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Chemistry > A - Acids And Bases > Flashcards

A - Acids And Bases Flashcards

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1
Q

What are the Bronsted-Lowry definitions of an acid and a base?

A

An acid is a proton donor and a base is a proton acceptor.

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2
Q

Acid-base equilibrio involve the transfer of what?

A

Protons.

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3
Q

What is an alkali?

A

A water-soluble base.

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4
Q

How can acids and bases be represented in equations in the general sense?

A

Acids - HA.

Bases - B.

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5
Q

What are the general equations for acids and bases reacting with water?

A

HA(aq) + H2O(l) —> H3O+(aq) + A-(aq)

B(aq) + H2O(l) —> BH+(aq) + OH-(aq)

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6
Q

What is H3O+ called?

A

The oxonium ion.

Also called hydroxonium ion or hydronium ion.

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7
Q

How is a proton formed?

A

When a hydrogen atom loses its only electron.

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8
Q

The H+ ion has no electrons of its own. What does this mean for the proton when it comes to bonding?

A

It can only form a bond with another species that has a lone pair of electrons.

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9
Q

What’s the differences between strong acids and bases and weak acids and bases?

A

Strong acids dissociate almost completely in water, and strong bases ionise almost completely in water. An equilibrium is set up in both cases where the position of equilibrium lies extremely far to the right.

Weak acids only dissociate very slightly in water, and weak bases only slightly ionise in water. The equilibriums formed from these reactions lie very far to the left.

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10
Q

What’s the general equation for the equilibrium reaction between an acid and a base?

A

HA(aq) + B(aq) {equilibrium arrows} BH+(aq) + A-(aq)

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11
Q

What do [ ] indicate?

A

The concentration of a species in mol dm-3.

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12
Q

What is the pH scale used to measure?

A

Hydrogen ion concentration.

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13
Q

Why is a logarithmic scale used for pH?

A

Because the concentration of hydrogen ions in aqueous solution can vary greatly.

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14
Q

What is the equation to find pH?

A

pH = -log10[H+]

Where log10 is log to the base 10.

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15
Q

How does the pH scale vary?

A

The smaller the pH, the greater the concentration of H+ ions so 0 on the scale would be very acidic.

pH 7 is regarded as being neutral.

pH 14 would be a very basic solution.

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16
Q

What does a difference of one pH number represent?

A

A tenfold difference in the [H+].

For example, pH 2 has ten times the [H+] of pH 3.

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17
Q

How do you find the [H+] if you have the pH of a solution?

A

[H+] = 10^-pH

(10 to the power of -pH).

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18
Q

What does an alkaline solution have more of?

A

Has a greater [OH-].

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19
Q

When is a solution neutral?

A

When [H+] = [OH-]

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20
Q

What is a monoprotic acid?

A

An acid thag releases one proton per molecule of acid when it dissociates.

So one mole of acid produces one mole of hydrogen ions meaning that the [H+] = [acid].

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21
Q

What is a diprotic acid?

A

Each molecule of a string diprotic acid releases 2 protons when it dissociates. So diprotic acids produce 2 mol if H+ for each mole of acid.

[H+] = 2[acid].

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22
Q

Water is slightly dissociated. Give the equilibrium equation which results from this dissociation.

Give the expression for the equilibrium constant for this equation.

A

H2O {equilibrium arrows} H+ + OH-

Kc = [H+][OH-]
—————-
[H2O]

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23
Q

What is Kw and where is it derived from?

A

Kw is the ionic product of water and it is derived from the equilibrium constant for the slight dissociation of water. Because water only dissociates a tiny amount, the equilibrium lies well over to the left so the concentration of water is considered to have a constant value.

Kw = [H+][OH-]

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24
Q

What are the units for Kw?

A

Mol2 dm-6

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25
Q

What happens to Kw when the temperature changes?

A

The value of Kw varies with temperature.

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26
Q

What is Kw for pure water and why?

A

Kw = [H+]^2 for pure water.

In pure water, there is always one H+ ion for every OH- ion so [H+]=[OH-] so you can say that Kw equals the above.

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27
Q

How do you find [OH-] from pH?

A

First work out the [H+] by rearranging the pH equation.

The substitute this value into the equation:

[H+][OH-]= 1.0 x 10-14 mol2 dm-6 to find [OH-].

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28
Q

What happens to weak acids and weak bases in aqueous solution?

A

They only dissociate slightly.

29
Q

What is the dissociation constant for a weak acid?

A

Ka

30
Q

What do the words strong and weak refer to when talking about acids and bases?

A

Only to the degree of dissociation, not the concentration of the acid or base.

31
Q

What assumptions can you make for a weak aqueous acid equilibrium?

