# Chemical Reaction Engineering Questions and Answers – Mole Balances

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

1. The unit for rate of reaction for a fluid-solid catalytic reaction is _______
a) mol/m3.s
b) mol/s. g catalyst
c) mol/s. mmHg
d) mol/dm3.s

Explanation: Rate of reaction for a fluid-solid catalytic reaction is defined as the number of moles of A reacted per unit time per unit mass of catalyst.

2. Chemical reaction rate is ___________
a) an extensive property
b) an intensive property
c) a chemical property
d) a physical property

Explanation: Chemical reaction rate depends on temperature and concentration. Hence, it is an intensive property.

3. –rA is a universal notation for reaction rate. What does the negative sign indicate?
a) Rate of formation of A
b) Rate of disappearance of A
c) Rate of dissociation of A
d) Rate of association of A

Explanation: rA indicates the rate of formation of A whereas –rA indicates the rate of disappearance of A.
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4. General mole balance equation of species ‘i’ at any instant is as follows: ________
a) Rate of moles of ‘i’ in = Rate of moles of ‘i’ out
b) Rate of moles of ‘i’ in + Rate of moles of ‘i’ out = Rate of moles of ‘i’ accumulated
c) Rate of moles of ‘i’ in + Rate of moles of ‘i’ accumulated = Rate of moles ‘i’ out
d) Rate of moles of ‘i’ in – Rate of moles of ‘i’ consumed by reaction – Rate of moles of ‘i’ out = Rate of moles of ‘i’ accumulated

Explanation: Governed by principles of conservation of mass and energy the following equation is the basic equation in Chemical engineering for design of all process units.
IN – OUT + Generation – Consumption = Accumulation.

5. Mole balance equation for a perfectly mixed batch reactor is _______
a) $$\frac{dNi}{dt}$$ = 0
b) $$\frac{dNi}{dt}$$ = ri V
c) $$\frac{dNi}{dt} = \int_0^V$$ri dV
d) $$\frac{dNi}{dt}$$ = ri

Explanation: General mole balance for batch reactor is $$\frac{dNi}{dt} = \int_0^V$$ri dV. For perfectly mixed batch reactors there are no spatial variations and above equation can be modified as
ri = $$\frac{1}{V} \frac{dNi}{dt}.$$

6. In ________ there are no spatial variations in concentration and temperature.
a) PFR
b) Tubular reactor
c) CSTR
d) Packed bed reactor

Explanation: CSTR is operated in such a way that the reaction mixture is perfectly mixed. Hence, there are no spatial variations.

7. The kinetic rate law is ______
a) a differential eqn
b) a linear eqn
c) an algebraic eqn

Explanation: Rate law is experimentally determined. It is a product of rate constant and algebra function of concentration.

8. In a PFR, the extent of reaction achieved does not depend on the shape of the reactor.
a) True
b) False

Explanation: From design eqn $$\frac{dFi}{dV}$$ = ri. The reaction rate doesn’t depend on shape of the reactor.

9. The unit for rate of reaction for a homogeneous reaction is __________
a) mol/s. g catalyst
b) mol/s. mmHg
c) mol/s. m3
d) mol/s. m2

Explanation: The reaction rate is defined in terms of reaction volume for homogeneous reactions.

10. Rate of gaseous phase reaction depends on __________
a) Temperature of reactants only
b) Pressure of reactants only
c) Concentration of reactants only
d) Temperature, Pressure and Concentration of reactants

Explanation: The reaction rate of gaseous phase reaction is dependent on all of them.

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