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A level Chemistry > Unit 3.8 - Equilibrium constants > Flashcards

Unit 3.8 - Equilibrium constants Flashcards

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1
Q

State Le Chatelier’s principle

A

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

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2
Q

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

A

The forward reaction

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3
Q

Under which circumstance will changing the pressure of a reaction have no effect on the position of equilibrium?

A

When the number of molecules are the same on both sides of the equation

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4
Q

Dynamic equilibrium

A

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

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5
Q

What are the conditions that affect the position of equilibrium?

A

Temperature
Pressure (gases)
Concentration

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6
Q

Explain what catalysts do to a reaction in terms of equilibrium

A

Don’t affect the position of equilibrium, just increase the rate at which the equilibrium is attained

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7
Q

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

A

Gaseous

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8
Q

Kc

A

The equilibrium constant expressed as concentration

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9
Q

Kp

A

The equilibrium constant expressed as partial pressure

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10
Q

What are the units used with Kp?

A

Pa or atm

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11
Q

Kc equation

A

Kc = [Products]
—————
[Reactants]

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12
Q

Kp equation

A

Kp = p(products)
——————
p(reactants)

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13
Q

What do the coefficients in an equation mean to us when dealing with equilibrium equations?

A

Show us to the power to which you raise the concentrations and pressures to

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14
Q

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

A

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

Kp = p^2HI/pH2.PI2

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15
Q

What is K known as?

A

The equilibrium constant

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16
Q

What is the equilibrium constant k independent of?

A

Concentration and pressure for a given reaction and temperature

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17
Q

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

A

ΔG

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18
Q

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

A

Gives us information about the position of equilibrium

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19
Q

What can we estimate if we know about the position of equilibrium for a specific reaction?

A

Enables the yield for a specific reaction to be estimated

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20
Q

What can ΔG be used to estimate?

A

The value of K

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21
Q

If ΔG is negative, describe…
-the position of equilibrium
-the value of K
-which side of the reaction predominates

A

-to the right
-K is large
-the products predominate

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22
Q

If ΔG is positive, describe…
-the position of equilibrium
-the value of K
-which side of the reaction predominates

A

-to the left
-the value of K is small
-the reactants predominate

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23
Q

When is K large and do the products predominate?

A

When the value of ΔG is negative and the position of equilibrium is to the right

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24
Q

When is K small and do the reactants predominate?

A

When the value of ΔG is positive and the position of equilibrium is to the left

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25
Q

Position of equilibrium if ΔG is negative

A

To the right

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26
Q

Value of K if ΔG is negative

A

Large

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27
Q

What predominate when the value of K is large?

A

Products

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28
Q

Homogenous system

A

All of the substances are in the same physical state

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29
Q

What is the concentration of a solid given by?

A

Its density

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30
Q

What does the density give for a solid?

A

Its concentration

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31
Q

Why are the concentrations of solids effectively constant in heterogenous systems?

A

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

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32
Q

In what type of systems are the concentrations of the solids effectively constant?

A

Heterogenous

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33
Q

what are effectively constant in heterogenous systems?

A

The concentrations of the solids

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34
Q

Since the concentration of a solid is given by its density, what does this lead to?

A

Densities of solids are not effected by changes in temperature and pressure; the concentrations of the solids are effectively constant

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35
Q

What are densities of solids not affected by?

A

Changes in temperature and pressure

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36
Q

CaCO3 (s) <——> CaO (s) + CO2 (g)
Kc and Kp equations

A

Kc = [CO2]
Kp = pCO2

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37
Q

aq meaning

A

In water

38
Q

When is the concentration of water removed from the Kc expression?

A

In an equilibrium system which is found in aqueous solution

39
Q

What is removed from the expression in an equilibrium system which is found in aqueous solution?

A

The concentration of water

40
Q

Why is the concentration of water removed from the expression of Kc in an equilibrium system which is found in aqueous solution?

A

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

41
Q

Write the Kc expression for the following reaction
2CrO4^2- (aq) + 2H+ (aq) <——> Cr2O7^2- (aq) + H2O (l)

A

Kc = [Cr2O7^2-]
——————
[CrO4^2-]^2[H+]^2

42
Q

What is removing of water true for?

A

All equilibrium systems in aqueous solution

43
Q

What type of equilibrium systems is water always removed from the Kc expression?

A

In aqueous solution

44
Q

When do we not remove water from the Kc expression when water has formed?

A

When the reaction has not taken place in aqueous solution

45
Q

Examples of reactions that form water but we do not remove it from the Kc expression

A

Not taken place in aqueous solution i.e:
1. Certain organic reactions
2. Water as steam

46
Q

Do we remove water from the Kc expression if it is as steam?

A

No

47
Q

Which 2 things are omitted from the expression for K?

