chemistry : class11 Equilibrium Concentrations : Equilibrium in chemical processes : Equilibrium MCQ Questions and Answers
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Equilibrium Concentrations
MCQ questions and answers.
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MCQ Questions
1. At chemical equilibrium, the concentrations of reactants and products are:
A.
Always equal
B.
Constant with time
C.
Always zero
D.
Continuously increasing
Correct Answer:
Constant with time
Explanation:
At equilibrium, the forward and reverse reactions continue, but the concentrations of reactants and products remain constant because both reaction rates are equal.
2. For the reaction A ⇌ B, the equilibrium concentration of A is 0.40 mol L⁻¹ and that of B is 0.60 mol L⁻¹. What is the total concentration of A and B at equilibrium?
A.
0.20 mol L⁻¹
B.
0.40 mol L⁻¹
C.
0.60 mol L⁻¹
D.
1.00 mol L⁻¹
Correct Answer:
1.00 mol L⁻¹
Explanation:
The total equilibrium concentration is the sum of the equilibrium concentrations: 0.40 + 0.60 = 1.00 mol L⁻¹.
3. For the reaction A ⇌ B, the initial concentration of A is 1.0 mol L⁻¹ and no B is present. If 0.3 mol L⁻¹ of A changes into B at equilibrium, what is the equilibrium concentration of A?
A.
0.3 mol L⁻¹
B.
0.7 mol L⁻¹
C.
1.0 mol L⁻¹
D.
1.3 mol L⁻¹
Correct Answer:
0.7 mol L⁻¹
Explanation:
The equilibrium concentration of A is its initial concentration minus the amount converted: 1.0 − 0.3 = 0.7 mol L⁻¹.
4. For the reaction A ⇌ B, the initial concentration of A is 0.8 mol L⁻¹ and no B is present. If 0.2 mol L⁻¹ of A reacts, what is the equilibrium concentration of B?
A.
0.2 mol L⁻¹
B.
0.6 mol L⁻¹
C.
0.8 mol L⁻¹
D.
1.0 mol L⁻¹
Correct Answer:
0.2 mol L⁻¹
Explanation:
For a 1:1 reaction, the amount of B formed equals the amount of A consumed. Therefore, the equilibrium concentration of B is 0.2 mol L⁻¹.
5. In a reaction mixture at equilibrium, the concentration of a reactant is 0.25 mol L⁻¹. Which statement is correct?
A.
The reactant concentration must become zero
B.
The reactant concentration remains approximately constant with time
C.
The reactant concentration must equal the product concentration
D.
The reactant concentration must continuously increase
Correct Answer:
The reactant concentration remains approximately constant with time
Explanation:
At equilibrium, the concentration of each substance remains constant with time, although the reaction continues in both directions.
6. For the reaction X ⇌ Y, the equilibrium concentration of X is 0.5 mol L⁻¹ and that of Y is 0.5 mol L⁻¹. Which statement is true?
A.
The forward reaction has stopped
B.
The reverse reaction has stopped
C.
The concentrations of X and Y are equal at equilibrium
D.
No molecules of X or Y are present
Correct Answer:
The concentrations of X and Y are equal at equilibrium
Explanation:
The given equilibrium concentrations of X and Y are both 0.5 mol L⁻¹. Equal concentrations do not mean that either reaction has stopped.
7. For the reaction A ⇌ 2B, the concentration of A decreases by 0.10 mol L⁻¹ before equilibrium is reached. By how much does the concentration of B increase?
A.
0.05 mol L⁻¹
B.
0.10 mol L⁻¹
C.
0.20 mol L⁻¹
D.
0.30 mol L⁻¹
Correct Answer:
0.20 mol L⁻¹
Explanation:
The reaction produces 2 mol of B for every 1 mol of A consumed. Thus, a decrease of 0.10 mol L⁻¹ in A produces an increase of 2 × 0.10 = 0.20 mol L⁻¹ in B.
8. For the reaction 2A ⇌ B, the concentration of A decreases by 0.20 mol L⁻¹ as equilibrium is established. What is the increase in the concentration of B?
A.
0.10 mol L⁻¹
B.
0.20 mol L⁻¹
C.
0.40 mol L⁻¹
D.
0.60 mol L⁻¹
Correct Answer:
0.10 mol L⁻¹
Explanation:
Two moles of A form one mole of B. Therefore, a decrease of 0.20 mol L⁻¹ in A corresponds to an increase of 0.20 ÷ 2 = 0.10 mol L⁻¹ in B.
9. Which quantity represents the concentration of a substance at equilibrium?
A.
The concentration before the reaction starts
B.
The concentration after the reaction has reached a constant value
C.
The concentration when all reactants are consumed
D.
The concentration immediately after mixing only
Correct Answer:
The concentration after the reaction has reached a constant value
Explanation:
An equilibrium concentration is the concentration of a reactant or product when it has become constant with time in an equilibrium mixture.
10. For the reaction A ⇌ B, the equilibrium concentrations are [A] = 0.30 mol L⁻¹ and [B] = 0.70 mol L⁻¹. What is the ratio of the equilibrium concentration of B to that of A?
A.
0.43
B.
1.00
C.
2.33
D.
3.00
Correct Answer:
2.33
Explanation:
The ratio is [B]/[A] = 0.70/0.30 = 2.33 approximately. This uses the concentrations of the substances at equilibrium.