3.2.3 Dynamic equilibrium and le Chatelier's principle Flashcards
What is the enthalpy change of reverse reactions
The enthalpy change for the forward and backwards reactions are equal and opposite.
Define dynamic equilibrium
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.
What are the 2 things that show a system is at equilibrium
- The rates are equal
- the concentrations are constant
When will a system reach equilibrium
When a reversible reaction is placed in a closed system
define closed system
a system isolated from its surroundings
nothing can enter or leave
State le Chatelier’s principle
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
What 3 factors can effect the position of equilibrium
- concentration
- pressure (gases)
- temperature
How does change in concentration effect the position of equilibrium
at dynamic equilibrium
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
How does change in pressure effect the position of equilibrium
at dynamic equilibrium
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
How does change in temperature effect the position of equilibrium
at dynamic equilibrium
Increasing the temperature favours the endothermic reaction
Decreasing the temperature favours the exothermic reaction
define exothermic reaction
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
define endothermic reaction
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
Outline a method to investigate changes to the position of equilibrium with concentration and explain
- Add a solution of (yellow) potassium chromate (K₂CrO₄), to a beaker.
- Add dilute sulfuric acid (H₂SO₄), dropwise until there is no further change. The solution turns an orange colour.
- 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
Outline a method to investigate changes to the position of equilibrium with temperature and explain
- 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.
- 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
If the forward reaction is exothermic what effect does change in temperature have
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
If the forward reaction is endothermic what effect does change in temperature have
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
Outline a method to investigate changes to the position of equilibrium with pressure and explain
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
What is the effect of a catalyst on equilibrium
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.
Which conditions should not be used for the Haber process and why
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.
What conditions are used for the Haber process
and why
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.
