Chapter No.1: Chemical Equilibrium -Chemistry For Class X (2022 and Onward) - Question & Answers

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Chemical Equilibrium
Short & Detailed Answer Questions

Q.1: What do you know about chemical reactions?
Ans: Chemical Reactions:
Many physical and chemical changes are occurring in our surroundings which may be due to chemical reactions. In these reactions reactants are converted into one or more products.
A chemical reaction is a chemical change which involves reactants and products
A chemical reaction contains two quantities; reactant and product which are separated by an arrow.
For example:

• The formation of water from hydrogen gas and oxygen gas.
• The decomposition reaction of sodium bicarbonate into sodium carbonate, water, and carbon dioxide etc.

Types Of Chemical Reaction:
The reactions may be:

1. Reversible
   (a) condensation
   (b) evaporation
   (c) freezing
   (d) melting
2. Irreversible
   (a) Combustion
   (b) rusting

Q.2: Define chemical equilibrium in terms of  reversible reaction with example.
Ans: Chemical Equilibrium:
The reversible chemical reaction never goes to completion because product reacts and reproduce reactants again and take place in forward and reverse direction. This state in which forward reaction rate and reverse reaction rate are equal known as equilibrium.
Equilibrium means a 'balance'. Equilibrium exists in many ways in our surroundings.
"It is a state where the rate of the forward reaction is equal to the rate of backward reaction."

OR

It is also defined as "A state where the concentration of reactant and product remains constant."
Example:

H_2(g) + I_2(g) ⇌ 2HI_(g)

Q.3: Give an example of a reversible reaction which never complete and exist in forward and reverse direction?
Ans: An example of a reaction which never complete and exist in forward and reverse directions is the conversion of ice into water by melting and conversion of water into ice by freezing. It is an example of reversible reaction.

Changing Of State Of Reversible Reaction

Q.4: How can the direction of the reaction be predicted from equation?
Ans: Direction Of Reaction:
The direction of a reaction can be predicted by type of arrow; single headed arrow (→) used for irreversible reactions and a double half headed arrows (⇌) are used for reversible reaction, that never goes to completion.

Q.5: Describe a reversible reaction with an example.
Ans: Reversible Reaction:
A reversible reaction never goes to completion. It containing two processes; forward and reverse. Hence, a reversible reaction can move in either direction depends upon conditions.
Example:
Let us take an example of manufacturing of ammonia.
Forward Reaction: When one mole of nitrogen gas reacts with three moles of hydrogen it produces two moles of ammonia gas. This is known as 'Forward reaction'.

N_2(g) + 3H_2(g) → 2NH_3(g)

Reverse Reaction: In contrast, two moles of ammonia gas may also be converted into one mole of nitrogen and three moles of hydrogen. This is known as 'Reverse reaction'.

N_2(g) + 3H_2(g) ← 2NH_3(g)

When both of these reactions are written together as a reversible reaction, they are represented as:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

The original laboratory apparatus designed by Fritz Haber and Robert Le Rossignol in 1908 for synthesizing ammonia Equilibrium exists in many ways in our

Q.6: Explain dynamic equilibrium with an example.
Ans: Dynamic Equilibrium:
The reaction rate depends on the concentration of the reactants. At the beginning the quantity of reactant is higher, and the rate of product formation is the higher. As the reactant amount decreases, the rate of reactant transformation also decreases, and the rate of product formation decreases. After o certain time, the concentrations of reactants and products become constant, and this state is called dynamic equilibrium.

Rate of forward reaction = Rate of reverse reaction

In a reversible reaction, dynamic equilibrium is established before the completion of reaction. The rate of both forward and reverse reaction becomes equal upon reaching the equilibrium point.
Graph:
The following graph which is of concentrations vs. time, shows that the concentrations of both reactants and product becomes constant at equilibrium.

Dynamic Equilibrium

Example:
An example of a reaction at equilibrium is a reaction of hydrogen and iodine in a closed container to produce hydrogen iodide. At the start of the reaction, there is a high concentration of hydrogen and iodine and, after that, the concentration decreases as hydrogen iodide is formed. The concentration of hydrogen iodide increases as the forward reaction proceeds. As hydrogen iodide is formed, the reverse reaction is then able to occur.

