# Bioprocess Engineering Questions and Answers – Concentration Gradients and Reaction Rates in Solid Catalysts

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This set of Bioprocess Engineering Questions and Answers for Entrance exams focuses on “Concentration Gradients and Reaction Rates in Solid Catalysts”.

1. Which of the following is not an example of convectiton?
a) After a car is turned on, the engine becomes hot.
b) Steaming cup of hot tea
c) Ice melting
d) Hot air balloon

Explanation: After a car is turned on, the engine becomes hot. The hood will become warm as heat is conducted from the engine to the hood. So it is an example of the process conduction.

2. In heterogeneous reaction, if a reaction proceeds slowly even in the presence of adequate substrate then mass transfer will be?
a) Slow
b) Very slow
c) Rapid
d) Negligible

Explanation: If a reaction proceeds slowly even in the presence of adequate substrate, it is likely that mass transfer will be rapid enough to meet the reaction demand. In this case, the observed rate would be determined more directly by the reaction process than mass transfer. Conversely, if the reaction tends to be very rapid, it is likely that mass transfer will be too slow to supply substrate at the rate required.

3. During a chemical reaction, the
a) mass of catalyst remains unchanged
b) physical state of catalysts remain unchanged
c) changes in chemical composition of catalysts may be observed
d) chemical composition of catalyst may change

Explanation: Catalyst is defined as a substance, which alters the rate of a chemical reaction, itself remaining chemically unchanged at the end of the reaction. The process is called Catalysis.
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4. Factors affecting speed of reaction does not include
a) Concentration
b) Particle size
c) Change in mass
d) Pressure

Explanation: A catalyst is a species that speeds up a chemical reaction without being chemically changed upon completion of the reaction. In other words, the mass of a catalyst is the same before and after a reaction occurs.

5. Which of the following is not an example of bulk catalyst?
a) Silica-alumina catalyst
b) Molybdenum oxide supported on alumina
c) Iron-molybdate
d) Iron doped with alumina and potassium oxide

Explanation: The supported catalysts are the catalytically active materials which are dispersed over the high surface area support material. For example, hydrodesulphurization is carried out over molybdenum oxide supported on alumina.

6. A catalyst provides an alternative pathway with a lower activation energy.
a) True
b) False

Explanation: A catalyst provides an alternative pathway with a lower activation energy. Therefore, more particles have enough energy to overcome the new activation energy requirement.

7. What is the unit of the reaction rate?
a) mol dm-3s-1
b) mol dm s
c) mol dm-3 s
d) mol-1 dm s-1

Explanation: The Reaction Rate for a given chemical reaction is the measure of the change in concentration of the reactants or the change in concentration of the products per unit time. Its units are mol dm-3s-1.

8. In which type of reactor, the perfect mixing can occur?
a) Ideal
b) Non-ideal
c) Constant
d) Variable

Explanation: In ideal reactor, composition and temperature are spatially uniform (i.e. perfect mixing).
Ideal (perfectly mixed) reactor: spatially uniform temp, conc, & reaction rate :
$$r_jV = \frac{dN_j}{dt}$$

9. The overall conversion of reactants can be increased in spite of increasing of the residence time.
a) True
b) False

Explanation: In heterogeneous reactors external mass transport can be increased by a better mixing of the fluid. A higher flow rate also contributes to a better mixing. Under the better mixing a laminar film around the catalyst particle is thinner and more reactants can be transported to the surface. Consequently, the overall conversion of reactants can be increased in spite of lowering of the residence time.

10. If the temperature of a reaction is decreased, what effect will this have on the rate of reaction?
a) The rate of reaction will increase
b) The rate of reaction will decrease
c) The reaction will stop
d) No effect on reaction

Explanation: Increasing the temperature increases reaction rates because of the disproportionately large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reaction.

11. In homogeneous catalysis_________
a) All reactants are in the same rate
b) The catalyst is in a different state to the reactants
c) The reactants and catalyst are in the same rate
d) All the products are in the same state

Explanation: Homogeneous catalysis refers to catalytic reactions where the catalyst is in the same phase as the reactants. Homogeneous catalysis applies to reactions in the gas phase and even in solids.

12. The minimum amount of energy needed to start a reaction is called the ______
a) Activation energy
b) Energy of reaction
c) Entropy of reaction
d) Reaction mechanism energy

Explanation: Activation energy is the minimum energy required for a reaction to occur. Catalysis is the increase in the rate of a chemical reaction by lowering its activation energy. Transition state is an intermediate state during a chemical reaction that has a higher energy than the reactants or the products.

13. The rate of a chemical reaction can be expressed in Molarity per seconds?
a) True
b) False

Explanation: There are many factors that can either slow or speed up the rate of a chemical reaction such as temperature, pressure, concentration, and catalysts. The Rate of a Chemical Reaction is always positive. It can be confusing since the Rate of Disappearance is negative, however when you think about it, a rate should never be negative since the rate is describing how fast the concentration changes with time. The units for the rate is Molarity per Seconds (M/s).

14. Factors affecting speed of reaction include ________
a) Volume of gas
b) Change in mass
c) Pressure
d) Temperature change

Explanation: Increasing the pressure of a reaction improves the likelihood reactants will interact with each other, thus increases the rate of the reaction. As you would expect, this factor is important for reactions involving gases, and not a significant factor with liquids and solids.

15. Concentration of the reactant is _____ proportional to the number of collisions?
a) Directly
b) Indirectly
c) Inversely
d) Not proportional

Explanation: Concentration of the reactants is directly proportional to the number of collisions and also directly proportional to the reaction rate. The more the collision, more will be the reaction rate and hence more will be the concentration of reactants.

Sanfoundry Global Education & Learning Series – Bioprocess Engineering.

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