A
  1. As only a tiny amount of HA dissociates, you can assume that the concentration of HA at equilibrium is roughly equal to that of HA at the start.
  2. You can also assume that the dissociation of acid is much greater the that of water so you can assume that all the H+ ions in solution come from the acid meaning that the concentration of H+ is roughly equal to the concentration of A-
32
Q

What is the Ka expression for a weak acid? What are the units for the expression in this case?

A

Ka = [H+]^2
————-
[HA]

Mol dm-3.

33
Q

Why can you use the pH calculation straight away when you know the concentration of a strong acid?

A

Because you can assume that they are fully dissociated and that the concentration of the acid is equal to that of the H+ ions.

34
Q

How can you calculate the pH of a weak acid?

A

You can no longer assume the acid is fully dissociates so can’t used the concentration of HA as the concentration of H+.

You must use the Ka expression to calculate H+ concentration first, and then put this into the pH formula.

35
Q

What is pKa? What do values of pKa mean?

A

It’s a measure of how strong a weak acid is.

The smaller the value of pKa, the stronger the acid is.

36
Q

What is the formula for pKa? How can this be rearranged to find Ka?

A

pKa = -log10 Ka

When log 10 is log to the base 10.

Ka = 10^-pKa

Where this means 10 to the power minus pKa.

37
Q

What is a titration used to find, and briefly, how?

A

The concentration of a solution.

By gradually adding a solution to a second solution with which it reacts. On of the solutions must be of known concentration and you must know the equation for the reaction.

38
Q

What are used in titrations to ensure you know exactly how much acid and base is used?

A

Pipettes and burettes.

39
Q

What is sued to show exactly when the solution is neutralised in a titration?

A

An indicator or a pH meter.

40
Q

What is the equivalence point in a titration?

A

The point at which sufficient base has been added to just neutralise the acid, or vice-versa.

41
Q

What do the following pH curves have in common which the pH curve for the weak acid/weak base titration do not:

  1. Strong acid/strong base
  2. Strong base/weak acid
  3. Weak base/strong acid?
A

They have a sharp incline or decline on the graph (depending on which type of titration is used) where a small amount of the solution added causes a big change in pH when the end point is reached.

42
Q

Why are you better off using a pH meter for a weak base/weak acid titration?

A

The end point isn’t such a sharp change in pH for a small amount of solution added. If you used an indicator, it’s colour would change very gradually and it would be very difficult to see the exact end point.

43
Q

What is the end point for a titration?

A

The volume of alkali or acid added when the indicator just changes colour.

44
Q

What are on the axes of pH curves?

A

pH on y axis and volume of either acid/base added on the x axis.

45
Q

Describe the pH curve for the titration where a strong monoprotic base is added to a strong acid.

A

The pH starts around 1, there’s an excess of strong acid. It then shoots up vertically at the equivalence point. It finishes up around pH 13, when there’s an excess of a strong base.

46
Q

Describe the pH curve for the titration where a weak base is added to a strong monoprotic acid.

A

The pH starts at around 1 where there’s an excess of strong acid. There’s a slightly smaller vertical portion of the graph, compared to that of the strong acid/strong base graph, at the equivalence point.

The pH finishes at 9 where there’s an excess of a weak base.

47
Q

Describe the pH curve for the titration where a strong base is added to a weak monoprotic acid.

A

The pH starts at around 5, where there’s an excess of weak acid. There’s a vertical portion of the graph at the equivalence point, this vertical section is slightly smaller than that of the strong acid/strong base graph.

The pH finishes around 13, where there’s an excess if a strong base.

48
Q

Describe the pH curve for the titration where a weak base is added to a weak acid.

A

The pH starts around 5 where there’s an excess of weak acid.

There is a slight vertical rise at the equivalence.

The pH finished around 9 where’s there’s an excess of a weak base.

49
Q

Name some properties of a suitable indicator for a titration.

A
  1. The colour change must be sharp than gradual at the end point (so no more than one drop of acid/alkali is needed to give a complete colour change).
  2. There should be distinct colour change, e.g colourless to pink with phenolphthalein.
50
Q

How can you use a pH curve to choose an indicator?

A

Pick an indicator where the colour changes over a narrow pH range that lies entirely in the vertical portion of the pH curve.

51
Q

What are the colour changes for phenolphthalein and methyl orange and at approximately what pH do these occur?

A

Methyl orange:
Red (low pH) to yellow (high pH) at 3.1-4.4.

Phenolphthalein:
Colourless (low pH) to pink (high pH) at 8.3-10.0.

52
Q

What should you use out of phenolphthalein and methyl orange as indicators for the following titrations:

  1. Strong acid/strong base
  2. Strong acid/weak base
  3. Weak acid/strong base
  4. Weak acid/weak base?
A
  1. You could use either indicator
  2. Only methyl orange
  3. Only phenolphthalein
  4. Neither - you need to use a pH meter.
53
Q

What things can you do to make sure titration results are as accurate as possible?

