19 Equilibrium constant Kp Flashcards
How is Kp different to Kc
eg write out kp expression for
2A + B <—> 3C +D
Although you can use Kc in solutions, many reversible reactions occur in the gas phase
So we have to express their concentrations in a different way, using partial pressure
(PpC)^3(PpD) / (PpA)^2(Pp(B)
So do NOT use square brackets for kp because we are not dealing with concentration.
Use rounded brackets, with P in them to show the pressure
What is partial pressure
What is total pressure
. It is the pressure that the gas would exert if it occupied the container on its own
. Each gas contributes to the partial pressure, so it is the sum of all the gases
Eg air has oxygen, nitrogen etc and has an overall pressure of 100Kpa
So partial pressure of oxygen in air is 20kpa bcs it makes up 20%
And nitrogen is 80kPa
Total pressure is the sum of all of these partial pressures combined
How do you find partial pressure
p of A = mol fraction of gas A multiplied by total pressure
How do you find mol fraction of a gas
Number of moles of A in the mixture divided by total number of moles of gas in mixture
How would you apply equilibrium law to gaseous equilibria
For example in 3H2 + N2 <–> 2NH3
. Find partial pressures of each reactant and product and multiply together in the kc equation
Where p is partial pressure
kp =
p^2(NH3) divided by (p^3(H2) x pN2)
In reaction
SO2Cl2 —> SO2 + Cl2
- Total pressure is 714kpa
- At equilibrium, vessel contains 263kpa of Cl2
Calculate the partial pressure of SO2Cl2
Total pressure is the sum of all the partial pressures so
714 = 263 + P(SO2) + P(SO2Cl2)
The chlorine and sulfur dioxide are in a 1:1 ratio so the partial pressure of SO2 is also 263
So pressure of SO2Cl2 is
714 - 526
Which is 188
In equation
2D(g) –> E(g) + F(g)
What would the mol fraction of D be
We can see that there are 4 mol of gas in total
So 2/4 is 1/2 so that is the mol fraction
How does temperature affect Kp
.Kp is only valid for one temperature
. Changing the temperature will change equilibrium pressures as more energy means the particles move around more and collide with walls etc so kp value will change
So if temperature increases, and if forward reaction is exothermic, the equilibrium will shift to the left, so there are more reactants.
So in Kp equation, the value of Kp will decrease
If temperature causes equilibrium to shift to the right, there will be more products made.
So value of Kp will increase
How does pressure affect Kp
Kp is not affected by change in pressure because the pressure will impact the conditions for all substances so it will even out
How would a catalyst impact Kp
Adding a catalyst would not affect Kp, it just increases rate of reaction by increasing speed of forward and backward reaction, decreasing activation energy for it to occur
