Chemistry Multiple Choice Questions 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
Answer: a
Clarification: 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.
2. Reactions whose molecularity is one but order is more than one are known as pseudo first-order reactions.
a) True
b) False
Answer: b
Clarification: 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
Answer: c
Clarification: 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.
4. Acid hydrolysis of esters such as CH3COOC2H5 in aqueous solution is an example of a pseudo first-order reaction.
a) True
b) False
Answer: a
Clarification: 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
Answer: d
Clarification: 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.
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]
Answer: b
Clarification: 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