Chemical Kinetics Important Questions with Solutions for CSIR NET, GATE, IIT JAM Entrance Exam
Chemical kinetics is an essential topic in physical chemistry, and it plays a crucial role in various competitive exams such as CSIR NET, GATE, IIT JAM, and other national-level entrance examinations. Understanding the principles of chemical kinetics is vital for predicting reaction rates, determining reaction mechanisms, and optimizing industrial processes. In this article, we will discuss and analyze some important questions related to chemical kinetics, providing explanations for the correct answers.
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| Chemical Kinetics Important Questions with Solutions For entrance Exams like CSIR NET, IIT JAM |
Q.1. The unit of the rate of reaction and constant are the same for
(A) zero-order reaction
(B) first-order reaction
(C) second-order reaction
(D) third-order reaction
Answer: (A) zero-order reaction
Hints:
The rate of a reaction is defined as the change in concentration of a reactant or product with respect to time. The unit rate of reaction is the concentration per unit time, typically moles per liter per second (mol/L/s).
We know that the unit of n^th order reaction of rate constant
k = mol^(1-n) L^(n-1) s^-1
So, n = 0 rate constant become k = mol^(1-0) L^(0-1) s^-1
k = mol^(1) L^(-1) s^(-1)
For a zero-order reaction, the rate expression is rate = k, where k is the rate constant with units of concentration per unit time. Therefore, the unit of the rate of reaction and the rate constant is the same for a zero-order reaction.
The correct answer is (A) zero-order reaction.
Q.2. For the reaction, 2A+B → C+2D which is first order in A and also first order in B, the rate is given by
(A) k [A]2[B]
(B) k [A][B]2
(C) k [A]2
(D) k [A][B]
Answer: (D) k [A][B]
Hints:
In a first-order reaction with respect to each reactant, the rate expression includes the concentration of the reactant raised to the power of 1. Here, the given reaction is first order with respect to both A and B. Hence, the rate expression will be proportional to [A] and [B]. The correct answer is (D) k [A][B].
Q.3. For the ideal gaseous reaction, the rate is generally expressed in terms of dp/dt instead of dc/dt or dn/dt (where C = concentration and n is the no. of moles). What is the relation among these three expressions if T and V are constants?
(A) dC/dt = dP/dt = dn/dt
(B) dC/dt = (1/V) dn/dt = (1/RT) dP/dt
(C) (RT) dC/dt = dP/dt = dn/dt
(D) none of these
Answer: (B) dC/dt = (1/V) dn/dt = (1/RT) dP/dt
Hints:
In an ideal gaseous reaction, the rate expression is generally expressed in terms of pressure (P) rather than concentration (C) or number of moles (n). The relation between the rate expressions involving concentration and pressure is given by the ideal gas law: PV = nRT. When the temperature (T) and volume (V) are constant, we can rewrite it as P = nRT/V.
Now, we need to differentiate the ideal gas law with respect to time (t):
d(PV)/dt = d(nRT)/dt
V dP/dt + P dV/dt = R dn/dt
Since V and R are constants, and for an ideal gas, dn/dt is the rate of change of moles of the gas, we can simplify the above expression as:
V dP/dt = R dn/dt
And, finally, rearranging the equation to solve for dP/dt:
dP/dt = (1/V) dn/dt
Furthermore, using the equation P = nRT/V, we can relate dP/dt to dC/dt:
dC/dt = (1/RT) dP/dt
Hence, the correct answer is (B) dC/dt = (1/V) dn/dt = (1/RT) dP/dt.
Q.4. If r = K [A]^2[B] for a reaction, by what factor is the initial rate multiplied if the [A] is multiplied by 1.5 and the [B] is tripled?
(A) 4.5
(B) 2.25
(C) 6.75
(D) none of these
Answer: (C) 6.75
Hints:
The rate law given is r = K [A]^2[B]. When [A] is multiplied by 1.5 and [B] is tripled, the new rate can be calculated as follows:
New rate = K (1.5[A])^2 (3[B])
New rate = K * 2.25[A]^2 * 3[B]
New rate = 6.75 K [A]^2[B]
The new rate is 6.75 times the initial rate, which means the initial rate is multiplied by a factor of 6.75. The correct answer is (C) 6.75.
Q.5. The rate expression for the reaction A(g)+B(g) → C(g) is rate = k[A]^1/2[B]^2. What changes in rate if the initial concentration of A and B increase by a factor of 4 and 2 respectively?
