# Chemical Reaction Engineering Questions and Answers – Autocatalytic Reactions

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This set of Chemical Reaction Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Autocatalytic Reactions”.

1. Autocatalytic reactions are the ones in which ____
a) One of the products poisons the catalyst
b) There is no catalyst employed
c) Only one reactant is involved
d) One of the products catalyses the reaction

Explanation: In autocatalytic reactions, the reactant is catalyzed by a product. One of the products is also a reactant.

2. State true or false.
An autocatalytic reaction is represented as A + R → R + R where, A is a reactant and R formed as the product catalyses the reaction.
a) True
b) False

Explanation: Initially, the progress of the reaction requires more R. As the reaction proceeds, A is consumed. Hence, more of A is required for the reaction to occur as time proceeds.

3. State true or false.
Autocatalytic reactions are single order reactions.
a) True
b) False

Explanation: The rate expression for autocatalytic reactions is non linear. In the reaction A + R → R + R, the concentration of R has to be initially increased. The concentration of R should be decreased as reaction progresses to favour product formation.

4. An example of autocatalytic reaction is ____
a) Ammonia formation reaction
b) Hydrogen peroxide decomposition reaction
c) Reaction of permanganate with oxalic acid
d) Oxidation of copper

Explanation: Permanganate in acidic medium oxidises the oxalic acid. This reaction produces Mn2+ ions, which in turn catalyses the reaction.

5. Which of the following correctly represents autocatalytic reactions?
a)
b)
c)
d)

Explanation: Initially rate increases with an increase in reactant concentration. After sometime, the reaction mixture consists more of the autocatalyst and the reaction is limited by the other reactant.

6. If the area under the curve of $$\frac{1}{-r_A}$$ versus XA for a PFR used to carry out an autocatalytic reaction is 5$$(\frac{m^3 s}{mol}),$$ then the ratio of change in volume to molar feed rate is ____
a) 5
b) 10
c) 15
d) 20

Explanation: For a PFR,
FA0dXA=(-rA)dV
The area under the curve of $$\frac{1}{-r_A}$$versus XA = $$\frac{dXA}{(-rA)} = \frac{dV}{F_{A0}}$$ = 5.

7. Which of the following reactors is used for carrying out autocatalytic reactions?
a) CSTR
b) PFR
c) Batch reactor
d) A combination of CSTR and PFR.

Explanation: A combination of reactors is used to optimize the reactor volume. The reactors are arranged such that the required conversion is achieved. The volume should be low due to economic considerations.

8. The combination of reactors to achieve the required conversion for the autocatalytic reaction depicted below is _______

a) PFR followed by batch reactor
b) CSTR followed by PFR
c) PFR followed by CSTR
d) Batch reactor followed by CSTR

Explanation:

To achieve the required conversion, CSTR followed by a PFR is used. This combination of reactors reduces the reactor volume compared to the other combination of reactors to achieve the same conversion.

9. The autocatalyst in microbial fermentation reactions is:
a) Organic feed
b) Microbes
c) Enzyme
d) Product chemical

Explanation: The microbes act on the organic feed to produce the product and more microbial cells. In microbial fermentation, the microbe which acts as the catalytic agent reproduces itself.

10. The graph shows the plot of $$\frac{1}{-r_A}$$ versus conversion for an autocatalytic reactor. If a single reactor is to be employed to achieve a conversion of 0.5, then which of the following reactors will best suit the given condition?

a) PFR
b) Batch reactor
c) CSTR
d) Fluidised bed reactor

Explanation: The volume of a PFR to achieve a conversion of 0.5 is given by the area under the curve from XA = 0 to XA = 0.5. This is much higher than the volume of CSTR to achieve the same conversion. The volume of the CSTR is shown as:

The blackened region gives the CSTR volume.

Sanfoundry Global Education & Learning Series – Chemical Reaction Engineering.

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