Punjab Board
Class 10th Chemistry
Chapter 09
“Chemical Equilibrium”
Exercise Solution
Exercise Mcqs
- The characteristics of reversible reactions are many except the following;
a. Products never combine to form reactants
b. They never complete
c. They proceed in both ways
d. They have double arrow between reactants and products
2. In the lime kiln, the reaction;
CaCO3 → CaO + CO2
Goes to completion because;
a. Of high temperature
b. CaO is more stable than CaCO3
c. CO2 escapes continuously
d. CaO is not dissociated
3. For the reactions,
2A + B ⇌ 3C
The expression of the equilibrium constant is;
a. [2A][B]/[3C]
b. [A}²[B]/[C]³
c. [3C]/[2A][B]
d. [C]³/[A]²[B]
4. When a system is at equilibrium state;
a. The conc. of reactants and products becomes equal
b. The opposing reactions(forward and reverse) stop
c. The rate of reverse reaction becomes very low
d. The rates of forward and reverse reactions become equal
5. Which one of the following statement is not correct about active mass.
a. Rate of reaction is directly proportional to active mass
b. Active mass is taken in molar concentration
c. Active mass is represented by square brackets
d. Active mass means total mass of substances
6. When magnitude of kc is very large it indicates;
a. Reaction mixture consists of almost all products
b. Reaction mixture has almost all reactants
c. Reaction has not gone to completion
d. Reaction mixture has negligible products
7. When magnitude of Kc is very small, it indicates;
a. Equilibrium will never established
b. All reactants will be converted to products
c. Reaction will go to completion
d. The amount of products is negligible
8. Reaction which have comparable amounts of reactants and products at equilibrium state have;
a. Very small kc value
b. Very large kc value
c. Moderate kc value
d. None of these
9. At dynamic equilibrium ;
a. The reaction stops to proceed
b. The amount of reactants and products are equal
c. The speed of the forward and reverse reactions are equal
d. The reaction can no longer be reverse
10. In an irreversible reaction, dynamic equilibrium;
a. Never establishes
b. Establishes before the completion of reactions
c. Establishes after the completion of reactions
d. Establishes readily
11. A reverse reaction is one;
a. Which proceed from left to right
b. In which reactants react to form products
c. Which slows down gradually
d. Which speeds up gradually
12. Nitrogen and hydrogen were reacted together to make a ammonia;
N2 + 3H2 ⇌ 2NH3
kc = 2.86 mol-²dm6
What will be present in the equilibrium mixture;
a. NH3 only
b. N2, H2 and NH3
c. N2 and H2 only
d. H2 only
13. For a reaction between PCl3 and Cl2 to form PCl5, the units of kc are;
a. mol dm-³
b. mol-¹ dm-³
c. mol-¹ dm³
d. mol dm³
SHORT Questions (Chemical Equilibrium)
-
What are reversible reactions? Give a few characteristics of them?
The type of chemical reactions in which the reactants combine to form products but the products do not recombine to form reactants are called reversible reactions.
Characteristics;
- Represented by( →) symbol.
- Goes to completion.
- Equilibrium is not attained.
- May be conducted in open or closed vessel.
- Can proceed only in forward direction.
-
Define chemical equilibrium state?
The state of a chemical reaction at which the rate of forward reaction becomes equal to the rate of reverse reaction is called chemical equilibrium state.
At equilibrium state, the composition of reactants and products always remains constant.
Examples;
a) N2 + 3H2 ⇌ 2NH3
Kc=2.86 mol-²/dm⁶
Where Kc is equilibrium constant for this reaction.
b) H2 + I2 ⇌ 2HI
Kc = 57.0 at 700K
Where Kc is equilibrium constant for this reaction.
-
Give the characteristics of reversible reaction?
The type of chemical reactions in which the reactants combine to form products and products recombine to form reactants are called reversible reactions.
Characteristics;
- Represented by (⇌ )
- Does not go to completion.
- Equilibrium is attained.
- Conducted in closed vessels.
- Can either go in forward direction or reverse direction.
-
How is dynamic equilibrium established?
Dynamic means “motion.”
When rate of forward reaction becomes equal to the rate of reverse reaction but the reaction does not stop, dynamic equilibrium is attained.
Examples;
Following are some of the reactions that show dynamic equilibrium.
a) N2 + 3H2 ⇌ 2NH3
b) CaCO3 ⇌ CaO + CO2
c) H2 + I2 ⇌ 2HI
-
Why at equilibrium state reaction does not stop?
