# Chemical Reaction Engineering Questions and Answers – Reversible Reactions Rate Laws

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

1. Choose the correct rate law for the elementary reversible reaction 3A ⇌ B + 2C.
a) –rA = kCA3 – CBCC2/KC
b) –rA = k[CA3 – CBCC2KC]
c) –rA = k[CA3 – CBCC2]/KC
d) –rA = k[CA3 – CBCC2/KC]

Explanation: Let’s say the rate constant for the forward reaction is k1 and for the backward reaction it is k2.
Rate of disappearance of A = k1CA3
Rate of formation of A = k2CBCC2
-rA = k1CA3 – k2CBCC2. We know that KC = k1/k2. So, -rA = k[CA3 – CBCC2/KC].

2. Choose the correct rate law for the set of elementary reactions.
2A ⇌ B + C [k1,-k2]
A + 5B → 6D [k3]
3C + 2D + B ⇌ 6E [k4, -k5]
5A + 3E ⇌ 2R [k6]
a) –rA = k1CA2 – k2CB2CC – k3CACB5 – k6CA5CE3
b) –rA = k1CA2 – k2CB2CC + k3CACB5 – k6CA5CE3
c) –rA = k1k4CACCCD – k2k5CE3 + k3k6CA6CB5CE3
d) –rA = k1k4CACCCD – k2k5CE3 – k3k6CA6CB5CE3

Explanation: A appears in reactions 1, 2 and 4.
Rate of disappearance of A = k1CA2 + k3CACB5 + k6CA5CE3
Rate of formation of A = k2CBCC

3. The equilibrium constant increases when _____________
a) Initial concentration of reactant increases in endothermic reactions
b) Initial concentration of reactant decreases in endothermic reactions
c) Temperature increases in exothermic reactions
d) Temperature decreases in exothermic reactions

Explanation: Equilibrium constant is independent of the initial reactant concentration. It only depends on temperature. As temperature rises, equilibrium constant increases for an endothermic reaction and decreases for an exothermic reaction. In both these cases the reverse happens when the temperature falls.
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4. For the reaction ½ A + 3B → 2C
the specific reaction rate will be maximum for which species?
a) A
b) B
c) C
d) Equal for all species

Explanation: -rA/( ½ ) = -rB/3 = rC/2
kB = 3kC/2 = 6kA.
Thus, kB has the biggest of the three rate constant values.

5. Which of the following is true for the rate law of a reversible reaction?
a) It reduces to irreversible form when product concentration is zero
b) It satisfies thermodynamic relationships at all times
c) Rate law can be written only in terms of partial pressures
d) Rate law can be written only in terms of concentrations

Explanation: Rate law can be written in terms of both concentrations or partial pressures. The thermodynamic relationships are satisfied only at equilibrium. If the product concentration is zero, the rate of the backward reaction becomes zero and the rate law reduces to irreversible form.

6. Which of the following constants is a type of equilibrium constant?
a) Dissociation constant
b) Association constant
c) Solubility
d) Decay constant

Explanation: Decay constant is actually a type of rate constant, for a given decay reaction. Solubility, association and dissociation constants are equilibrium constants. These are reaction quotients calculated at the stag of ionic equilibrium w.r.t various ionic and non-ionic species present in a system.

7. Choose the correct statement.
a) A + C → B
B + C → D
D → A + 2C
The above set represents a reversible reaction system
b) An irreversible reaction is just an extreme case of reversible reaction
c) A reversible reaction always reaches completion
d) Reversible reactions are spontaneous but very slow

Explanation: As strange as it may sound, it is actually true. All chemical systems have a natural tendency to attain an equilibrium. All reactions are technically reversible to some degree. The ones which we generally name irreversible reactions are nothing but extremely one-sided cases of reversible reactions, thus, making it almost impossible to observe the equilibrium stage due to only trace presence of certain species remaining.

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