Chapter 19 Equilibrium Flashcards
Write the expression for Kc?
[C]^c [D]^d
Kc —————-
[A]^a [B]^b
What is dynamic equilibrium?
Equilibrium where the rate of the forward reaction is equal to the rate of the reverse reaction so there is no overall change in the concentrations of the reactants or products
Occurs in a closed system
What is homogeneous equilibrium?
When all the reactants and products in the equilibrium system are in the same physical state
What is heterogeneous equilibrium?
When the reactants and products in an equilibrium system are in different physical states
Outlines the rules of using heterogeneous equilibrium states in Kc?
If the system is heterogeneous, only gases and aqueous substances are substituted into the Kc expression (solids and liquids are omitted)
State the Kc expression for
C (s) + H2O (g) <-> CO (g) + H2 (g)
[CO (g) ] [H2 (g) ]
Kc = —————————
[H2O (g) ]
What kind of table is used to calculate Kc from equilibrium amounts?
ICE table:
Initial
Change
Equilibrium
What is Kp?
The equilibrium constant in terms of partial pressures
How is the total pressure of a gas mixture calculated?
Sum of all partial pressures of individual gases
What is a mole fraction of a gas?
The proportion of a gas mixture that is a particular gas
How can the mole fraction of a gas be calculated?
Mole fraction of gas in mixture=
Number of moles of the gas
———————————————
Total number of moles in gas mixture
What is partial pressure?
The contribution that the gas makes towards the total pressure
So the sum of the partial pressures of each gas = total pressure
How is the partial pressure of a gas calculated?
Partial pressure of gas p(A)=
Mole fraction of gas (A) x total pressure of mixture
Write the expression for Kp?
p(C)^c x p(D)^d
Kp = ————————-
p(A)^a x p(B)^b
State the only condition when the value of Kp can change?
Temperature
How does K change when the temperature is increased?
N2 + 3H2 ⇌ 2NH3
Δ H= -92 KJmol-1
The forward reaction is exothermic so increasing the temperature will shift equilibrium in the endothermic reverse reaction.
Therefore, less product will from, partial pressure of the products will decrease.
This causes the equilibrium constant to decrease.
How does K change when temperature is decreased?
N2 + 3H2 ⇌ 2NH3
Δ H= -92 KJmol-1
The forward reaction is exothermic so decreasing the temperature will favour the forward reaction.
Therefore, more products will form so the partial pressure of the products will increase.
This causes the equilibrium constant to increase.
How does the equilibrium position change when
Kp = 1?
Equilibrium position is half way between reactants and products
How does the equilibrium position change when
Kp > 1?
Equilibrium position will shift in forward direction/ with products
How does the equilibrium position change when
Kp < 1?
The equilibrium position will shift in the reverse reaction/ with reactants
How does the position of equilibrium change when the concentration of the reactants is increased?
Equilibrium will shift in the forwards direction to form more products and reduce the concentration of reactants.
This keeps equilibrium constant the same.
How does equilibrium change when pressure is increased?
Equilibrium will shift to the side with the fewest gaseous moles to reduce pressure and keep the equilibrium constant the same.
How does equilibrium change when pressure is decreased?
Equilibrium will shift to the side with the most gaseous moles to raise the pressure again.
This keeps equilibrium constant the same.
How does the presence of a catalyst affect the position of equilibrium?
Catalysts have no effect on the position of equilibrium or the value of Kc or Kp.
They increase the rate that equilibrium is reached.
What is the effect of concentration and pressure on equilibrium and equilibrium constant?
Concentration and pressure do not affect the values of Kc or Kp but can change the concentrations of reactants and products present at equilibrium.
What does Kc indicate?
The position of equilibrium
What does a larger Kc value show?
The larger the Kc value the further the equilibrium lies towards the products side (right)
State the only condition which allows the value of Kc to change?
Temperature
How does the value of Kc change when temperature is increased in an exothermic forward reaction?
Increasing temperature, decreases the value of Kc for exothermic reactions
Therefore the equilibrium concentrations need to change to make Kc lower
This means the [Products] needs to decrease and the [Reactants] needs to increase
So equilibrium shifts to the left
How does the value of Kc change when temperature is decreased in an endothermic forward reaction?
Increasing temperature, increases the value of Kc for an endothermic forward reaction
Therefore, the equilibrium concentrations need to change in order to make Kc higher
This means the [Products] needs to increase and the [Reactants] needs to decrease
Equilibrium can shift to the right
How does the position of equilibrium change when the concentration of a reactant is increased?
The value of Kc remains unchanged as long as temperature remains constant, therefore in order to maintain the value of Kc the [Reactants] needs to decrease and the [Products] needs to increase so equilibrium shifts to the right
How does the position of equilibrium change when the concentration of a product is increased?
The value of Kc remains unchanged as long as temperature remains constant, therefore in order to maintain the value of Kc the [Reactants] needs to increase and the [Products] needs to decrease so equilibrium shifts to the left
How does the value of Kp change when temperature is increased in an exothermic forward reaction?
Increasing temperature decreases the value of Kp for an exothermic forward reaction
Therefore, the ratio of partial pressures need to change in order to make Kp lower
So the pp(reactants) needs to increase and pp(products) needs to decrease
Equilibrium shifts to the left
How does the value of Kp change when temperature is increased in an endothermic forward reaction?
Increasing temperature, increases the value of Kp for an endothermic forward reaction
Therefore, the ratio of partial pressures need to change in order to make Kp higher
So the pp(products) needs to increase and the pp(reactants) needs to decrease
Equilibrium shifts to the right
How does the position of equilibrium change when the pressure of a system is increased?
The value of Kp remains the same when pressure is changed so if the pressure of the system is increased then the partial pressures will also increase.
This means equilibrium needs to shift to bring Kp back to its original value
Equilibrium will need to shift to side with fewest number of gaseous moles
How does the position of equilibrium change when the pressure of a system is decreased?
The value of Kp remains the same when pressure is changed so if the pressure of the system is decreased then the partial pressures will also decrease.
This means equilibrium needs to shift to bring Kp back to its original value
Equilibrium will need to shift to side with most number of gaseous moles
