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Advanced Higher Chemistry: Unit 1- Inorganic And Physical Chemistry > Chemical Equilibrium > Flashcards

Chemical Equilibrium Flashcards

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1
Q

What is chemical equilibrium defined as?

A

the state where the concentration of reactants and products remain constant with time.

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2
Q

On a molecular level, describe the dynamic situation of equilibrium?

A

the reactants are being converted to products and products are being converted to reactants at the same rate.

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3
Q

When is system said to have reached equilibrium?

A

Once the concentrations of A and B reach levels where the forward rate equals the reverse rate

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4
Q

Once equilibrium is reached, how is it maintained?

A

by all relevant factors remaining the same

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5
Q

To determine the amount of each compound that will be present at equilibrium what must you know?

A

the equilibrium constant

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6
Q

What is the equilibrium constant, K given by?

A

K= [C]c [D]d / [A]a [B]b

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7
Q

What are equilibrium constants independent of?

A

the concentrations or pressures of the species involved in the equilibrium reaction

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8
Q

In terms of equilibrium concentrations and value of K, what is important to note?

A

the value of k is independent of the concentration (i.e. unaffected)

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9
Q

Describe the effect of a catalyst on the value of K.

A

A catalyst will not affect the position of equilibrium, therefore has no effect on K.

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10
Q

In terms of temperature and value of K, what is important to note?

A

K values are dependent on temperature

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11
Q

For an exothermic reaction, describe the effect of an increase in temp on the position of equilibrium and therefore the value of K.

A

equilibrium will shift to the left. The yield of product is decreased, causing a decrease in K.

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12
Q

For an endothermic reaction, describe the effect of an increase in temp on the position of equilibrium and therefore the value of K.

A

equilibrium will shift to the right. The yield of product is increased, causing an increase in K

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13
Q

What does the water molecule exist in?

A

equilibrium with its ions

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14
Q

Describe the equilibrium of water? Explain this.

A

it lies far to the left. Very few water molecules dissociate - they can effectively be treated as having a constant concentration

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15
Q

What is [H20] given the value of?

A

1

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16
Q

For any neutral solution, what value are [H2+] and [OH-] given?

A

1x10-7 mol l-1

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17
Q

What is the value of Kw?

A

1x10-14 at 24 degrees Celsius

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18
Q

What does the value of Kw vary with?

A

temperature

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19
Q

What kind of process is the formation of hydrogen ions and hydroxide ions from water?

A

endothermic (forward reaction)

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20
Q

Explain why the value of Kw varies with temperature?

A

According to Le Chatelier’s principle, if you increase the temperature of the water, the forward reaction will be favoured, and more hydrogen ions and hydroxide ions will be formed. The effect of that is to increase the value of Kw as temperature increases.

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21
Q

What does the value of K provide indication of?

A

the extent of a reaction once it has reached equilibrium i.e. whether the position of equilibrium lies to the left pr right.

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22
Q

If K is large (>1) describe the position of equilibrium.

A

[products]>[reactants] and equilibrium lies to the right

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23
Q

If K is small (<1) describe the position of equilibrium.

A

[reactants]>[products] and equilibrium lies to the left.

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24
Q

What do K values gives no indication of?

A

the rate of reaction

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25
Q

How do you know its a strong acid?

A

fully dissociated in aqueous solution

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26
Q

Give 3 examples of strong acids.

A

hydrochloric acid
sulphuric acid
nitric acid

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27
Q

How do you know its a weak acid?

A

partially dissociated in aqueous solution

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28
Q

Give an example of a weak acid.

A

ethanoic acid

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29
Q

Describe an acid as part of Bronsted Lowry Definition.

A

An acid is a proton donor i.e. will donate H+ ions to another substance.

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30
Q

Describe a base as part of Bronsted Lowry Definition.

A

A base is a proton acceptor i.e. a substance that accepts hydrogen ions.

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31
Q

What equation is used to find the pH of a strong acid?

