3.2.3 Dynamic equilibrium and le Chatelier's principle

Question 1

What is the enthalpy change of reverse reactions

Answer 1

The enthalpy change for the forward and backwards reactions are equal and opposite.

Question 2

Define dynamic equilibrium

Answer 2

The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrationds do not change.

Question 3

What are the 2 things that show a system is at equilibrium

Answer 3

1. The rates are equal
2. the concentrations are constant

Question 4

When will a system reach equilibrium

Answer 4

When a reversible reaction is placed in a closed system

Question 5

define closed system

Answer 5

a system isolated from its surroundings

nothing can enter or leave

Question 6

State le Chatelier’s principle

Answer 6

When a system in dynamic equilibrium is subjected to external change, the system readjusts itself to minimise the effect of the change to restore equilibrium

When a system at equilibrium is subjected to change, the equilibrium will shift to minimise the change

Question 7

What 3 factors can effect the position of equilibrium

Answer 7

• concentration
• pressure (gases)
• temperature

Question 8

How does change in concentration effect the position of equilibrium

at dynamic equilibrium

Answer 8

A ↔ B
To effect equilibrium using concentration you must add or remove reactants or products
If A is added (↑[A]), equilibrium shifts to the right to remove A (by turning it into B)
If B is removed (↓[B]), equilibrium shifts to the right to produce more B

↔ is representing the reverse reaction arrow from reactants to products and the other way around

Question 9

How does change in pressure effect the position of equilibrium

at dynamic equilibrium

Answer 9

ONLY count the moles of gases
2A ↔ B
If pressure is increased, equilibrium shifts to the side with fewer moles of gas.
Here inceased pressure shifts equilibrium to the right

Question 10

How does change in temperature effect the position of equilibrium

at dynamic equilibrium

Answer 10

Increasing the temperature favours the endothermic reaction
Decreasing the temperature favours the exothermic reaction

Question 11

define exothermic reaction

Answer 11

A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings
𝚫H = negative

Question 12

define endothermic reaction

Answer 12

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken from the surroundings
𝚫H = positive

Question 13

Outline a method to investigate changes to the position of equilibrium with concentration and explain

Answer 13

1. Add a solution of (yellow) potassium chromate (K₂CrO₄), to a beaker.
2. Add dilute sulfuric acid (H₂SO₄), dropwise until there is no further change. The solution turns an orange colour.
3. Add aqueous sodium hydroxide, (NaOH), until there is no further change. The solution chnages back to a yellow.
   (steps 2 and three can be repeated to continue changing the colour)

When dilute NaOH (aq) is added it increases the [H⁺ ions], which increases the rate of the forward reaction and so causes the equilibrium to shift to minimise the change in [H⁺].
This shift decreases the the concentration of teh added reactants (H⁺ ions).
The position of equilibrium shifts to the right of the equation - making more products.
A new position of equilibrium is established favouring the products. Which turns the solution orange as Cr₂O₇²⁻ forms

2CrO₄²⁻ + 2H⁺ ↔ Cr₂O₇²⁻ + H₂O

→ = add acid - the [H⁺] increases - turns orange
← = alkali added - the [H⁺] decreases - turns yellow

Question 14

Outline a method to investigate changes to the position of equilibrium with temperature and explain

Answer 14

1. dissolve cobalt chloride in water in a boiling tube. Add a small quantity of hydrochloric acid. Place the boiling tube in some iced water. The solution is a pink colour.
2. Set up a water bath and transfer the boiling tube into the boiling water. The solution turns blue.
   (Transfer between the two temperatures to continue changing the colour)

In the boiling water you are increasing the heat energy of a system. This causes th e position of equilibrium to shift to minimise the change. As the forward reaction is endothermic (𝚫H is positive), the position of equilibrium shifts to the right in the endothermic direction, to take heat energy in and minimise the increase in temperature. This turns the solution blue

(Co(H₂O)₆)²⁺ + 4Cl⁻ ↔ CoCl₄²⁻ + 6H₂O

→ = increase temperature - shifts in endo direction - turns blue
← = decrease temperature - shifts in exo direction - turns pink

Question 15

If the forward reaction is exothermic what effect does change in temperature have

Answer 15

Increased temp: position of equilibrium shifts to the left - more reactants are made.
decreased temp: position of equilibrium shifts to the right - more products are made

Question 16

If the forward reaction is endothermic what effect does change in temperature have

Answer 16

Increased temp: position of equilibrium shifts to the right - more products are made
Decreased temp: position of equilibrium shifts to the left - more reactants are made

Question 17

Outline a method to investigate changes to the position of equilibrium with pressure and explain

Answer 17

Increasing the pressure of the system will shoft the position of equilibrium to the side with fewer molecules, reducing the pressure of the system.
As there are fewer gaseous moles on the right-hand side of the equilibrium, the position of equilibrium shifts to the right, reducing the number of gaseous miles to minimise the increase in pressure.
So more colourless N₂O₄ is formed and the brown colour fades.

2NO₂ ↔ N₂O₄

→ = increased pressure shifts towards fewer gaseous molecules - turns colourless
← = decreased pressure shifts towards more gaseous molecules - turns brown

Question 18

What is the effect of a catalyst on equilibrium

Answer 18

A catalyst does not change the position of equilibrium. It meerly speeds up the rates of the forward and reverse reactions equally. A catalyst will, however, increase the rate at which an equilibrium is established.

Question 19

Which conditions should not be used for the Haber process and why

Answer 19

The best conditions to produce ammonia would be low temperature and high pressure but these are not used because:
- low temperature would produce a high yeild of product, but would do so very slowly. if the temperature is too low, the rate maybe so slow that equilibrium is never established.
- A high pressure not only increases the yeild but also forces the molecules together, increasing the concentration and rate of reaction. However a very high pressure requires a very strong container and a large quantity of energy, increasing the cost of the process. Safety is also a concern, as failure could lead to hot gases, including toxic ammonia, leaking and endangering the workforce and surrounding area.

Question 20

What conditions are used for the Haber process

and why

Answer 20

A typical ammonia plant operates under compromised conditions using a high enough temperature to give a reasonable rate without shifting the equilibrium too far away from the products. This ensures a good yeild is achieved quickly, cheaply and safely. Usually between 350 and 500 ℃. and pressures of 100-200atm.
An iron catalyst is used to speed up the reaction so that a lower teperature can be used and operating costs are reduced.