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Advanced Higher Chemistry > 1.6. Chemical Equilibrium > Flashcards

1.6. Chemical Equilibrium Flashcards

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1
Q

What is K?

A

The equilibrium constant.

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2
Q

What are the units of K?

A

There are none.

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3
Q

What is the general equation for K?

A

For the reaction:

aA + bB <=> cC + dD

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

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4
Q

What do the capital letters in the square brackets symbolise?

A

The concentrations of the letter inside it.

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5
Q

What is homogenous and heterogenous equilibrium?

A

Homogenous equilibrium is an equilibrium reaction in which all the reactants and products are in the same state. Heterogenous has different states.

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6
Q

What is the value of the concentration of a pure liquid or solid in the equilibrium constant equation?

A

1 as the concentration is taken as a constant.

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7
Q

What is the only thing that can affect the value of K?

A

Temperature.

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8
Q

If you increased the temperature in an ENDOthermic reaction, what would happen?

A

The products would be favoured so the P:R ratio would increase. K would also increase as a result as the equilibrium constant equation would be top heavy. The opposite would happen if you decreased the temperature.

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9
Q

If you increased the temperature in an EXOthermic reaction, what would happen?

A

The reactants would be favoured so the P:R ratio would decrease. K would also decrease as a result as the equilibrium constant equation would have a higher denominator. The opposite would happen if you decreased the temperature.

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10
Q

What is the water equilibrium equation?

A

H20 (l) + H20 (l) <=> H30+ (aq) + OH- (aq)

or

H20 (l) <=> H+ (aq) + OH- (aq)

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11
Q

What is a hydronium ion?

A

A hydrated proton.

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12
Q

What is Kw and what is its formula?

A

Kw is the ionic product of water (the equilibrium constant of water) and has a formula of Kw = [H+][OH-].

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13
Q

What is the value of Kw at room temperature?

A

1x10^-14

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14
Q

Water can act as both an acid and a base. What is this characteristic called?

A

Amphoteric.

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15
Q

How can you calculate the pH knowing just the H+ concentration?

A

Through using the formula:
pH = -log10 [H+]

This can be rearranged to find the H+ concentration:
[H+] = 10 ^ -pH

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16
Q

For every change in 1 in the pH scale, what factor does the [H+] change by?

A

A factor of 10.

pH change of 1 = [H+] change of 10
pH change of 2 = [H+] change of 100

and so on…

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17
Q

How can you calculate the pOH?

A

Using the formula:

pOH = -log10 [OH-]

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18
Q

If you knew the pH, how could you work out the pOH?

A

By subtracting the pH from 14. This goes for finding the pH from the pOH too.

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19
Q

What classifies a strong acid?

A

The fact that it can completely ionise in solution.

An example would be HCl
HCl (aq) -> H+ (aq) + Cl- (aq)

A solution of a strong acid exists only as its dissociated ions. Other molecules include nitric and sulfuric acid.

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20
Q

What classifies a weak acid?

A

The fact that it only partially dissociates in solution.

An example would be CH3COOH.
CH3COOH (aq) <=> CH3COO- (aq) + H+ (aq)

A weak acid dissociation is an equilibrium reaction.

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21
Q

What position does the equilibrium lie in weak acid dissociations?

A

It lies over to the left so there will be much more undissociated acid molecules than its ionic components.

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22
Q

What is the value of K in weak acid equilibria?

A

K < 1.

23
Q

Define a diprotic acid.

A

An acid that has two hydrogen atoms that can become hydrogen ions.

24
Q

Define a monoprotic acid.

A

An acid that has only one hydrogen atom that can ionise.

25
Q

What classifies a strong base?

A

The fact that it can fully dissociate in solution.

An example would be NaOH.

NaOH (s) -> Na+ (aq) + OH- (aq)
Oxides and hydroxides are all strong bases.

26
Q

What classifies a weak base?

A

The fact that it can only partially dissociate in solution.

An example is NH3.

NH3 (aq) + H20 (l) <=> NH4 (aq) + OH- (aq)

The equilbrium lies over to the left again meaning there are fewer products than reactants.
Amines are weak acids.

27
Q

Why do strong acids and bases make good conductors?

A

Since they fully ionise in water their ions are all dissociated and can conduct much easier. There are more H+ ions and OH- ions.

28
Q

What is the pH of a salt of a strong acid and strong base?

A

Neutral as they are both strong and both fully ionise in water meaning the equilibrium is unaffected.

29
Q

What is the pH of a salt of a strong acid and weak base and why?

A

Acidic.

Take the example of the salt of HCl and NH3. The salt of this is NH4+Cl-.
In solution the NH4+Cl- is acidic and therefore reacts with some of the OH- ions in the water equilibrium. This reaction between NH4+ and OH- form NH3 and H2O. As ammonia is a weak base there will be more reactants than products.