A

-log10[H+]

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32
Q

What equation is used to calculate the hydrogen ion concentration when given the pH?

A

10*-pH

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33
Q

How do you find out the hydroxide ion concentration from the hydrogen ion concentration?

A

Kw/ [H+]

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34
Q

How do you find out the hydrogen ion concentration from the hydroxide ion concentration?

A

Kw/[OH-]

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35
Q

What is important to note about hydrogen ions in solution?

A

they are too small to exist on their own

36
Q

What are hydrogen ions always associated with? What is the formula?

A

Hydrogen ions are always associated with a water molecule and have the formula H30+

37
Q

What is the name of the H3O+ ion?

A

Hydronium ion

38
Q

What is the shape of a hydronium ion?

A

pyramidal

39
Q

What to ways can protons be represented by?

A

H+ or H3O+ in equilibrium

40
Q

Write the equation for the ionisation of water.

A

H20(l) + H2O(l) > H3O+(aq) + OH-(aq)

41
Q

What is water described to be?

A

amphoteric

42
Q

What is meant by amphoteric?

A

it can act as both an acid and a base

43
Q

What equation can be written for a weak acid in solution at equilibrium?

A

HA(aq) + H2O(l) <> H3O+(aq) + A-(aq)

44
Q

Give the expression for the acid dissociation constant?

A

Ka= [H3O+] [A-] / [HA]

45
Q

What is the equation for pKa?

A

pKa = -log10Ka

46
Q

What can both pKa and Ka be used to give an indication of?

A

the strength of an acid

47
Q

State the values of Ka and pKa when the acid is strong? Also, state the position of equilibrium.

A

Ka = large
pKa = small
Equilibrium lies to the RHS

48
Q

State the values of Ka and pKa when the acid is weak? Also, state the position of equilibrium.

A

Ka= small
pKa = large
Equilibrium lies to LHS

49
Q

What equation is used to calculate the pH of a weak acid?

A

pH= 1/2pKa - 1/2log10c

50
Q

Describe strong bases? Give example.

A

they are fully ionised in aqueous solution e.g. metal hydroxides

51
Q

Describe weak bases? Give example.

A

they are partially ionised in aqueous solution e.g. ammonia, amines

52
Q

What would the pH of a salt from a strong acid and strong alkali be?

A

7

53
Q

What would the pH of a salt from a strong acid and weak alkali be?

A

<7

54
Q

What would the pH of a salt from a weak acid and strong alkali be?

A

> 7

55
Q

Use HCl and NH3 to explain why the pH of a salt from a strong acid and weak alkali would be <7?

A

HCl + NH3 > NH4Cl pH=5
When ammonium chloride dissolves in water the following ions are formed:
NH4Cl > NH4+ + Cl-
The NH4+ ion reacts with the OH- ions from the dissociation of water.
H2O <> H+ + OH-
NH4+ + OH- <> NH3 + H20
This leaves an excess of H+ ions in solution, hence the solution is acidic

56
Q

Use CH3COOH and NaOH to explain why the pH of a salt from a weak acid and strong alkali would be >7?

A

CH3COOH + NaOH > CH3COO-Na+ + H2O
When sodium ethanoate dissolves in water the following ions are formed:
CH3COONa > CH3COO- + Na+
The CH3COO- ion reacts with the H+ ions from the dissociation of water to form ethanoic acid (weak)
H20 <> H+ + OH-
CH3COO- + H+ <> CH3COOH
This leaves an excess of OH- ions in solution, hence the solution is alkaline.

57
Q

What is a buffer solution?

A

A buffer solution is one in which the pH remains constant on addition of small amounts of acid or alkali i.e. buffer solutions are resistant to pH change

58
Q

What is an acid buffer? Give example.

A

A solution of a weak acid and one of its salts. e.g. benzoic acid and sodium benzoate

59
Q

What is a basic buffer? Give example.

A

A solution of a weak base and one of its salts. e.g. ammonia and ammonium chloride

60
Q

What do acid buffers contain?

