Unit 3.8 - Equilibrium constants Flashcards
State Le Chatelier’s principle
If a system under equilibrium undergoes a change, the position of equilibrium moves to counteract the change imposed
The enthalpy change of what reaction will always be the one quoted in the question?
The forward reaction
Under which circumstance will changing the pressure of a reaction have no effect on the position of equilibrium?
When the number of molecules are the same on both sides of the equation
Dynamic equilibrium
The rate of the forwards reaction is equal to the rate of the backwards reaction
What are the conditions that affect the position of equilibrium?
Temperature
Pressure (gases)
Concentration
Explain what catalysts do to a reaction in terms of equilibrium
Don’t affect the position of equilibrium, just increase the rate at which the equilibrium is attained
What type of reactions are those affected by changes in pressure?
Gaseous
Kc
The equilibrium constant expressed as concentration
Kp
The equilibrium constant expressed as partial pressure
What are the units used with Kp?
Pa or atm
Kc equation
Kc = [Products]
—————
[Reactants]
Kp equation
Kp = p(products)
——————
p(reactants)
What do the coefficients in an equation mean to us when dealing with equilibrium equations?
Show us to the power to which you raise the concentrations and pressures to
Write the equation H2(g) + I2(g) <——> in terms of Kc and Kp
Kc = [HI]^2/[H2][I2]
Kp = p^2HI/pH2.PI2
What is K known as?
The equilibrium constant
What is the equilibrium constant k independent of?
Concentration and pressure for a given reaction and temperature
What can be used to estimate the value of K (equilibrium constant)?
ΔG
What information do we obtain from a reaction if we know the value of K?
Gives us information about the position of equilibrium
What can we estimate if we know about the position of equilibrium for a specific reaction?
Enables the yield for a specific reaction to be estimated
What can ΔG be used to estimate?
The value of K
If ΔG is negative, describe…
-the position of equilibrium
-the value of K
-which side of the reaction predominates
-to the right
-K is large
-the products predominate
If ΔG is positive, describe…
-the position of equilibrium
-the value of K
-which side of the reaction predominates
-to the left
-the value of K is small
-the reactants predominate
When is K large and do the products predominate?
When the value of ΔG is negative and the position of equilibrium is to the right
When is K small and do the reactants predominate?
When the value of ΔG is positive and the position of equilibrium is to the left
Position of equilibrium if ΔG is negative
To the right
Value of K if ΔG is negative
Large
What predominate when the value of K is large?
Products
Homogenous system
All of the substances are in the same physical state
What is the concentration of a solid given by?
Its density
What does the density give for a solid?
Its concentration
Why are the concentrations of solids effectively constant in heterogenous systems?
The concentration of a solid is given by its density
Densities of solids are not affected by changes in temperature and pressure; the concentrations of the solids are effectively constant
In what type of systems are the concentrations of the solids effectively constant?
Heterogenous
what are effectively constant in heterogenous systems?
The concentrations of the solids
Since the concentration of a solid is given by its density, what does this lead to?
Densities of solids are not effected by changes in temperature and pressure; the concentrations of the solids are effectively constant
What are densities of solids not affected by?
Changes in temperature and pressure
CaCO3 (s) <——> CaO (s) + CO2 (g)
Kc and Kp equations
Kc = [CO2]
Kp = pCO2
aq meaning
In water
When is the concentration of water removed from the Kc expression?
In an equilibrium system which is found in aqueous solution
What is removed from the expression in an equilibrium system which is found in aqueous solution?
The concentration of water
Why is the concentration of water removed from the expression of Kc in an equilibrium system which is found in aqueous solution?
The concentration of water is effectively constant as we already started with a significant amount of water (aq) and the amount of water present is very large compared to the other species present
Write the Kc expression for the following reaction
2CrO4^2- (aq) + 2H+ (aq) <——> Cr2O7^2- (aq) + H2O (l)
Kc = [Cr2O7^2-]
——————
[CrO4^2-]^2[H+]^2
What is removing of water true for?
All equilibrium systems in aqueous solution
What type of equilibrium systems is water always removed from the Kc expression?
In aqueous solution
When do we not remove water from the Kc expression when water has formed?
