10–5 L mol–1 s–1 (B) k = 4 × 10–4 s–1? (2 marks)
Ans. (A) The unit of second order rate constant is L mol–1 s–1, therefore k = 4.3 × 10–5 L mol–1 s–1 represents a second order reaction.
(B) The unit of a first order rate constant is s–1 therefore k = 4 × 10–4 s–1 represents a first order reaction.
Ques. The initial concentration of N2O5 in the following first order reaction N2O5(g) → 2 NO2(g) + 1/2O2 (g) was 2.24 × 10–2 mol L–1 at 318 K. The concentration of N2O5 after 40 minutes was 0.20 × 10–2 mol L–1. Calculate the rate constant of the reaction at 318 K? (3 marks)
Ans. For a first order reaction it is given as:
- Log [R]1/[R]2 = k(t2-t1)/2.303
- k = 2.303/ (t2-t1) x Log [R]1/[R]2
- k = 2.303/ (40 – 0) x Log 2.24 × 10–2 /0.20 × 10–2
- k = 2.303 / 40 x log 11.2
- k = 0.0598
Ques. A chemical reaction is found to be in first order in X and second order in Y.
(A) What would be the effect on rate if concentration of Y is four times?
(B) What would be the effect on rate if concentration of both X and Y is increased by five times? (2 Marks)
Ans. (A) Concentration of Y is increased 4 times:
Rate, R 2 = K [X] 1 [4Y] 2
On solving R 2 = 16*R 1
Hence the rate of reaction increases by 16 times on tripling Y concentration
(B) When concentration of both X and Y are increased 5 times
R 3 = K [2X] 1 [5Y] 2
On solving R 3 = 25*R 1
Hence the rate of reaction increases by 25 times on five the concentration of X and Y
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