At equilibrium state, the reaction does not stop because at that point, rate of forward reaction is equal to the rate of reverse reaction. Due to equal rate, it seems that reaction has been stopped but actually both the forward and reverse reactions are occurring with same speed.
Examples;
a) N2 + 3H2 ⇌ 2NH3
Kc = 2.86 mol-²/dm⁶
b) H2 + I2 ⇌ 2HI
Kc=57.0 at 700K
Where Kc is equilibrium constant for these reactions.
-
Why is equilibrium state attainable from either way?
Equilibrium state is attained from either way because in forward reaction, reactants combine to form products while in reverse reaction, products recombine to form reactants.
Reactants ⇌ Products
Examples;
a) H2 + I2 ⇌ 2HI
b) CaCO3 ⇌ CaO + CO2
In these chemical reactions, the dynamic equilibrium can be achieved by combining the reactants to form products or recombining the products to form reactants.
-
What is relationship between active mass and rate of reaction?
The relationship between active mass and rate of reaction is explained by law of mass action.
Statement;
According to law of mass action, “The rate of a chemical reaction is exactly proportional to the product of the active masses of the reacting substances, and the rate at which a substance reacts is directly proportional to its active mass.”
For example take a look at reversible reaction;
A + B ⇌ C + D
Assume that the molar concentrations (mol dm-³) of A, B, C, and D are, respectively, [A], [B], [C], and [D].
The Law of Mass Action states that:
Rate of forward reaction α [A] [B]
Rate of forward reaction = kf [A] [B]
Similarly;
Rate of reverse reaction α [C] [D]
Rate of reverse reaction = kf [C] [D]
Where kf and kr are the proportionality constants.
At equilibrium state;
Rate of forward reaction = Rate of reverse reaction
kf [A] [B] = kr [C] [D]
kf/kr = [C] [D]/ [A] [B]
Where Kc = kf/kr
Kc is equilibrium constant which is;
Kc = [C][D]/[A][B]
-
Derive equilibrium constant expression for the synthesis of ammonia from nitrogen and hydrogen?
For the reaction of nitrogen with hydrogen to form ammonia, the balanced or equilibrium equation is;
N2 + 3H2 ⇌ 2NH3
Rate of forward reaction = kf [N2][H2]³
Rate of reverse reaction = kr [NH2]³
Equilibrium constant expression;
Kc = [NH2]³/ [N2][H2]³
-
Write the equilibrium constant expression for the following reactions;
i) H2 + I2 ⇌ 2HI
ii) CO + 3H2 ⇌ CH4 + H2O
For (i);
Rate of forward reaction = kf [H2][I2]
Rate of reverse reaction = kr [HI]²
Equilibrium constant expression;
Kc = [HI]²/ [H2][I2]
For (ii);
Rate of forward reaction = kf [CO][H2]³
Rate of reverse reaction = kr [CH4][H2O]
Equilibrium constant expression;
Kc = [CH4][H2O] / [CO][H2]³
-
How direction of a reaction can be predicted?
The direction of a reaction can be determined by putting the concentration of reactants and products in a specific amount in equilibrium expression.
Kc = molar conc. of products / molar conc. of reactants.
From this equation, we get a specific value of equilibrium constant (Kc). From this value of Kc, we can predict the direction of a chemical reaction.
-
How can you know that a reaction has achieved an equilibrium state?
At equilibrium state, the rate of forward reaction becomes equal to the rate of reverse reaction. Thus, due to equal rates of both reactions, the concentration of reactants and products no more changes. From these constant concentrations of reactants and products, we can predict that the reaction has attained equilibrium state.
-
What are the characteristics of a reaction that establishes equilibrium state at once?
Characteristics;
- The reaction does not reach completion.
- These reactions can either move in forward or reverse directions.
- In these reactions
Rate of forward reaction= Rate of reverse reaction.
-
If reaction quotient (Qc) of a reaction is more than Kc, what will be the direction of the reaction?
If the Qc is greater than Kc, the reaction will move from right to left ( in the reverse direction ) to gain equilibrium state.
-
An industry was established based upon a reversible reaction. It failed to achieve products on commercial level. Can you point out the basic reasons of its failure being a chemist?
In reversible reaction, the composition (or amount) of reactants and products remains constant when equilibrium state is achieved. That is why, the industry will fail to achieve the desired products on commercial level.
