# The Chemical Equilibrium or Dynamic Equilibrium

The equilibrium is a state which some reactions reach

It is also called dynamic equilibrium because both forward and backward reactions are going on simultaneously

A+B ⇌ C+D

## Features of dynamic equilibrium

• The rate of both forward and backward reaction are equal
• The amount of reactants and products are constant

## How the dynamic equilibrium is established • At first, the rate of forward reaction is the highest and the rate of the backward reaction is zero.
• As the reaction progresses the rate of forward reaction decreases and the rate of the backward reaction increases.
• The equilibrium is reached when the rate of both forward and backward reaction becomes equal. • At the beginning, the reactants are turning to products so the concentration of reactants are decreasing.
• The reactants are producing products so the concentration of product is increasing.
• When the equilibrium is reached, the rate of forward and backward reaction becomes equal. So the concentration of reactants and products becomes constant.

## Factors which affects the equilibrium position

### Le Chatelier’s Principle

Whenever a system which is in dynamic equilibrium is disturbed it tend to respond in such a way as to oppose the disturbance and so restore equilibrium.

## Factors affecting the equilibrium position

• Temperature
• Pressure
• Concentration

### Temperature

Suppose:

A+B⇌ C+D ΔH= -90KJmol-1

• The enthalpy change given is for the forward reaction
• Therefore the enthalpy change for the backward reaction is +90 KJmol-1
• If the temperature is increased the equilibrium would want to decrease the temperature by absorbing heat, by doing the endothermic reaction. Hence the equilibrium position would shift to the left.
• If the temperature is decreased, the equilibrium would want to increase the temperature, by doing the exothermic reaction. Hence shifting the equilibrium to the right.

### Pressure

• It is a factor for the equilibrium position if anyone of the products or reactants are gas.

Example: A(g) + 2B(g) → C(g) + D(g)

• An increase in pressure would shift the equilibrium position to the right because there are more number of gaseous molecules on the right-hand side ( 3 molecules)

### Concentration

A+B ⇌ C+ D

• If the concentration of B is increased, the equilibrium would shift to the right in order to decrease the concentration of B
• If the concentration of C is increased, the equilibrium position would shift to the left, to decrease the concentration of C

Examples:

1. 2NO2(g) ⇌ N2O4(g)
• NO2 is brown in color, whereas N2O4 is colorless.
• The equilibrium mixture is light brown in color, as there are both gases present.
• If the pressure is increased, the equilibrium shifts to the right, less number of gaseous molecules on right. Producing more colorless N2O4 , turning the mixture lighter in color.
• If the temperature in increased, the equilibrium shifts to the left-hand side, because the forward reaction is exothermic. Producing more brown NO2 , turning the mixture darker.

### Iodine(I)Chloride and Iodine(II)Chloride

• Chloride reacts with iodine to form Iodine(I)Chloride

I2 (s) + Cl2(g) → 2ICl(l)

• If excess chlorine is passed ICl reacts with more chlorine to form Iodine(III)Chloride ICl3

ICl(l) +Cl2(g) ⇌ ICl3(s)

• ICl3 →Yellow crystal

ICl à Brown liquid

• When excess chlorine is passed, the equilibrium shifts to the right producing and more yellow solid is formed.
• When the U-tube is tipped, chlorine gas is removed, the equilibrium shifts to the left, forming more brown liquid. iii) Color Change:  1. iv) Methane Hydrate (s) ⇌ methane (g) + water (l)
• The forward reaction is exothermic
• Due to the recent climate change and increase in temperature, the equilibrium is shifting to the right and more methane gas is formed.

## Haber process

• It is the name given for the production of ammonia gas.

N2 + 3H2 ⇌ 2NH3 ΔH = -92 KJmol-1

### Conditions for Haber process

Temperature: 450°C

Pressure: 200 atm

Catalyst used: Iron

#### Reasons for such decision:

Temperature: Even though a low temperature is preferred, but still a high temperature of 450°C is used. Because a low temperature would decrease the rate of reaction. But a temperature higher than 450°C would result in a lower yield which would not be beneficial.

Pressure: A high pressure of 200 atm is understandable as it shifts the equilibrium position to the right. As there are less number of gaseous molecules on the right-hand side. But a higher pressure than 200 atm would be expensive also risky.

Catalyst: The ruthenium is 40% more efficient than iron but still iron is used because ruthenium does not work at a lower temperature.

#### Things done during the Haber process:

• The ammonia gas produced is liquefied and removed. Shifting the equilibrium position to the right producing more ammonia gas.
• The nitrogen and hydrogen gas are recycled continuously, leading to all reactants being used up. Atom economy is therefore 100%.