H_2(g) + I_2(g) ⇌ 2HI_(g)

Hydrogen-Iodine Equilibrium System

So, there is no observable changes although both forward and reverse reactions occurring. The reaction has not stopped but reached dynamic equilibrium.

Q.7: Describe the macroscopic characteristics of forward and reverse reaction?
Ans: Macroscopic Characteristics Of Forward And Reverse Reaction:
FORWARD REACTION:

1. It is always directed from left to right in a chemical reaction.
2. Reactants produce products( Reactants → Products).
3. Initially rate is fast but gradually slow down.

REVERSE REACTION:

1. It is always directed from right to left in a chemical reaction.
2. Product produce reactant (Reactants ← Products).
3. Initially rate is slow but gradually speed up.

Q.8: What are the macroscopic characteristics of forward and reverse reaction in dynamic equilibrium?
Ans: Macroscopic Characteristics Of Forward And Reverse Reaction In Dynamic Equilibrium:

1. A dynamic equilibrium can only exists in a closed system — neither reactants nor products can enter or leave the system.
2. At equilibrium, the concentrations of reactants and products remain constant.
3. At equilibrium, the forward and reverse reactions are taking place at equal and opposite rates.
4. Equilibrium can be approached from either side of the reaction equation.
5. An equilibrium state can be disturbed and again achieved under the given condition of concentration, pressure, and temperature.

Q.9: What is Active Mass? OR Define active mass? and give its unit.
Ans: ACTIVE MASS:
The concentration of reacting substance is called Active mass.
Unit:
The unit of active mass is mol.dm^-3 or mol/L and its value is expressed in square brackets.

Q.10: Define law of mass action and derive the expression for the equilibrium constant.
Ans: LAW OF MASS ACTION:
Statement:
The law of mass action state that:

"The rate of a reaction is directly proportional to the product of the concentration of each reactant."

Explanation:
The rate at which a substance reacts is directly proportional to its active mass and the rate of a reaction is directly proportional to the product of the active masses of the reacting substances. The law of mass action also suggests that the ratio of the reactant concentration and the product concentration is constant at a state of chemical equilibrium.

Derivation Of Equation For Equilibrium Constant:
Let us apply law of mass action on a hypothetical reversible reaction.

aA + bB ⇄ cC + dD

Forward Reaction:
First let us discuss forward reaction, where A and B are reactants whereas 'a' and 'b' are number of moles needed to balance a chemical equation. The rate of forward reaction according to law of mass action is:

R_f ∝ [A]^a [B]^b

R_f = K_f [A]^a [B]^b

Where K_f is the rate constant for forward reaction.

Reverse reaction:
Likewise, rate of reverse reaction is directly proportional to product of molar concentrations of C and D whereas 'c' and 'd' are number of moles needed to balance a chemical reaction.

R_r ∝ [C]^c [D]^d

R_r = K_r [C]^c [D]^d

Where K_r is the rate constant for reverse reaction.

At Equilibrium:
We know that, at equilibrium rate of forward and reverse reaction becomes equal. So,

R_f = R_r

Putting the values of R_f and R_r, we have:

K_f [A]^a [B]^b = K_r [C]^c [D]^d

By taking constants on L.H.S and variables on R.H.S, we have

Where K_c is called equilibrium constant.
Hence proven that law of mass action describe relation between active masses of reactants and products with rate of reaction. All the reversible reactions can be expressed in this form.

Q.11: Define equilibrium Constant? How can equilibrium constant be determined.
Ans: Equilibrium Constant:
An equilibrium constant is used to show the relation between molar concentration of product and molar concentration of reactant. Or It can be define as the ratio between molar concentration of product to the molar concentration of reactant. It is denoted by K_c.
Equation:

Unit: As K_c is a ratio so it has no unit.
K_c is independent of initial concentration of reactant and product. K_c is temperature dependent.

Determine The equilibrium Constant:
Equilibrium constants are determined by observing the concentrations of each ingredient in a single reaction until it reaches equilibrium and then calculating its numerical value. We have to figure out the ratio of product to reactant concentrations. It is impossible to change a given reaction's equilibrium constant since concentrations are measured at equilibrium, and regardless of initial concentrations.

