Equilibrium reactions become dynamic equilibriums
Dynamic Equilibrium is when the rate of both the forward reaction and the reverse reaction is equal with the reactions occurring at the same time and the concentrations of reactants and products constant
Dynamic Equilibrium is when the rate of both the forward reaction and the reverse reaction is equal with the reactions occurring at the same time and the concentrations of reactants and products constant
Equilibrium Constant
The equilibrium is the ratio of the concentrations of products to reactants
The equilibrium constant is given the symbol Kc
Kc is also a way of working out which side the equilibrium lies toward
The equilibrium constant is given the symbol Kc
Kc is also a way of working out which side the equilibrium lies toward
Kc only changes for a specific reaction if the temperature changes
Working out Kc
The equilibrium constant's relationship between the chemical equation and the Kc equation is:
Working out Kc units
Take this example:
Now plug in the units you know
If there are units you need to know the volume if you are only given moles
Discovering Equilibrium Concentrations
Sometimes you are only given the starting concentrations in a reaction
In the reversible reaction PCl5 ⇌ PCl3 + Cl2
At the start there was a mixture of 1 mole of PCl5
When equilibrium was reached only 0.5 moles of PCl5 remained
This means that 0.5 moles of PCl5 was lost during the reaction
The reactions stoichiometry shows a one to one reaction, meaning 0.5 moles for each of PCl3 and Cl2 was gained
In the reversible reaction PCl5 ⇌ PCl3 + Cl2
At the start there was a mixture of 1 mole of PCl5
When equilibrium was reached only 0.5 moles of PCl5 remained
This means that 0.5 moles of PCl5 was lost during the reaction
The reactions stoichiometry shows a one to one reaction, meaning 0.5 moles for each of PCl3 and Cl2 was gained
Don't worry that the total number of moles has changed from 1 to 1.5. This can occur because the type of species has changed.
A simple analogy can explain this:
A simple analogy can explain this:
So far so simple. Now lets change the Stoichiometry.
N2 + 3H2 ⇌ 2NH3
At the start there was 1 mole of N2 and 1 mole of H2
At equilibrium 0.8 moles of N2 remained
This messes up the results as N2 and H2 are 1:3 and N2 and NH3 are 1:2
This means that as 0.2 moles of N2 have been lost:
0.6 moles of H2 will have been lost and 0.4 moles of NH3 have been made
N2 + 3H2 ⇌ 2NH3
At the start there was 1 mole of N2 and 1 mole of H2
At equilibrium 0.8 moles of N2 remained
This messes up the results as N2 and H2 are 1:3 and N2 and NH3 are 1:2
This means that as 0.2 moles of N2 have been lost:
0.6 moles of H2 will have been lost and 0.4 moles of NH3 have been made
Algebra in Equilibrium
You can get asked to work out the value of 'x' in an equilibrium equation
Kc = 0.5 moldm^-3
Plug in values and cancel the Vs (Leave as V instead of 2 so not to complicate things)
Plug in the value of V and rearrange to one line
Now rely on your GCSE Maths skills to work out the quadratic
x must equal 1
-2 would give you negative concentration of the products which can't happen
This means a minimum of 1 mole of PCl5 is required to provide 1 mole of each product
-2 would give you negative concentration of the products which can't happen
This means a minimum of 1 mole of PCl5 is required to provide 1 mole of each product
Changing the ConditionsUsing Le Chatalier's Principle
Temperature
Remember that Kc changes with temperature
Increase in Temperature
Decrease in Temperature
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Concentration
Remember that Kc doesn't change with concentration or pressure
Lets look at the following equation
- Increase in products means equilibrium moves to left temporarily.
- This forces the reactants to decrease until the Kc fraction restores itself.
- Decrease in products means equilibrium moves to right temporarily.
- This forces the reactants to increase until the Kc fraction restores itself.