Chemistry

Chapter Chemical Equilibrium

(Solved Exercise)

Q1:Encircle the correct answer:

1. Which statement is true about the state of equilibrium?
   • Forward reaction stops
   • Reverse reaction stops
   • Forward and reverse both reactions stop
   • Both forward and reverse reactions continue at the same rate

2. When a mixture of H₂ and I₂ is sealed in a flask and the temperature is kept at 25 ^oC, the following equilibrium is established.

Which substance will be present in the equilibrium mixture?

• H₂ and I₂
• HI only
• H₂ only
• H₂ , I₂ and HI

Solution: The reaction can go either way since it is reversible, and we know that reversible reactions can go both ways. Therefore, equilibrium will be reached once the rates of forward and reverse reactions are equal. At this point, the concentrations of the reactants and products will be equal. As a result, the equilibrium mixture will contain both the reactants (H₂, I₂) and the product (HI) at the same concentration.

3. Derive the Kc unit for

• mol^-1.dm³
• mol² / dm⁶
• dm³ / mol
• No unit

Solution:

Solution: We can see that there are two moles of products and three moles of reactants in this reaction. As a result, one mole of reactant will be left behind after cutting two moles of reactants into two moles of products, which will serve as the unit of Kc.

4. For which of the following reactions Kc will have unit less?

• 1
• 2
• 3
• All of the above

Solution:

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Because there are exactly the same amounts of reactants and products in each of the three reactions, they all cancel one another out and leave no remaining unit for Kc.

5.Concentration of reactants and products at equilibrium remains unchanged if

• Concentration of any reactant or product is not changed
• Temperature of the reaction is not changed
• Pressure or volume of the system is not changed
• All of the above are observed

6. Which of the following does not happen when a system is at an equilibrium state?

• Forward and reverse reactions stop
• Forward and reverse reaction rates become equal
• Reactants and products concentration stops changing
• Reaction occur in both the directions

7.For which one reaction the unit of Kc will be mol/dm³

• 1
• 2
• 3
• 4

Solution:

8. How equilibrium established, in an irreversible reaction

• Established quickly
• Established slowly
• Never established
• Established when reaction stops

9. Active mass can be justified as

• Total mass of the reactants
• Total mass of products
• Total mass of reactants and products
• Mass of substance in mole per dm³ in a dilute solution

10. For a reverse reaction

Which substance is the product of the reaction?

• A
• B
• Both A and B
• C

Solution: We know that Kc is the product of the ratio of products and reactant concentration in light of the law of mass action. Thus, [C]² is the end result.

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Q2.Give Short Answers.

Q1): Differentiate between forward and reverse reactions.

Ans): When a reaction proceeds from left to right then this is called a forward reaction. In this reaction synthesis of the product is favoured. While a reaction that proceeds from the right to the left side is called a reverse reaction, the synthesis of reactants is favoured in this reaction. e.g.

Example :

Forward Reaction:

Backward Reaction:

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Q2): What is meant by chemical equilibrium?

Ans): Chemical equilibrium is the condition of a system at which the rates of forward and reverse reactions are equal. As soon as the concentration of the reactants and products is equal, the reactants and products will be in equilibrium.

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Q3): Describe the law of mass action?

Ans): The rate of the reaction is directly proportional to the active mass of the reactant that is actively participating in the reaction, according to this law, which was put forth by C.M. Gulberg and P. Waage. We may conclude that the rate of reaction is directly proportional to the product of the active masses of the reactants if there are several reactants on the left side.

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Q4): Point out the conditions of equilibrium.

Ans): When forward and reverse reaction rates become equal it means that reactants and products concentration are balanced and this state of a system is called a state of equilibrium. There are some conditions of equilibrium that are given below.

• Reactants and products concentration will be the same/balanced/constant
• Three factors of a system namely; temperature, pressure, and volume of the system will also not change (balanced/constant).

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Q5): What is the importance of Kc?

Ans): The equilibrium constant can help us understand some crucial facts about a reaction. When we don’t know the equilibrium concentration of the equilibrium mixture, Kc uses the initial concentration of the reactants to determine the equilibrium concentration of the equilibrium mixture.

When a reaction is underway and we are unsure of its direction—whether it will move ahead or backward—Kc will decide the reaction’s course.

When a reaction is underway and we are unsure of its magnitude or the rate at which it is progressing, Kc will determine the reaction’s size or velocity.

When a reaction is in equilibrium, any changes that take place at this point in the reaction will have an impact on the reaction depending on Kc. Industrial chemists also determine the impact of changes in concentration, temperature, pressure, and other variables before determining the impact of changes that take place at equilibrium.

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Q3): Following reaction can occur during lightning storms

For this reaction write

• Expression of the equilibrium constant
• Determine the units of the equilibrium constant
• Forward and reverse reactions

Ans): (I)

(II)

(III)

Forward Reaction:

Reverse Reaction:

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Q4): To form CO and H₂ coal reacts with hot steam. In the presence of a catalyst, these substances react further to give methane and water vapour.

• Write forward and reverse reactions for it
• Derive the expression of Kc for the reaction
• Determine units for Kc

Ans): (I)

Forward Reaction:

Reverse Reaction:

(II)

(III)

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Q5): Derive the expression of Kc for each of the following reactions.

Ans): (I)

(II)

(III)

(IV)

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Q6) Find out the units of Kc for the following reactions.

Ans):

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Q7): State the ways with the help of which we can recognize that the system is at equilibrium.

Ans): We know that in a state of equilibrium forward and reverse reaction rates become equal which means that reactants and products concentration become balanced. So how we will find physically and chemically that yes now reactants and product concentration become balanced.

We can find out the state of equilibrium by the most commonly used methods

1) Titration 2) Spectroscopy

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Q8): Explain the macroscopic characteristics of an equilibrium reaction.

Ans): The macroscopic characteristics of an equilibrium reaction are given below:

• At a state of equilibrium reactants and products concentration will become equal to each other.
• At the state of equilibrium forward and reverse reaction rates will become equal to each other.
• The addition of a catalyst does not affect the state of equilibrium it only speeds up the reaction.

Think-Tank

1.Bromine chloride (BrCl) decomposes to form chlorine and bromine. For this reaction write

• Chemical equation
• Kc expression
• Unit of Kc

Ans):

(I)

(II)

(III)

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2): Kc expression for a reaction is given below

Choose reactant and product to derive the unit of Kc for this reaction.

Ans):

Q14): For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete. Justify.

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3)Cobalt chloride forms pink crystals (CoCl₃.6H₂O). Water evolved from (CoCl₃.6H₂O). Upon heating and turns blue (CoCl₃). Can you use Cobalt chloride as a test for water, argue

Ans): The reaction which is given below is reversible.

We know that reversible reaction proceeds in both forward and reverse directions so, obviously yes both reactants and products would likely be found at the state of equilibrium when forward and reverse reaction rates become equal then equilibrium will be established and a state of equilibrium between reactants and products concentration will become equal. While the first reaction (formation of CO₂) is a combustion reaction that is irreversible in nature and will proceed only in one direction (forward direction) means left to right and reactants will be completely consumed and will convert to the product.

Anhydrous cobalt (II) chloride is a blue substance. On reaction with water, it converted to pink color (hydrated cobalt (II) chloride). So, the change of color from blue to pink is a very useful thing and due to this color change, cobalt chloride can be used as a test for water.

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