Topic 7: Equilibrium Flashcards
Reversible reaction
Has a forward reaction and a reverse reaction
Closed system
In a closed system, matter can escape - but energy can’t
At equilibrium…
the rate of the forward reaction is equal to the rate of the reverse reaction
How does the concentration of products/reactants change over time in equilibrium
Equilibrium can be reached from either direction.
At equilibrium the concentrations of reactants and products are constant
Physical equilibrium
Differs from chemical equilibrium in that it involves a change of state rather than a chemical reaction
For example in a flask (closed system) where a volume of water evaporates and condenses at the same rate (they have reached physical equilibrium)
Equilibrium constant (Kc)
Write the Kc value of this reaction aA+bB ⇌ cC+dD
A constant for a reaction at a specific temperature - it is temperature dependant
It’s value give an idea of how far a reaction proceeds
[C]ᶜ[D]ᵈ Kc = ————
[A] ᵃ[B]ᵇ
Interpreting position of equilibrium from Kc value
What is the position of equilibrium if the Kc is:
1. 1.9 x10^-23
2. 3.2 x10^81
3. 4.61 x10^-3
4. 640
- Equilibrium is to the left (forward reaction hardly proceeds)
- Equilibrium lies to the right (goes to completion)
- Lies to the left (reaction contains mostly reactants)
- Lies to the right (reaction contains mostly products)
Interpreting extent of reaction from Kc
- Kc > 1
- Kc = 1
- Kc < 1
- Equilibrium lies to the right - equilibrium mixture contains higher concentration of products
- Equilibrium mixture contains equal concentrations of reactants and products
- Equilibrium lies to the left - equilibrium mixture contains higher concentration of reactants
Reaction quotient Q
Tells us in which direction the reaction will proceed to reach equilibrium
When Q = Kc the reaction is at equilibrium
Interpreting Q vs Kc
- Q > Kc
- Q < Kc
- Q = Kc
- Reaction will proceed to the left to reach equilibrium
- Reaction will proceed to the right to reach equilibrium
- Reaction is at equilibrium
Manipulating Kc
What happens when:
1. The reaction is reversed
2. The reaction coefficients are doubled
3. The reaction coefficients are halved
4. The reactions are added together
- Invert the expression to make it 1/Kc
- Square the value to make it Kc^2
- Square root the value
- Multiple the expression to make it Kc^1 x Kc^2
Le Chatelier’s principle
When a system at equilibrium is subjected to a change, the system will respond to minimise the effect of the change.
The changes can be changes in concentration, pressure or temperature (and temperature is the only one that affects Kc)
Change in concentration and effect on equilibrium
If the concentration of a reactant is increased, equilibrium shifts to the right (more products are produced ie. increase in yield)
If the concentration of a product is increased, equilibrium shifts to the left
Kc does not change
Change in pressure and effect on equilibrium
When pressure is increased, equilibrium is shifted to the side with fewer molecules
When pressure is decreased, equilibrium is shifted to the side with more molecules
If number of molecules of reactants and products are equal there is no change to position of equilibrium
Kc does not change
Change in temperature and effect on equilibrium
For exothermic reactions:
When temperature is increased, equilibrium is shifted to left and Kc decreases.
When temperature is decreased, equilibrium is shifted to right and Kc increases.
For endothermic reactions:
When temperature is increased, equilibrium is shifted to right and Kc increases.
When temperature is decreased, equilibrium is shifted to left and Kc decreases.
