Chemistry Questions and Answers – Chemical Kinetics – Pseudo First Order Reaction

«
»

This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Chemical Kinetics – Pseudo First Order Reaction”.

1. For a pseudo first-order reaction, what is the unit of the rate of the reaction?
a) s-1
b) mol L-1s-1
c) mol-1 L s-1
d) mol-2 L2 s-1
View Answer

Answer: a
Explanation: The unit of the rate of the reaction (k) is (mol L-1)1-n s-1, where n is the order of the reaction.
For a pseudo first-order reaction n=1.
So, (mol L-1)1-n s-1 = (mol L-1)1-1 s-1 = s-1.
advertisement

2. Reactions whose molecularity is one but order is more than one are known as pseudo first-order reactions.
a) True
b) False
View Answer

Answer: b
Explanation: Pseudo first-order reactions are the reactions whose molecularity is more than one but follows first order kinetics. Acid hydrolysis of esters such as CH3COOC2H5 in aqueous solution is an example for pseudo first order reaction.

3. Which of the following reactions is an example of a pseudo first-order reaction?
a) H2 + Br2 → 2HBr
b) CH3CHO → CH4 + CO
c) C12H22O11 + H2O → glucose + fructose
d) PCl5 → PCl3 + Cl2
View Answer

Answer: c
Explanation: The order of the reaction may be sometimes altered sometimes by changing the conditions, for example, taking one or more reactants in excess compared to the other. The molecularity of inversion of cane sugar is 2 but follows first order kinetics because water is present in excess.
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!
advertisement
advertisement

4. Acid hydrolysis of esters such as CH3COOC2H5 in aqueous solution is an example of a pseudo first-order reaction.
a) True
b) False
View Answer

Answer: a
Explanation: The reaction involved in acid hydrolysis of CH3COOC2H5 in aqueous solution = CH3COOC2H5 + H2O → CH3COOH + C2H5OH. The rate law for the reaction is R=k[CH3COOC2H5][H2O], however, water is present in large excess and is assumed to be constant during the course of the reaction. So the rate law becomes R=k [CH3COOC2H5] and follows first order kinetics.

5. The rate law for inversion of cane sugar is R=k [C12H22O11][H2O]. Find the concentration of sucrose if the rate of the reaction is 0.032 s-1 and rate constant k=0.005.
a) 5.8 M
b) 6 M
c) 6.2 M
d) 6.4 M
View Answer

Answer: d
Explanation: Given, rate constant k=0.005, rate of the reaction R=0.032 s-1.
Inversion of cane sugar is a pseudo first order reaction and water is present in large excess so the rate law becomes R=k [C12H22O11]
0.032= 0.005 [C12H22O11]
[C12H22O11]=6.4 M.
advertisement

6. What is the rate law of hydrolysis of CH3COOC2H5 in aqueous solution?
a) R=k [CH3COOC2H5]2
b) R=k [CH3COOC2H5]
c) R=k [CH3COOC2H5] [H2O]
d) R=kk’ [CH3COOC2H5][H2O]
View Answer

Answer: b
Explanation: The reaction involved in acid hydrolysis of ester = CH3COOC2H5 + H2O → CH3COOH + C2H5OH. So the rate law becomes R=k’ [CH3COOC2H5][H2O], since water is present in large excess it is assumed to be constant throughout the reaction.
R=k[CH3COOC2H5], where k is k’ [H2O].

7. Which of the following is not true for a pseudo first-order reaction?
a) The reaction follows first order kinetics
b) Molecularity of the reaction should be one
c) The reactants such as water are present in excess and are assumed to be constant throughout the reaction
d) Unit of the rate constant is s-1
View Answer

Answer: b
Explanation: The order of the reaction can be sometimes altered by taking one of the reactant in excess compared to the other. Pseudo first order reactions are those reactions in which the molecularity is more than one but follows first order kinetics.
advertisement

8. Which of the following is not similar between a first order and pseudo first order reaction?
a) The molecularity is one in both the reactions
b) Both follows first order kinetics
c) The unit of rate constant is s-1
d) The rate of the reaction depends only on one reactant
View Answer

Answer: a
Explanation: Both first order and pseudo first order reactions follow first order kinetics as in the rate of the reaction depends only on one reactant and the unit for rate constant is s-1 but the molecularity in first order reaction should be one where as in pseudo first order reaction the molecularity can be more than one. For example: Inversion of cane sugar.

9. Which of the following is not an example of a pseudo first-order reaction?
a) CH3COOC2H5 + NaOH → CH3COOH + H2O
b) CH3COOC2H5 + H2O → CH3COOH + C2H5OH
c) C2H5COOC2H5 + H2O → C2H5COOH + C2H5OH
d) C12H22O11 + H2O → glucose + fructose
View Answer

Answer: a
Explanation: The reactions CH3COOC2H5 + H2O → CH3COOH + C2H5OH, C2H5COOC2H5 + H2O → C2H5COOH + C2H5OH and C12H22O11 + H2O → glucose + fructose are examples of pseudo first order reaction because water is present in excess and is assumed to remain constant is all the reactions but the reaction CH3COOC2H5 + NaOH → CH3COOH + H2O is an example of second order reaction and follows second order kinetics with rate law R=k [CH3COOC2H5] [NaOH].
advertisement

10. For the reaction C2H5COOC2H5 + H2O → C2H5COOH + C2H5OH, find the concentration of [C2H5COOC2H5] if the rate constant k=0.2 and the rate of the reaction is 0.39 s-1.
a) 2.87 M
b) 2.01 M
c) 1.99 M
d) 1.95 M
View Answer

Answer: d
Explanation: The reaction C2H5COOC2H5 + H2O → C2H5COOH + C2H5OH is a pseudo first order reaction and follows first order kinetics with rate law R=k[C2H5COOC2H5].
Given, rate of the reaction R=0.39 s-1 and rate constant k=0.2
So, 0.39 = 0.2 × [C2H5COOC2H5]
[C2H5COOC2H5]=1.95 M.

Sanfoundry Global Education & Learning Series – Chemistry – Class 12.

To practice all areas of Chemistry, here is complete set of 1000+ Multiple Choice Questions and Answers.

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & technical discussions at Telegram SanfoundryClasses.