Chemical equilibrium

Question 1

Define dynamic equilibrium

Answer 1

Dynamic equilibrium is the equilibrium that exists in a closed system when the rate of a forward reaction is equal to the rate of the reverse reaction.

Question 2

Define le Chatelier’s principle

Answer 2

le Chatelier’s principle states that when a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise the change.

Question 3

When a system is in a state of equilibrium, there is no observable change, but what is actually happening?

Answer 3

The system is dynamic: - Reactants are being converted into products as fast as the products are being converted into reactants.

Question 4

What two factors must remain the same for a chemical system to be in dynamic equilibrium?

Answer 4

• The concentrations of the reactants and the products remain the same. - The rate of the forward reaction is the same as the rate of the reverse reaction.

Question 5

What is meant by equilibrium only applies as long as the system remains isolated?

Answer 5

In an isolated system, no materials are being added or taken away and no external conditions, such as temperature or pressure, are being altered.

Question 6

The position of equilibrium can be altered by changing what?

Answer 6

• Concentrations of the reactants and products. - Pressure in reactions involving gases - Temperature

Question 7

For the following equilibrium:

CH₃CH₂OH(l) + CH₃CH₂COOH(l) ⇔ CH₃CH₂COOCH₂CH₃(l) + H₂O(l)

Where will the position of equilibrium move if you increase the concetration of reactants?

Answer 7

The position of equilibrium will move in the direction that decreases the increased reactant concentration.

• The system opposes the change by decreasing the concentration of the reactant by removing it.
• The position of equlibrium moves to the right-hand side, forming more products.

Question 8

Consider this equilibrium:

N₂(g) + 3H₂(g) ⇔ 2NH₃(g)

• What happens to the equlibrium when you increase the total pressure of the system?

Answer 8

• Increasing the total pressure of the system causes the position of equilibrium to move to the side with fewer gas molecules, as this will decrease the pressure. In the equation, the position of equilibrium will move to the right.

Question 9

Consider the reaction:

N₂(g) + 3H₂(g) ⇔ 2NH₃(g) H=-92KJ mol^-1

Explain what happens to the equilibrium when the temperature is increased.

Answer 9

• The system opposes the change by taking in heat and the position of equilibrium moves to the left.
• The position of equilibrium moves in the endothermic direction.

Question 10

What is the effect of a catalyst on an equilibrium reaction?

Answer 10

A catalyst does not alter the position of equlibrium of the composition of an equilibrium system.

• A catalyst speeds up the rate of the forward and reverse reactions equally.
• A catalyst increases the rate at which the equilibrium is established.

Question 11

In the haber process, were do the raw materials come from?

Answer 11

• Nitrogen is obtained from the air by fractional distillation.
• Hydrogen is prepared by reacting together methane

( from natural gas) and water.

Question 12

What conditions(theoretical) favourproduction of ammonia?

Answer 12

Ammonia is produced by the forward reaction in this equilibrium.

• The forward reaction produces fewer gas molecules favoured by using high pressure.
• The forward reaction is exothermic, favoured by using low temperature.

Question 13

What are the drawbacks of the theoretical(perfect) conditions for the ammonia process?

Answer 13

• Although a low temperature should produce a high equilibrium yield, the reaction would take place at a very low rate. At low temperatures, comparatively few N₂ and H₂ molecules have energy equal to, or greater than the activation energy.
• A high pressure increases the concentration of the gases,increasing the rate. However, large quantities of energy are required to compress gases, adding significantly to the process’s running costs.
• There are all also safety implications-any failure in the system could potentially allow chemicals to leak into the environment, endangering those working on site.

Question 14

Explain all the actual conditions in a modern ammonia plant.

Answer 14

• Temperature - A temperature of 400*C is used. This is a compromise between having a low temperature for a high yield and a high temperature for fast rate of reaction.
• Pressure - A high pressure benefits high yield and rate. Only 200 atmoshperes are used to reduce the danger of the work force or environment.
• Catalyst - An iron catalyst is added to speed up the rate of reaction, this reduces temperatures and means less energy is used to generate heat, reducing costs.