Chapter 19 - Equilibrium Flashcards
How do you work out the units for Kc?
Substitute units into the expression for Kc
Cancel common units and show the final units on a single line
What is homogeneous equilibria?
Where equilibrium species all have the same state or phase
What is heterogeneous equilibria?
When equilibrium species have different states or phases
What changes must you make to the Kc expression when it is a heterogeneous equilibrium?
Any species that are solids and liquids and omitted from the Kc expression (as their concentration is essentially constant)
Kc only includes species that are (g) or (aq)
What does ICE stand for in an ICE table for experimental results?
Initial moles
Change moles
Equilibrium moles
What is the mole fraction of a gas?
The proportion of a gas to the total volume of gases in a gas mixture
How do you calculate mole fraction?
For Gas A:
mole fraction x(A) = number of moles of A/total number of moles in gas mixture
What is partial pressure?
The contribution that a gas makes in a gas mixture towards the total pressure
How do you calculate partial pressure?
For Gas A:
partial pressure p(A) = mole fraction of A x total pressure
What is the expression for the equilibrium constant Kp?
Kp = p(C)^c p(D)^d/p(A)^a p(B)^b
Where p = equilibrium partial pressure
small c/d/a/b = balancing number in chemical equation
What does the value of K give?
The exact position of the equilibrium
What happens to K at a set temperature when concentration or pressure changes, or a catalyst is present?
K value remains constant
How does K change with increasing temperature if the forward reaction is exothermic? (and why)
The equilibrium constant decreases with increasing temperature
BECAUSE:
Raising the temp decreases the equilibrium yield of product as the equilibrium position shifts to the left
Therefore the denominator (reactants) becomes larger and the K value will decrease
How does K change with increasing temperature if the forward reaction is endothermic?
The equilibrium constant increases with increasing temperature
BECAUSE:
Raising the temp decreases the equilibrium yield of reactant as the equilibrium position shifts to the right
Therefore the numerator (products) becomes larger and the K value will increase
How is the equilibrium constant affected by changes in concentration/pressure?
K remains constant
What is still affected by changing concentrations and pressure?
Equilibrium position (moves to ensure K stays constant)
How does Kp change when pressure increases (and there are less gaseous mol on the product side)?
Equilibrium shifts to the right as there are less gaseous mol on products side
Increased pressure increases the denominator of Kp (reactants)
So the equilibrium shifts to restore the Kp ratio by increasing the numerator
How does a catalyst affect equilibrium constants?
They are unaffected
Why do catalysts not affect the equilibrium constant?
Catalysts affect the rate of a chemical reaction but not the position of equilibrium
They speed up both the reverse and forward reactions by the same factor.
Equilibrium is therefore reached much quicker, but the constant is not changed
What are 2 features of a system that is in dynamic equilibrium?
rate of forward reaction = rate of reverse reaction
concentrations of reactants and products are constant but they are constantly interchanging
In the Haber process, why is it easier to increase N2 than H2?
N2 is obtained from the air whereas H2 must be manufactured