When the reaction has not taken place in aqueous solution
Examples of reactions that form water but we do not remove it from the Kc expression
Not taken place in aqueous solution i.e:
1. Certain organic reactions
2. Water as steam
Do we remove water from the Kc expression if it is as steam?
No
Which 2 things are omitted from the expression for K?
-concentration of pure solids
-water in aqueous solution
Why are the concentrations of pure solids and water in aqueous solution omitted from the expression for K?
They’re effectively constant
When does K have no units?
Same number of molecules on each side of the equation
what do we do when K has no units?
Specifically write out that it has no units
When does K have units?
When there are a different number of molecules on each side of the equation
What can equilibrium constants give us a guide to?
The degree that an equilibrium lies towards products or starting materials
Kc for an equilibrium that has equal amounts of starting materials and products
1
What kind of equilibrium would have a Kc value of 1?
An equilibrium that has equal amounts of starting materials and products
What is formed if Kc is a lot less than one?
Very few products
What is most of the mixture if Kc is a lot less than 1?
Starting material
What is the value of ΔG when Kc is less than 1?
Positive
Why is Kc less than 1 when ΔG is positive?
As the reaction does not occur spontaneously
What does it mean if the Kc of a reaction is a lot more than 1?
Most of the reactants have been converted into products
When is the Kc of a reaction more than 1?
When ΔG is negative
What does a negative ΔG value led to in terms of the value of Kc?
Kc is a lot more than 1
Why is the Kc for a reaction more than 1 when ΔG for the reaction is negative?
The reaction will occur spontaneously
Where is the position of equilibrium is K is more than 1?
To the right
Where is the position of equilibrium if K is less than 1?
To the left
If the forward reaction is exothermic, what will the reverse reaction be?
Endothermic
What is the only thing that affects equilibrium constants?
Temperature
What is the value given for when the enthalpy change is stated for a reversible reaction?
For the forward reaction
How do we know if the forward reaction of a reaction is exothermic?
Negative enthalpy change given
Which side of the equation is favoured by the position of equilibrium when the temperature is increased and why?
The position of equilibrium will move to reduce the temperature
Endothermic reactions absorb heat and so are favoured when the temperature is increased
What side of the equation is favoured by the position of equilibrium when the temperature is decreased and why?
The position of equilibrium will move to increase the tempaeture
Exothermic reactions release heat and so are favoured when the temperature is decreased
When will the value of Kc and Kp decrease?
If the forward reaction is exothermic
What will happen to the value of Kc and Kp is the forward reaction is exothermic?
Will decrease
What will happen to the value of Kc and Kp if the forward reaction is endothermic?
Increase
When will the value of Kc and Kp increase?
If the forwards reaction is endothermic
For an exothermic reaction, what does increasing the temperature do to Kc and Kp and why?
Decreases them
Force the position of equilibrium to the endothermic side
For an endothermic reaction, what does increasing the temperature do to Kc and Kp?
Increases them
When do we use ICE tables?
When we don’t have equilibrium concentrations
What does ICE stand for?
Initial
Change
Equilibrium
How do we form an ice table?
Write in the values given in the question in the table, with each value in the correct row for that specific element
What is put in the “initial” row for any products in an ice table?
Always zero
When are molar ratios relevant in ice tables?
In the “change” row
What does the “change” row in an ice table show?
Whatever was added (products) or subtracted (reactants) from the initial concentration to reach the final concentration
What do we need to do if we’re working with moles with ice tables?
Need to convert to concentration after figuring out E values
What are the values we put into the Kc or Kc expression when using ICE tables?
The “equilibrium” values
How do we use the Kc or Kp expression after forming an ICE table?
Using the “equilibrium” values, do any squaring etc like usual even though we have already consider the molar ratios
Describe the reactants “change” column in an ice table and explain this
Negative (used up)
Describe the products “change” column in an ice table and explain this
Positive (produced)
When do we need an ICE table?
When we don’t know equilibrium concentrations
What does each element need in an ice table?
Its own column
How do we work out the Kc or Kp values using an ice table?
Exactly the same as any other Kp or Kc expression, using equilibrium values
What do we do when we don’t have equilibrium concentrations for a Kp calculation?
Exactly the same principles as working out Kc using an ICE table
What do we do if we’re given moles when working with an ICE table?
Convert to concentration after working out E to input into Kc