A
  1. Measure neutralisation volume as accurately as possible (to the nearest 0.05 cm3).
  2. Repeat the titration at least three times and take a mean titre value.
  3. Don’t use anomalous results (all results should be within 0.1cm3 of each other.
54
Q

How does the pH curve with a diprotic acid differ from that of a monoprotic acid?

A

There are two equivalence points on the graph as the two protons which can be removed from the acid are done so separately.

55
Q

What is the half-neutralisation point?

A

This is the point on the pH curve half way between zero and the equivalence point (the first horizontal part that has very little change in pH for a big change in volume).

56
Q

How does the half-neutralisation point allow you to find pKa of a weak acid?

A

HA + OH- —> H2O + A-

At the half-neutralisation point, half the HA has been converted into A- and half remains so:

[HA] = [A-] which means Ka = [H+]

57
Q

What are buffers?

A

Solutions that maintain an approximately constant pH, despite dilution or the addition of small amounts of acid or base.

58
Q

Does a buffer stop pH from changing completely?

A

Not completely, but it does make changes very slight.

59
Q

Do buffers work for a small or large amount of acid or base?

A

Only for small amounts of acid or base.

60
Q

What are the two types of buffer?

A

Acidic buffers and basic buffers.

61
Q

What do acidic buffers and basic buffers contain?

A

Acidic buffer solutions contain a weak acid and a salt of that weak acid.

Basic buffer solutions contain a weak base and the salt of that weak base.

62
Q

What pH’s do acidic and basic buffers maintain?

A

Acidic buffers maintain a pH below 7 and basic buffers maintain a pH greater than 7.

63
Q

How does an acidic buffer maintain a constant pH when an alkali is added?

A

The buffer is made from a weak acid which only slightly dissociates forming the equilibrium:

HA(aq) {equilibrium arrows} H+(aq) + A-(aq)

And from a salt of the acid which fully dissociates into its ions when dissolved.

When an alkali is added, the OH- ions from the alkali react with HA to produce water molecules and A-:

HA(aq) + OH-(aq) —> H2O(aq) + A-(aq)

This removes the added OH- so pH tends to remain almost the same.

Or, the OH- ions can react with the H+ ions from the dissociation of the acid, forming water. The position of equilibrium shifts to replace the H+ ions, and the increase of OH- ions from the original addition of the alkali, is then decreased by the formation of water.

64
Q

How does an acidic buffer maintain a constant pH when a small amount of acid is added?

A

The buffer is made from a weak acid which only slightly dissociates forming the equilibrium:

HA(aq) {equilibrium arrows} H+(aq) + A-(aq)

And from a salt of the acid which fully dissociates into its ions when dissolved.

When an acid is added, the [H+] increases, the equilibrium shifts to the left where H+ ions combine with A- ions to form HA.

65
Q

How does a basic buffer maintain a constant pH when a small amount of base is added?

A

A basic buffer is made from a weak base and the salt of the base. The salt fully dissociates in solution and some of the base will react with water molecules to form B+ and OH- .

The overall equilibrium equation is:

B(aq) + H2O(l) {equilibrium arrows} NH4+(aq) + OH-(aq)

When a base is added, the [OH-] increases, most of these OH- ions react with NH4+ ions to form NH3 and H2O. The equilibrium shifts to the left, removing OH- ions from solution and resisting much change in pH.

66
Q

How does a basic buffer resist change in pH when a small amount of acid is added?

A

A basic buffer is made from a weak base and the salt of the base. The salt fully dissociates in solution and some of the base will react with water molecules to form B+ and OH- .

The overall equilibrium equation is:

B(aq) + H2O(l) {equilibrium arrows} NH4+(aq) + OH-(aq)

When an acid is added, the [H+] increases. Some of the H+ ions react with OH- ions to make H2O. When this happens, equilibrium shifts to the right to replace Oh- ions which have been used up. Some of the H+ ions will react with NH3 molecules to form NH4+ ions, again resisting change in pH.

67
Q

How can you find the pH of acidic buffer solutions, given Ka and the concentrations of the weak acid and its salt?

A

Rearrange the Ka expression to find [H+] and use the data given in the question. Then use the pH calculation now you have [H+].

68
Q

Name and explain 3 applications of buffer solutions.

A
  1. Most shampoos contain a pH 5.5 buffer. Human hair becomes tougher if exposed to alkaline conditions, buffer solutions prevent this from happening.
  2. Biological washing powders contain buffers to keep pH at the right level for enzymes to work most efficiently.
  3. Our bodies contain lots of biological buffer systems making sure our tissues are kept at the right pH. E.g - our blood need to shah ah a pH close to 7.4 so it contains a buffer solution.

Chemistry (34 decks)

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