Here below some basic MCQ’s about “Order and Molecularity” with Answer which is explained in details. let’s check one by one which is given below.
Question 1
What is the order of a reaction if its rate is proportional to the concentration of one reactant raised to the power of 2?
A) 1
B) 2
C) 3
D) None of the above
Answer: B) 2
Explanation: The order of a reaction is determined by the sum of the powers of the concentration terms in the rate law. If the rate is proportional to the concentration of one reactant raised to the power of 2, the reaction is second-order with respect to that reactant.
Question 2
Which of the following statements is true regarding molecularity?
A) Molecularity can be a fractional value.
B) Molecularity refers to the number of molecules involved in the rate-determining step.
C) Molecularity is the sum of the orders of all reactants.
D) Molecularity can only be zero or a whole number.
Answer: B) Molecularity refers to the number of molecules involved in the rate-determining step.
Explanation: Molecularity is defined as the number of reactant molecules that collide simultaneously in an elementary reaction. It can only be a whole number (1, 2, 3, etc.) and cannot be fractional.
Question 3
A reaction has a rate law of Rate=k[A]1[B]2. What is the overall order of the reaction?
A) 1
B) 2
C) 3
D) 4
Answer: C) 3
Explanation: The overall order of the reaction is the sum of the exponents in the rate law. Here, it is 1+2=3.
Question 4
If a reaction is said to be zero-order, what does that imply about the concentration of the reactant?
A) The concentration of the reactant remains constant.
B) The reaction rate increases with the concentration of the reactant.
C) The reaction rate is dependent on the concentration of the reactant.
D) The reaction will stop once the concentration is zero.
Answer: A) The concentration of the reactant remains constant.
Explanation: In a zero-order reaction, the rate of reaction is constant and does not depend on the concentration of the reactants. Therefore, even if the concentration decreases, the rate remains the same until the reactants are depleted.
Question 5
Which of the following reactions is most likely to be bimolecular?
A) A → Products
B) A + B → Products
C) 2A → Products
D) A + B + C → Products
Answer: B) A + B → Products
Explanation: Bimolecular reactions involve two reactant molecules in the rate-determining step. The option A + B → Products indicates that two different molecules are involved, making it bimolecular. Option C could also be bimolecular if it involves two molecules of A colliding, but it is less clear than the direct two different reactants in option B.
Question 6
The molecularity of a reaction that involves the simultaneous collision of three reactant molecules is:
A) Unimolecular
B) Bimolecular
C) Trimolecular
D) Zero
Answer: C) Trimolecular
Explanation: Molecularity refers to the number of reactant molecules that participate in an elementary reaction. If three molecules collide simultaneously, the reaction is trimolecular.
Question 7
Which of the following rate laws indicates a first-order reaction?
A) Rate=k[A]0
B) Rate=k[A]1
C) Rate=k[A]2
D) Rate=k[A]3
Answer: B) Rate=k[A]1
Explanation: A first-order reaction has a rate that is directly proportional to the concentration of one reactant raised to the first power. Therefore, Rate=k[A]1 indicates a first-order reaction.
Question 8
In a reaction mechanism, which step is usually the slowest?
A) Fast step
B) Intermediate step
C) Rate-determining step
D) Catalytic step
Answer: C) Rate-determining step
Explanation: The rate-determining step is the slowest step in a reaction mechanism, which determines the overall rate of the reaction.
Question 9
What is the molecularity of a reaction represented by the equation 2A→Products?
A) 1
B) 2
C) 3
D) None of the above
Answer: B) 2
Explanation: In this reaction, two molecules of A are involved in the rate-determining step, making it bimolecular.
Question 10
Which of the following statements about zero-order reactions is true?
A) The reaction rate decreases as the reactant concentration decreases.
B) The reaction can proceed without any reactant.
C) The concentration of the reactant affects the rate.
D) The rate is constant regardless of the concentration of reactants.
Answer: D) The rate is constant regardless of the concentration of reactants.
Explanation: In a zero-order reaction, the rate is independent of the concentration of the reactants, remaining constant until the reactants are exhausted.
Question 11
A reaction is said to be second order if:
A) The rate is proportional to the square of the concentration of one reactant.
B) The rate is independent of the concentration of reactants.
C) The rate is proportional to the concentration of one reactant.
D) The rate is proportional to the sum of the concentrations of the reactants.
Answer: A) The rate is proportional to the square of the concentration of one reactant.
Explanation: A second-order reaction can either involve one reactant raised to the second power or two different reactants, each raised to the first power.
Question 12
Which of the following scenarios would likely result in a higher reaction rate?
A) Increased activation energy
B) Decreased temperature
C) Increased concentration of reactants
D) Increased volume
Answer: C) Increased concentration of reactants
Explanation: Increasing the concentration of reactants generally increases the frequency of collisions, which leads to a higher reaction rate.
Question 13
What type of reaction mechanism is represented by a single elementary step?
A) Complex mechanism
B) Overall reaction
C) Elementary reaction
D) Multi-step reaction
Answer: C) Elementary reaction
Explanation: An elementary reaction is a single step that describes a single reaction event, as opposed to a multi-step or complex mechanism.
Question 14
If doubling the concentration of a reactant in a first-order reaction results in a doubling of the rate, this is consistent with which of the following?
A) Zero order
B) First order
C) Second order
D) Third order
Answer: B) First order
Explanation: In a first-order reaction, the rate is directly proportional to the concentration of the reactant, so doubling the concentration will double the rate.
Question 15
A reaction’s rate constant kk increases with temperature due to:
A) Decreased activation energy
B) Increased frequency of collisions
C) Increased activation energy
D) Changes in molecularity
Answer: B) Increased frequency of collisions
Explanation: An increase in temperature generally increases the kinetic energy of the molecules, resulting in more frequent and more energetic collisions, which can enhance the rate of reaction.
Question 16
If a reaction is fourth order with respect to reactant A, how does the rate change if the concentration of A is tripled?
A) The rate increases by a factor of 3
B) The rate increases by a factor of 9
C) The rate increases by a factor of 27
D) The rate remains unchanged
Answer: C) The rate increases by a factor of 27
Explanation: For a fourth-order reaction, if the concentration of A is tripled, the rate increases by 34=8134=81, making it a factor of 81.
Question 17
Which reaction type is characterized by a rate that is independent of reactant concentration?
A) First-order reaction
B) Second-order reaction
C) Zero-order reaction
D) Pseudo-first-order reaction
Answer: C) Zero-order reaction
Explanation: In a zero-order reaction, the rate does not change with the concentration of the reactants, remaining constant until the reactants are exhausted.
Question 18
In the reaction A+B→C, if the rate law is Rate=k[A][B]2, what is the order with respect to reactant B?
A) 1
B) 2
C) 3
D) 0
Answer: B) 2
Explanation: The order of a reaction with respect to a reactant is given by the exponent of that reactant in the rate law. Here, the exponent of B is 2.
Question 19
For a bimolecular reaction, the molecularity can be defined as:
A) 1
B) 2
C) 3
D) 0
Answer: B) 2
Explanation: Bimolecular reactions involve two molecules colliding in the rate-determining step, so the molecularity is 2.
Question 20
If a reaction has an activation energy of 50 kJ/mol and is run at a higher temperature, what effect will this have on the rate constant k?
A) k will decrease
B) k will remain the same
C) k will increase
D) Cannot be determined
Answer: C) k will increase
Explanation: Generally, an increase in temperature leads to an increase in the rate constant k due to the Arrhenius equation, which states that k increases exponentially with temperature.
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