(A) 4
(B) 6
(C) 8
(D) 2
Answer: (C) 8
Hints:
The rate law given is rate = k[A]^1/2[B]^2. When the initial concentrations of A and B increase by a factor of 4 and 2 respectively, the new rate can be calculated as follows:
New rate = k * (4[A])^1/2 * (2[B])^2
New rate = k * 2[A] * 4[B]^2
New rate = 8 k [A] [B]^2
The new rate is 8 times the initial rate, which means the rate increases by a factor of 8. The correct answer is (C) 8.
Q.6. Reaction A → B follows second-order kinetics. Doubling the concentration of A will increase the rate of formation of B by a factor of
(A) 1/4
(B) 1/2
(C) 2
(D) 4
Answer: (C) 2
Hints:
For a second-order reaction, the rate expression is given by rate = k[A]^2. If we double the concentration of A, the new rate can be calculated as follows:
New rate = k * (2[A])^2
New rate = k * 4[A]^2
The new rate is 4 times the initial rate, which means the rate increases by a factor of 4. The correct answer is (C) 2.
Q.7. An elementary reaction between A and B is a second-order reaction. Which of the following rate equations must be correct?
(A) r = k[A]2[B]0
(B) r = k[A]3/2[B]1/2
(C) r = k[A]0[B]2
(D) r = k[A][B]
Answer: (D) r = k[A][B]
Hints:
An elementary reaction is a reaction that occurs in a single step and follows the stoichiometry of the balanced equation. For an elementary second-order reaction, the rate expression will involve the concentration of each reactant raised to the power of 1 (first-order). The correct answer is (D) r = k[A][B].
Q.8. For an elementary reaction 2A+B → A2B, if the volume of the vessel is quickly reduced to half of its original volume, then the rate of reaction will
(A) unchanged
(B) Increase four times
(C) Increase eight times
(D) Decrease eight times
Answer: (C) Increase eight times
Hints:
In an elementary reaction, the reaction rate is determined by the stoichiometric coefficients of the reactants in the balanced equation. For the given elementary reaction, 2A + B → A2B, the stoichiometric coefficients are 2 and 1, respectively.
According to the rate expression for this elementary reaction, the rate is directly proportional to the concentrations of reactants raised to their respective stoichiometric coefficients. Therefore, when the volume of the vessel is quickly reduced to half, the concentrations of both A and B will effectively double.
The rate of reaction is expected to increase by a factor equal to the ratio of the new concentration to the original concentration of A and B. Since both A and B concentrations double, the rate of reaction will increase by a factor of 2 (for A) * 2 (for B) = 4.
Hence, the correct answer is (C) Increase eight times.
Q.9. If the rates of a reaction are R1 and R2 at concentrations C1 and C2 of the reactant respectively, the order of reaction n (assuming that the concentrations of all order reactants and T remain constant) with respect to that reactant is given by
(A) n = (log (R1 / R2)) / (log (C1 / C2))
(B) n = (log (R2 / R1)) / (log (C1 / C2))
(C) n = (log (C1 / C2)) / (log (R1 / R2))
(D) n = (log (R1 / C2)) / (log (C1 / R2))
Answer: (A) n = (log (R1 / R2)) / (log (C1 / C2))
Hints:
To determine the order of reaction with respect to a reactant, we can use the method of initial rates. According to this method, the order of reaction with respect to a particular reactant can be calculated using the following equation:
n = (log (R1 / R2)) / (log (C1 / C2))
where R1 and R2 are the initial rates of the reaction at concentrations C1 and C2 of the reactant, respectively.
rate R1 = k [C1]^n ..................eq-1
rate R2 = k [C2]^n .......................eq-2
From equations 1 and 2, we get.
(R1 / R2) = (C1 / C2)^n
Now both sites take the log, then we get.
log (R1 / R2) = n log (C1 / C2)
n = (log (R1 / R2)) / (log (C1 / C2))
By comparing the initial rates of the reaction at two different concentrations of the reactant, we can determine the order of the reaction with respect to that reactant.
Hence, the correct answer is (A) n = (log (R1 / R2)) / (log (C1 / C2)).
In conclusion, chemical kinetics is a fundamental topic in physical chemistry, and understanding its principles is crucial for success in various competitive exams. By mastering the concepts related to reaction rates, rate laws, and reaction orders, students can confidently approach questions on chemical kinetics in exams like CSIR NET, GATE, IIT JAM, and others. Practice and understanding the underlying principles of chemical kinetics will enable students to tackle a wide range of questions on this topic effectively.