Q.12: What factors affects the equilibrium constant?
Ans: Factors Affects The Equilibrium Constant:
The temperature is the single factor affecting the equilibrium constant's value.

Q.13: Describe the important characteristics of equilibrium constant expression?
Ans: Characteristics Of Equilibrium Constant Expression

1. Important characteristics of equilibrium constant expression are as follows:
2. K_c only works in equilibrium.
3. It represents the equilibrium concentration of the reactant and product in mol.dm^-3
4. K_c is independent of reactant and product concentrations.
5. K_c varies with temperature.
6. K_c is a balanced chemical equation coefficient. In a balanced chemical equation, each reactant and product has a concentration equal to its coefficient.
7. K_c represents equilibrium position. If K_c is larger than 1, the reaction is forward. If K_c is less than 1, the reaction is a reverse reaction.
8. Equilibrium constant K_c is a ratio of reactant to product that is utilized to define chemical behavior.

Q.14: Describe the units of equilibrium constant?
Ans: Unit Of Equilibrium Constant:
At equilibrium:

Rate of the forward reaction = Rate of the backward reaction

An equal number of moles on both sides of the equation has no unit in K_c, because K_c expression uses concentration units that cancel. The unit of concentration is mol.dm^-3.
Consider a reaction:

CO_2(g) + H_2(g) ⇌ CO_(g) + H₂O_(g)

For reactions when the number of motes of reactants and product are not equal, K_c has a unit. Let us consider the following reaction:

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

Q.15:Write down the importance of the equilibrium constant?
Ans: Importance Of Equilibrium Constant:
The value of K_c varies depending on the response. K_c isn't only a calculating constant. It affects both the direction and the extent of a chemical reaction.

1. Direction Of A Chemical Reaction:
When dealing with reversible reactions, it is critical to determine the reaction's direction at any given time. For example, to produce ammonia efficiently from nitrogen and hydrogen, we must optimize the process. So it's critical to forecast the reaction's condition at any given time. The reaction quotient, Q_c, can help make such predictions.
It has the some mathematical structure as K_c, but Q_c is a ratio of real concentrations computed at a given moment (not a ratio of equilibrium concentrations). Comparing K_c and Q_c values predicts response direction. We have three categories:

1. If Q_c = K_c:
   The actual product and reactant concentrations are equal to the equilibrium concentrations, and the system is stable.
2. If Q_c < K_c:
   Then there is increase in product concentration for equilibrium. So the forward reaction occurs, forming additional products.
3. If Q_c > K_c:
   There is decrease in product concentration & to achieve equilibrium. As, the process reverses, forming more reactants.

2. Extent Of Chemical Reaction:
At a certain temperature, the extent of a reaction is measured. The magnitude of an equilibrium constant can predict the scope of a chemical reaction. As magnitude may be very high, very low, or moderate, so can be extent of a chemical reaction.

1. K_c is very small:
   Reactions with low K_c never finish. That is, maximum reactant concentration and minimum product concentration. These are called 'reverse or backward responses.
   
   F_2(g) ⇌ 2Fe_(g) K_c = 7.4 x 10^-13 at 227 °C 
   
   
2. K_c is very large:
   Reactions with high K_c values are virtually complete. That is, maximum product concentration and minimum reactant concentration. This type of reaction is known as 'Forward reaction.
   
   2H_2(g) + O_2(g) ⇌ 2H₂O_(g) K_c= 2.4 x 10⁴⁷ at 227°C
   
   
3. K_c is neither very small nor very large:
   Reactions which have moderate value of K_c are considered to be at equilibrium. The concentration of reactants and products is almost same. For example:
   
   N₂O_4(g) ⇌ 2NO_2(g) K_c = 0.36 at 25 °C

Q.16: Define reaction quotient Or Q_c?
Ans: Reaction Quotient Or Q_c
Under non-equilibrium condition, reaction quotient 'Q_c' is define as the ratio of the product of active masses of reactant and products raised to the respective stoichiometric coefficients in the balanced chemical equation to that of the reactant.