Chemical Equilibrium

Question 1

What is Forward Reaction?

Answer 1

A forward reaction is a reaction in which products are produced from reactants and it goes from left to right in a reversible reaction.

Question 2

What is backward Reaction?

Answer 2

The Backward reaction is when the reaction goes from products to reactants and it goes from right to left in a reversible reaction.

Question 3

What is Reversible Reaction?

Answer 3

A reaction which can go in the forward and backward direction simultaneously is called a Reversible reaction.
A+B ⇋C+D

Question 4

What is Chemical Equilibrium?

Answer 4

The state of a reversible reaction when the two opposing reactions occur at the same rate and the concentrations of reactants and products do not change with time.
The equilibrium of a reaction can be attained from both sides. Thus, the equilibrium concentrations of the reactants and products are the same whether it start with reactants or products.

Question 5

Why Chemical Equilibrium is Dynamic Equilibrium?

Answer 5

At equilibrium the concentration of reactants and products are maintained constant. This does not mean that the reaction stops. But both forward and backward reactions continue at the same rate, even after the equilibrium is reached. That’s why equilibrium is known as dynamic equilibrium.

Question 6

Chemical equilibrium on the basis of kinetic molecular model.

Answer 6

A+B ⇋C+D
The molecules of in the equilibrium mixture collide with each other to form C and D. Likewise C and D collide to give back A and B. The collisions of molecules in a closed system are a ceaseless phenomenon. Therefore, collisions of A and B giving C and D (Forward reaction) and collisions of C and D giving back A and B (reverse reaction) continue to occur even at equilibrium, while concentrations remain unchanged.

Question 7

Characteristics of Chemical Equilibrium

6 steps

Answer 7

1. Constancy of concentrations
2. Equilibrium can be initiated from either side
3. Equilibrium cannot be Attained in an Open Vessel
4. A catalyst cannot change the equilibrium point
5. Value of Equilibrium Constant does not depend upon the initial concentration of reactants
6. At Equilibrium ΔG = 0

Question 8

1. Constancy of concentrations

Answer 8

When a chemical equilibrium is established in a closed vessel at constant temperature, concentrations of the various species in the reaction mixture become constant. The reaction mixture at equilibrium is called Equilibrium mixture. The concentrations at equilibrium are called Equilibrium concentrations.

Question 9

2. Equilibrium can be initiated from either side

Answer 9

The state of equilibrium of a reversible reaction can be approached whether the reaction start with reactants or products, for example, the equilibrium
H_2 (g)+ I_2 (g) ⇋ 2HI (g)
is established if the reaction starts the reaction with H2 and I2, or 2HI.

Question 10

3. Equilibrium cannot be Attained in an Open Vessel

Answer 10

The equilibrium can be established only if the reaction vessel is closed and no part of the reactants or products is allowed to escape out. In an open vessel, the gaseous reactants and/or products may escape into the atmosphere leaving behind no possibility of attaining equilibrium.
However, the equilibrium can be attained when all the reactants and products are in the same phase i.e., ethanol and ethanoic acid.

Question 11

4. A catalyst cannot change the equilibrium point

Answer 11

When a catalyst is added to a system in equilibrium, it speeds up the rate of both the forward and the reverse reaction to an equal extent. Therefore, a catalyst cannot change the equilibrium point.

Question 12

5. Value of Equilibrium Constant does not depend upon the initial concentration of reactants

Answer 12

It has been found that the equilibrium constant must be the same when the concentrations of reacting species are varied over a wide range.

Question 13

6. At Equilibrium ΔG = 0

Answer 13

At equilibrium, the Gibbs free energy (G) is minimum and any change taking place at equilibrium proceeds without change in free energy, ΔG = 0.

Question 14

Law Of Mass Action

Answer 14

The rate of a chemical reaction is proportional to the active masses of the reactants.
Here, active mass means the number of moles per liter. The rate of a reaction is proportional to the molar concentrations of the reactants.

Question 15

What is the relationship between rate of reaction and molar concentration?

Answer 15

The rate of a reaction is proportional to the molar concentrations of the reactants.

Question 16

Definition of equilibrium constant.

Answer 16

The product of the equilibrium concentrations of the products is divided by the product of the equilibrium concentrations of the reactants, with each concentration term raised to a power equal to the coefficient of the substance in the balanced equation.

Question 17

Relationship between Kc and Kp:

Answer 17

Kp = Kc × RT^∆n

Question 18

What is ΔG°?

Answer 18

ΔG° is the free energy of the reaction in the standard state and is constant at a given temperature.

Question 19

Importance of ΔG°

Answer 19

The sign of ΔG° indicates whether the forward or reverse reaction is spontaneous. There are three possibilities depending on the sign of ΔG° for the reaction.

1. If ΔG° is negative, log K must be positive and the reaction proceeds spontaneously in the forward reaction.
2. If ΔG° is positive, log K must be negative and K is less than one. The reverse reaction is then spontaneous.
3. If ΔG° = 0, log K = 0 and K = 1. The reaction is at equilibrium.

Question 20

Meaning of the Magnitude of the Free Energy Change

Answer 20

If ΔG° is a large negative number, K will be much greater than one and the forward reaction will proceed nearly to completion. On the other hand, if ΔG° is a large positive number, K will be a small fraction. Then the reverse reaction will proceed almost to completion.

Question 21

What is Liquid equilibria?

Answer 21

The chemical equilibrium in which all the reactants and products are in the liquid phase is referred to as the liquid equilibrium. Like the gas-phase equilibria, the liquid equilibria are also called homogeneous equilibria. For example, alcohols and acids react to form esters and water.

Question 22

What is Heterogeneous Equilibria?

Answer 22

The equilibria in which the reactants and products are not all in the same phase are called heterogeneous equilibria. The decomposition of calcium carbonate upon heating to form calcium oxide and carbon dioxide is an example of heterogeneous equilibrium.

Question 23

Why the concentration of pure solid are not included in the equilibrium constant expression.

Answer 23

The concentration (moles per unit volume) of a pure solid (or liquid) is fixed and cannot vary. Thus, the concentrations of pure solids or liquids are not included in the equilibrium constant expression.

Question 24

Le Chatelier’s Principle

Answer 24

When stress is applied to a system in equilibrium, the system tends to adjust itself so as to reduce the stress.

Question 25

Le Chatelier’s Principle in a chemical reaction in equlibrium

Answer 25

If a change in concentration, pressure or temperature is caused to a chemical reaction in equilibrium, the equilibrium will shift to the right or the left so as to minimize the change.

Question 26

What will happen if a inert gas is added in a chemical equilibrium?

Answer 26

Addition of inert gas to an equilibrium mixture while the volume of the reaction vessel remains the same. The addition of the inert gas increases the total pressure but the partial pressures of the reactants and products are not changed. Thus, the addition of inert gas has no effect on the position of the equilibrium.

Question 27

La chatelier’s principle details

Answer 27

Effect of Change in Concentration: When the concentration of any of the reactants or products is changed, the equilibrium shifts in a direction so as to reduce the change in concentration that was made.

Effect of Change in Pressure: when pressure is increased on a gaseous equilibrium reaction, the equilibrium will shift in a direction which tends to decrease the pressure. The reactions in which the number of product molecules is equal to the number of reactant molecules are unaffected by pressure changes.

Effect of Change of Temperature: If the forward reaction proceeds by the evolution of heat (exothermic), the reverse reaction occurs by the absorption of heat (endothermic). Both these reactions take place at the same time and equilibrium exists between the two. If the temperature of a reaction is raised, heat is added to the system. The equilibrium shifts in a direction in which heat is absorbed in an attempt to lower the temperature.