Equilibrium 2 - Kp Flashcards
If a reaction under equilibrium contains gases…
Then an alternative reaction can be set up using the partial pressures of gases instead of concentrations
Partial pressure of a gas is…
The pressure of a single gas (that’s in a mixture) that the gas would exert if it occupied the same volume as the gaseous mixture
What is the total gas mixtures pressure?
Every gas in the mixture’s partial pressure
Added together
What is Kp?
An equilibrium constant that uses partial pressures in the expression instead of concentrations
So can tell us the position of equilibrium
What do equilibrium constants (Kp, Kc) tell us?
The ratio of products to reactants at equilibrium, thus how far to the right or left the position of equilibrium is
How do we write an expression for Kp?
partial pressure of products
——————————————— = Kp
partial pressure of reactants
How do we represent the partial pressures in the expression for Kp?
NO SQUARE BRACKETS
p (gas formula)
To the power of the stoichiometry
Units for Kp
Substitute in the units for each partial pressure into the Kp expression and work out the units by cancelling/multiplying
The mole fraction
The number of moles of a gas
————————————————
Total number of moles of all gases
What does the mole fraction show?
The percentage of how much of one gas we have in a mixture out of the total amount of gas in the mixture
How to find the partial pressure of a gas from the mole fraction?
Multiply the specific gas’s mole fraction by the total pressure of gas
To find partial pressure
If given a percentage instead of mole fraction…
The percentage shows the percent of that gas in the gas mixture so we can work out the partial pressure by multiplying partial pressure by total pressure of gas
Then take away this partial pressure from total pressure to find the pressures of unknown gases
And we can share this value in the ratio depending on their stoichiometry
What changes the value of Kp?
Only temperature
The value of Kp stays constant at…
1 temperature
If you increase the temperature…
The pressures of products of the endothermic reaction increases
The pressures of reactants of exothermic reaction decrease
To overcome the change in equilibrium by taking heat energy in, POE shifts to favour endothermic reaction
If you decrease the temperature…
The pressures of products of the exothermic reaction increase
The pressures of products of endothermic reaction decrease
Because POE shifts to favour exothermic reaction to oppose the decrease in heat energy so heat energy released
What factors don’t change the Kp value?
Pressure
Catalysts
How does changing pressure affect value of Kp?
Has no affect because
The position of equilibrium will shift in order to keep the ratios of partial pressures of reactants to gases the same for each given temperature. So Kp stays constant
How do catalysts affect the value of Kp?
No affect because…
The ratio of partial pressures of reactants to gases stays the same (Kc value constant) at this temp
But rate of reaction increases equally for both forward and backward reaction, the pressures remain same but equilibrium achieved quicker
