10. Equilibrium constant Kp
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What is Kp?
The equilibrium constant for a reversible reaction where some or all of the reactants and products are gases
Why do solids, liquids and aqueous solutions not appear in the expression for Kp?
They don’t contribute to the pressure of a system
What substances appear in the expression for Kp?
Gases only
What is the expression for Kp when aA + bB ⇋ cC + dD?
p(C)ᶜ p(D)ᵈ / p(A)ᵃ p(B)ᵇ
In the expression for Kp, what does the p before the brackets stand for?
Partial pressure
How are the units for Kp worked out?
The same way as those for Kc
What can be used the same way as Kc to calculate the composition of an equilibrium mixture?
The gas equilibrium expression
What is the mole fraction?
The proportion of gaseous mixture that is made up of one particular gas
How to calculate the mole fraction of a gas in a mixture?
Number of moles of a gas / Total number of moles of gas in the mixture
What is partial pressure (p)?
In an equilibrium reaction involving gases, when each individual gas exerts its own pressure
How to calculate the partial pressure of a gas in a mixture?
Mole fraction of gas x Total pressure of the mixture
How to calculate total pressure of a gas mixture?
Sum of all partial pressures of each gas in the reaction
What is Le Chatelier’s principle?
If a factor affecting the position of equilibrium is altered, the position of equilibria will shift to oppose the effect of the change
What happens to equilibrium if the temperature is decreased?
It will shift in the direction of the exothermic reaction
What happens to equilibrium if the temperature is increased?
It will shift in the direction of the endothermic reaction
What happens to Kp if a change in temperature shifts the position of equilibrium to the right?
Kp will increase
What happens to Kp if a change in temperature shifts the position of equilibrium to the left?
Kp will decrease
What happens to equilibrium if the pressure is increased?
It will shift in the direction with the least moles of gas
What happens to equilibrium if the pressure is decreased?
It will shift in the direction with the most moles of gas
Does changing pressure make any difference to Kp?
No
Does adding a catalyst have any effect on Kp?
No
Why does adding a catalyst have no effect on Kp?
Increases rate of forward and reverse reactions by the same amount - so position of equilibrium will be reached quicker but remain unchanged