A

large reserves of undissociated acid, HA, and the conjugate base of the acid, A-, i.e. salt

61
Q

Using ethanoic acid and sodium ethanoate as an example, describe the contents of an acid buffer.

A

The acid is weak therefore only partially dissociated (equilibrium lies to LHS).
CH3COOH <> CH3COO- + H+
There is a high concentration of CH3COOH molecules present.
The salt is fully ionised.
CH3COO-Na > CH3COO- + Na+
There is a high concentration of CH3COO-

62
Q

Using ethanoic acid and sodium ethanoate as an example, what would happen if H+ ions were added to the buffer?

A

H+ ions will react with CH3COO-
CH3COO- + H+ > CH3COOH
The CH3COO- will remove the added H+ ions so the pH will remain constant.

63
Q

Using ethanoic acid and sodium ethanoate as an example, what would happen if OH- ions were added to the buffer?

A

The added OH- ions will combine with the H+ ions from the weak acid
CH3COOH <> CH3COO- + H+
H+ + OH- > H20
More CH3COOH will dissociate to restore equilibrium; so pH will remain constant

64
Q

What do basic buffers contain?

A

large reserves of undissociated base, B, and the conjugate acid of the base, BH+ i.e. the salt

65
Q

Using ammonia and ammonium chloride as an example, what would happen if H+ ions were added to the buffer?

A

NH3 + H+ <> NH4+

Added H+ ions will react with undissociated base B, so pH will remain constant

66
Q

Using ammonia and ammonium chloride as an example, what would happen if OH- ions were added to the buffer?

A

NH4+ + OH- <> NH3 + H20

Added OH- ions will react with the conjugate acid BH+.

67
Q

What equation is used to calculate the pH of a buffer solution?

A

pH = pKa -log10[acid]/[salt]

68
Q

What are indicators?

A

weak acids which change colour according to the pH of a solution

69
Q

What are indicators used to detect?

A

the end-point in titrations

70
Q

What are indicators usually?

A

organic dyes which are also weak acids - the undissociated weak acid is a different colour from the conjugate base

71
Q

When is the colour change distinguishable?

A

when [HIn] and [In-] differ by a factor of 10

72
Q

What is the expression from equilibrium constant KIn?

A

KIn = [H+][In-]/[HIn]

73
Q

What is important to consider when choosing an indicator?

A

choose one whose pKIn (or pKa) is close to the pH at the end point i.e. when [H+] = KIn

74
Q

Why is it impossible to use an indicator for a titration with a weak acid and a weak base?

A

the end-point is not sharp

75
Q

At the end-point of a titration what must the indicator show?

A

a sharp colour change

76
Q

At the exact end-point, what will the indicator colour be?

A

midway between the acid colour of HIn and the alkaline colour of In-

77
Q

What is pKIn equal to?

A

pH

78
Q

What is the range of indicator?

A

the pH range over which it changes colour

79
Q

What is pH range of an indicator given by?

A

pH = pKIn +/- 1

80
Q

What are liquids which don’t mix known as?

A

immiscible liquids

81
Q

Depending on their density, what will immiscible liquids form?

A

two layers when added together (e.g. oil and water)

82
Q

What happens if a solute is added to the layers and the mixture is shaken?

A

the solute will dissolve and distribute itself between the two liquids in a ratio known as the partition coefficient. The solute will be more soluble in one liquid than the other.

83
Q

What is the partition coefficient expressed as?

A

the ratio of the concentrations of a solute in the organic layer and the aqueous layer.

84
Q

Give examples of factors that affect the partition coefficient value

A

the nature of the solute
the nature of the two liquids (type of solvents)
temperature

85
Q

What is solvent extraction?

A

an application of the partition of a solute between two liquids.

86
Q

What can solvent extraction be used in?

A

separation and purification of a desired product.

87
Q

How do u calculate Pc?

A

[products] / [reactants]

Advanced Higher Chemistry: Unit 1- Inorganic And Physical Chemistry (12 decks)

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