A

-concentration of pure solids
-water in aqueous solution

48
Q

Why are the concentrations of pure solids and water in aqueous solution omitted from the expression for K?

A

They’re effectively constant

49
Q

When does K have no units?

A

Same number of molecules on each side of the equation

50
Q

what do we do when K has no units?

A

Specifically write out that it has no units

51
Q

When does K have units?

A

When there are a different number of molecules on each side of the equation

52
Q

What can equilibrium constants give us a guide to?

A

The degree that an equilibrium lies towards products or starting materials

53
Q

Kc for an equilibrium that has equal amounts of starting materials and products

A

1

54
Q

What kind of equilibrium would have a Kc value of 1?

A

An equilibrium that has equal amounts of starting materials and products

55
Q

What is formed if Kc is a lot less than one?

A

Very few products

56
Q

What is most of the mixture if Kc is a lot less than 1?

A

Starting material

57
Q

What is the value of ΔG when Kc is less than 1?

A

Positive

58
Q

Why is Kc less than 1 when ΔG is positive?

A

As the reaction does not occur spontaneously

59
Q

What does it mean if the Kc of a reaction is a lot more than 1?

A

Most of the reactants have been converted into products

60
Q

When is the Kc of a reaction more than 1?

A

When ΔG is negative

61
Q

What does a negative ΔG value led to in terms of the value of Kc?

A

Kc is a lot more than 1

62
Q

Why is the Kc for a reaction more than 1 when ΔG for the reaction is negative?

A

The reaction will occur spontaneously

63
Q

Where is the position of equilibrium is K is more than 1?

A

To the right

64
Q

Where is the position of equilibrium if K is less than 1?

A

To the left

65
Q

If the forward reaction is exothermic, what will the reverse reaction be?

A

Endothermic

66
Q

What is the only thing that affects equilibrium constants?

A

Temperature

67
Q

What is the value given for when the enthalpy change is stated for a reversible reaction?

A

For the forward reaction

68
Q

How do we know if the forward reaction of a reaction is exothermic?

A

Negative enthalpy change given

69
Q

Which side of the equation is favoured by the position of equilibrium when the temperature is increased and why?

A

The position of equilibrium will move to reduce the temperature
Endothermic reactions absorb heat and so are favoured when the temperature is increased

70
Q

What side of the equation is favoured by the position of equilibrium when the temperature is decreased and why?

A

The position of equilibrium will move to increase the tempaeture
Exothermic reactions release heat and so are favoured when the temperature is decreased

71
Q

When will the value of Kc and Kp decrease?

A

If the forward reaction is exothermic

72
Q

What will happen to the value of Kc and Kp is the forward reaction is exothermic?

A

Will decrease

73
Q

What will happen to the value of Kc and Kp if the forward reaction is endothermic?

A

Increase

74
Q

When will the value of Kc and Kp increase?

A

If the forwards reaction is endothermic

75
Q

For an exothermic reaction, what does increasing the temperature do to Kc and Kp and why?

A

Decreases them
Force the position of equilibrium to the endothermic side

76
Q

For an endothermic reaction, what does increasing the temperature do to Kc and Kp?

A

Increases them

77
Q

When do we use ICE tables?

A

When we don’t have equilibrium concentrations

78
Q

What does ICE stand for?

A

Initial
Change
Equilibrium

79
Q

How do we form an ice table?

A

Write in the values given in the question in the table, with each value in the correct row for that specific element

80
Q

What is put in the “initial” row for any products in an ice table?

A

Always zero

81
Q

When are molar ratios relevant in ice tables?

A

In the “change” row

82
Q

What does the “change” row in an ice table show?

A

Whatever was added (products) or subtracted (reactants) from the initial concentration to reach the final concentration

83
Q

What do we need to do if we’re working with moles with ice tables?

A

Need to convert to concentration after figuring out E values

84
Q

What are the values we put into the Kc or Kc expression when using ICE tables?

A

The “equilibrium” values

85
Q

How do we use the Kc or Kp expression after forming an ICE table?

A

Using the “equilibrium” values, do any squaring etc like usual even though we have already consider the molar ratios

86
Q

Describe the reactants “change” column in an ice table and explain this

A

Negative (used up)

87
Q

Describe the products “change” column in an ice table and explain this

A

Positive (produced)

88
Q

When do we need an ICE table?

A

When we don’t know equilibrium concentrations

89
Q

What does each element need in an ice table?

A

Its own column

90
Q

How do we work out the Kc or Kp values using an ice table?

A

Exactly the same as any other Kp or Kc expression, using equilibrium values

91
Q

What do we do when we don’t have equilibrium concentrations for a Kp calculation?

A

Exactly the same principles as working out Kc using an ICE table

92
Q

What do we do if we’re given moles when working with an ICE table?

A

Convert to concentration after working out E to input into Kc

A level Chemistry (39 decks)

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