Unit 3.8 - Equilibrium constants Flashcards by Amaia Gooding

State Le Chatelier’s principle

If a system under equilibrium undergoes a change, the position of equilibrium moves to counteract the change imposed

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The enthalpy change of what reaction will always be the one quoted in the question?

The forward reaction

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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

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Dynamic equilibrium

The rate of the forwards reaction is equal to the rate of the backwards reaction

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What are the conditions that affect the position of equilibrium?

Temperature
Pressure (gases)
Concentration

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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

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What type of reactions are those affected by changes in pressure?

Gaseous

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The equilibrium constant expressed as concentration

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The equilibrium constant expressed as partial pressure

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What are the units used with Kp?

Pa or atm

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Kc equation

Kc = [Products]
—————
[Reactants]

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Kp equation

Kp = p(products)
——————
p(reactants)

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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

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Write the equation H2(g) + I2(g) <——> in terms of Kc and Kp

Kc = [HI]^2/[H2][I2]

Kp = p^2HI/pH2.PI2

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What is K known as?

The equilibrium constant

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What is the equilibrium constant k independent of?

Concentration and pressure for a given reaction and temperature

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What can be used to estimate the value of K (equilibrium constant)?

ΔG

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What information do we obtain from a reaction if we know the value of K?

Gives us information about the position of equilibrium

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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

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What can ΔG be used to estimate?

The value of K

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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

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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

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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

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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

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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?

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

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