11: Equilibrium 2 Flashcards
When is the Kp and Kc value applicable?
Only applies to a system in equilibrium
What are the stoichiometric amounts?
Number of molecules in the equation
What is the Kc equation?
Kc = ([C]^c[D]^d) / ([A]^a[B]^b)
What is Kc called?
The equilibrium constant
What state of elements are found in the Kc equations?
Solids and pure liquids are ignored as they cannot have variable concentrations
When does Kc not change?
When the temperature is constant Kc is constant
This explains Le Chatelier’s principle
What are the units of Kc?
Depends on the reaction involved
Each concentration is mol/dm^3
What is it called when the Kc has no units?
Dimensionless
What does it mean when Kc is large?
Much higher concentration of products than reactants at equilibrium
What does it mean when Kc is small?
Very little amount of products and a large amount of reactants at equilibrium
What does it mean when Kc is about 1?
Roughly equal amounts of products and reactants at equilibrium
What happens to equilibrium constants when you change the concentrations of things at equilibrium?
Changing concentrations has no effect on Kc
Only thing that changes Kc is a change in temperature
What happens to the position of equilibrium when concentrations are changed?
Le Chatelier’s principle states that concentration change means that the position of equilibrium moves to undo the change made
This is done so Kc is maintained
What occurs to the equilibrium constants when the pressure is changed?
Pressure changes causes no change in the equilibrium constants
What occurs to the position of equilibrium when the pressure is changed?
May be changed according to Le Chatelier’s Principle to undo the change which you have made
If pressure is increased will move to the side with less moles of gas decreasing it
What is the equation of Kp?
Kp = (Pc x Pd) / (Pa x Pb) P = partial pressure
When should Kp be used?
When there is an all-gas equilibrium
What is the equation to calculate partial pressure?
Partial pressure = Mole fraction of A x total pressure at equilibrium
What occurs in the Kp equation when there are the same number of moles on each side?
The pressures cancel out so the equation is just the mole fractions
Therefore pressure has no effect on position of equilibrium
If the forward reaction is endothermic and the temperature is increased, what is the effect on Kc & Kp?
Increase temp increases the products
Kc and Kp increases
Opposite for exothermic
If the forward reaction is endothermic and the temperature is decreased, what is the effect on Kc & Kp?
Decreased temp decreases the products
Kc and Kp decreases
Opposite for exothermic
In the reaction N2O4 -> 2NO2 the pressure is lowered, what is observed?
(NO2 is a brown gas)
Firstly the colour lightens as the brown gas is spread over a larger volume
Then the colour darkens as the equilibrium shifts to the right producing more brown
What is the total pressure in a mixture?
Sum of all the partial pressures
What is the equation for mole fraction?
Mole fraction of a gas = number of moles of gas / total number of moles of gas in the mix
What states are included in Kp in a heterogeneous reaction?
Only the gases
What happens to the equilibrium constant if the temp changes?
If temp change means less product forms, equilibrium constant decreases
If temp change means more product forms, equilibrium constant increases
What does a catalyst do to equilibrium?
No effect on position of equilibrium
Equilibrium is reached faster
What are the equilibrium moles in this reaction?
N2 + 3H2 -> 2NH3
Initial moles: 10, 0, 10
Equilibrium:x,y,9
x = 10.5
y = 1.5
As lost on the right side, there is a gain on the left side
How can a colorimeter be used to measure equilibrium?
When a system is in equilibrium, the colour will remain constant
How do you explain a dimensionless Kc?
Equal number of moles on either side of the reaction so the units cancel
What does it show when two values are very similar in a titration related to equilibrium?
Results are within error / concordant
Shows the fact the equilibrium can be reached from either direction, rate of forward = rate of backwards reaction
