Chemical Equilibrium

What is chemical equilibrium

A reaction where the forward and reverse reactions are occurring at the same rate and the concentrations of all reagents are stable

What are the differences between reversible and irreversible reactions?

• Reversible → can go forwards and backwards

• Irreversible → can only go forwards

• Reversible → does not go into completion, so yield is never 100%

• Irreversible → reaction continues until the limiting reagent is used up and the reaction goes into completion

Examples of reversible reactions

• H2 (g) + I2 (g) ⇌ 2HI [formation and decomposition]

• Haber process

• N2O4 (g) ⇌ 2NO2 [decompostion and dimerization]

Examples of irreversible reactions

• Precipitation

• Combustion

Differences between dynamic and static equilibrium

• Dynamic → reversible reaction

• Static → irreversible reaction

• Dynamic → reactants are products still participating in chemical reactions

• Static → no further chemical reaction in the system

• Dynamic → forward and backward reaction rates are equal

• Static → forward and backward reaction rates are 0

• Dynamic → only occurs in closed systems

• Static → occurs in both open and closed systems

Difference between closed and open system

• Open: can exchange both matter and energy with its surroundings

• Closed: can only exchange energy, not matter

What is a similarity between irreversible and reversible reactions?

Concentration of reactant and products in total remains constant, though the concentration of reactant as compared to products is different

Graphs of dynamic equilibrium in a closed system

What is Le Châtelier’s Principle?

When a system in dynamic equilibrium is subjected to a change in conditions which disturb the equilibrium, the system will respond in such a way as to counteract that change to establish a new equilibrium

How does concentration of gases or aqueous substances affect equilibrium?

Increase: Equilibrium shifts to the opposite side of the reaction to remove the increased substance

Decrease: Equilibrium shifts to the same side of the reaction to produce more substance

How does pressure of reactants affect equilibrium?

Increase: Volume decreases. Equilibrium shifts to the side with fewer moles to reduce pressure

Decrease: Volume increases. Equilibrium shifts to the side with more moles to increase pressure

How does temperature affect equilibrium?

Increase: Favours endothermic reactions 

Decrease: Favours exothermic reactions

*Exo. occurs at the opposite direction of end.

How does the presence of a catalyst affect equilibrium?

• Quickens the attainment of equilibrium

• Does not affect position of equilibrium

What is the equation for Haber Process?

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Describe the Haber process

1. N2 and H2 are mixed in the ratio of 1:3 b volume

2. The gas mixture is compressed

3. Compressed gases flow over the catalyst and are heated

4. A mixture of NH3, N2 and H2 is obtained and cooled

5. NH3 condenses into a liquid pumped into tanks and stored under pressure

6. Unreacted N2 and H2 are transferred back into the converter to be recycled

What are the conditions for the Haber process?

• Pressure of 250atm

• Temperature of 450ºc

How is hydrogen obtained for the Haber process?

From the cracking or breaking down of crude oil fractions

How is nitrogen obtained for the Haber process?

From the fractional distillation of liquid air

Why is an Fe catalyst used in the Haber process?

• Speeds up the forward and reverse reactions, which are slow due to the compromise temperature and pressure

• Cost of using catalyst is lower in the long run since it can be reused and regenerated

What is the effect of pressure in the Haber process?

• Higher pressure → higher ammonia yield and faster reaction

*250atm compromised pressure as maintaining a high pressure is costly

What is the effect of temperature in the Haber process?

• Lower temperature leads to higher yield of ammonia as it reduces the decomposition of ammonia

*450ºc is a compromised temperature as lower temperature leads to a slower reaction

How does temperature affect rate and yield of reactions in general?

Low temp: Low rate, high yield

High temp: High rate, low yield

Moderate temp: Moderate yield and rate

With high rate, frequency of effective collisions is lower