For any salt of a strong acid and weak base the solution is acidic due to the positive ions of the salt removing the OH- ions.

30
Q

What is the pH of a salt of a weak acid and a strong base and why?

A

Basic.

Take the example of CH3COOH and NaOH and their salt Na+CH3COO-.

The negative ions in the sodium ethanoate react with the H+ ions in the water equilibrium and form ethanoic acid molecules. This is due to ethanoic acid being a weak acid so it’s equilibrium is on the left hand side meaning there is more reactants than products.

Due to the negative ethanoate ions removing H+ ions, the solution will be alkaline.

31
Q

What is the pH of a soap?

A

Basic as soaps are made of fatty acids (weak) and strong bases. A typical salt is sodium stearate.

32
Q

What did Brønsted and Lowry define acids and bases as?

A

Acids = Proton donors.

Bases = Proton acceptors.

33
Q

What is a conjugate base and a conjugate acid?

A

A conjugate base is a base which is formed after an acid donates a proton.

A conjugate acid is an acid which is formed after a base accepts a proton.

34
Q

What is Ka?

A

The acid dissociation constant.

35
Q

How do we know the concentration of a strong acid?

A

It is equal to the concentration of H+ ions.

36
Q

What is the equilibrium equation for the dissociation of a weak acid?

A

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

HA is an acid and A- is its conjugate base.

This equation can be simplified to:

HA (aq) <=> H+ (aq) + A- (aq)

37
Q

What is the formula for finding out Ka?

A

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

38
Q

How can the Ka equation be simplified to Ka = [H+]^2 / c

A

[HA] is taken to be the same value as the unionised acid concentration given the symbol c.

When one molecule of HA ionises, one H+ ion and one A- ion are created meaning that [H+] = [A-] so we can write [H+][A-] as [H+]^2.

39
Q

What is the formula for working out the pKa of something?

A

pKa = -log10 [Ka]

40
Q

What is the equation for working out the pH of a weak acid using pKa?

A

pH = 1/2 pKa - 1/2 log10c

41
Q

What is a buffer solution?

A

A solution in which the pH remains around the same when small amounts of acid and alkali are added.

42
Q

What is an acid buffer?

A

An acid buffer is a buffer which consists of a weak acid and one of its salts.

43
Q

What happens when you add an acid to an acid buffer solution?

A

Look at the equilibrium for CH3COOH

CH3COOH (aq) <=> CH3COO- (aq) + H+ (aq)

When an acid is added to this buffer solution the newly added H+ (aq) ions react with the ethanoate ions from the salt which forms more ethanoic acid molecules to counteract the addition of the H+ ions so the pH stays the same.

44
Q

What happens when you add a base to an acid buffer solution?

A

CH3COOH (aq) <=> CH3COO- (aq) + H+ (aq)

The new OH- ions from the base react with the H+ ions from the ethanoic acid. More ethanoic acid molecules dissociate to account for these lost H+ ions so the pH stays the same.

45
Q

What happens when you add an acid to a basic buffer?

A

NH3 (aq) <=> NH4+ (aq) + OH- (aq)

When an acid is added to this buffer solution the H+ ions can react with the ammonia molecules to form more NH4+, or they can react with the OH- ions to form ammonia and water.

46
Q

What happens when you add a base to a basic buffer?

A

NH3 (aq) <=> NH4+ (aq) + OH- (aq)

When you add a base to this basic buffer solution the newly added OH- ions react with the positive ammonium ion to form ammonia and water.

47
Q

What is the formula for finding out the pH of a buffer solution?

A

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

48
Q

What are indicators?

A

Indicators are weak acids which have a different colour than their conjugate bases.

49
Q

What is the equilibrium equation for the dissociation of an indicator?

A

HIn (aq) + H20 (l) <=> H3O+ (aq) + In- (aq)

50
Q

What is KIn?

A

The equilibrium constant for an indicators dissociation equilibrium.

51
Q

What is the formula for KIn?

A

KIn = [H3O+][In-] / [HIn]

This can be rerranged to:

[In-]/[HIn] = KIn/[H3O+]

52
Q

How do we know that pKIn = pH?

A

As in the rearranged equation if [In-][HIn] = 1, then KIn/[H3O+] = 1 too. This means they must be the same value. [H3O+] is another way of saying pH. Therefore pKIn = pH.

We can deduce that a colour change occurs when pKIn = pH, or when concentrations of the weak acid and conjugate base are the same.

53
Q

The colour change is only distinguishable when [HIn] and [In-] are different by a factor of ~10. What does this mean?

A

The pH range over which the colour change of an indicator occurs is approx. pH = pKIn +/- 1.

54
Q

Where does the pH change occur most rapidly?

A

Around the end point of the reaction.

Advanced Higher Chemistry (15 decks)

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