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chemistry sem 2 > Acids, Bases and Buffers > Flashcards

Acids, Bases and Buffers Flashcards

1
Q

what is a bronzed-lowry acid?

A

a proton (H+) donor

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2
Q

What is the definition of a strong acid?

A

A proton donor that fully dissociates into it’s ions in a solution

e.g. HA –> H+ + A-

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3
Q

What are examples of common strong acids?

A

Hydrochloric acid (HCl)
Sulphuric acid (H2SO4)
Nitric acid (HNO3)
Phosphoric acid (H3PO4)

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4
Q

What is the definition of a weak acid?

A

A proton donor that partially dissociates into its ions in a solution

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5
Q

What are examples of common weak acids?

A

methanol acid (HCOOH)
Ethanoic acid (CH3COOH)

E.G. HA ⇌ H+ + A-

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6
Q

What is concentration?

A

the measure of how much substance is present in a particular volume of solution, in moldm-³

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7
Q

What is the strength of acid?

A

a measure of the degree of dissociation of the acid into its ions in solution. We can have a dilute solution of a strong acid or a concentrated solution of a weak acid

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8
Q

What is a bronty-lowry base?

A

is a proton (H+) acceptor

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9
Q

what are examples of common bases?

A

any metal oxide (MgO), metal hydroxide, ammonia solution

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10
Q

what is the definition of an alkali?

A

an alkali is a base that dissolves in water forming OH- ions (not all alkali bases are soluble)

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11
Q

what are examples of common alkalis?

A

NaOH, KOH, NH3, Ca(OH)2

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12
Q

what is the definition of a salt?

A

a salt is an acid that has had its H+ ion replaced by a metal ion or another positive ion.

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13
Q

what are examples of salts?

A

KCl, NaCl

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14
Q

what do the terms mono, di and tribasic acids mean?

A

the total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction

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15
Q

what are conjugate acid-base pairs?

A

a set of two species that transform into each other by loss or gain of a proton.

i.e. they consist of the same species apart from a H+

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16
Q

how do you identify conjugate acid-base pairs?

A
  • identify acid on left, and conjugate base on right (by seeing which loses a H+)
  • identify base on the left and conjugate acid on right (by seeing which gains a H+)
  • assign a number: acid 1/2 base 1/2
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17
Q

how do you find out the pH?

A

pH = -log10 [H+]

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18
Q

how do you calculate the H+ concentration?

A

[H+] = 10 -^pH

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19
Q

what is the background behind the pH scale

A

the more manageable pH scale of approx 1-14 was introduced by Soren sorenson to replace the use of hydrogen ion concentration ranging form 10-¹ to 10-¹⁴

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20
Q

how do you calculate the pH of strong acids?

A

[acid] = [H+] due to the acid fully dissociating

therefore we can work out [H+] using the normal pH equation (pH = -log10 [H+])

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21
Q

what does amphoteric mean, and what molecule is this?

A

it can act as an acid and a base, pure water is amphoteric

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22
Q

what is the ionic product of water?

A

Kw

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23
Q

what is the Kw of pure water?

A

1 x 10-¹⁴ mol²dm-⁶

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24
Q

what is the only change that affects Kw?

A

temperature

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25
Q

how do you calculate the pH of water?

A

[H+] =√kw
pH = -log10 [H+]

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26
Q

what does the Kw tell you about the enthalpy change of the dissociation of water?

A
  • as the temp increases, the pH decreases
  • this means that the forward reaction is favoured so equilibrium shifts right
  • increasing temp always favours the endothermic reaction which means the endothermic reaction is forward
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27
Q

How to we calculate the pH of a strong base?

A

strong bases fully dissociate in solution,

if we know the conc. of a strong base we know the conc of the OH- ions

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28
Q

what is the expression to work out the pH from a strong base?

A

Kw = [H+] [OH-]

Kw / [OH-] = [H+]

pH = -log10 [H+]

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29
Q

What is the Ka?

A

Ka is the acid dissociation constant for a weak acid

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30
Q

What is the expression for Ka?

A

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

31
Q

what is the only change that affects Ka?

A

temperature

32
Q

the larger the numerical value of ka…

A

the more the equilibrium lies to the right, the greater the conc. of the dissociated ions H+ & A- (numerator) and the stronger the acid

33
Q

the smaller the numerical value of ka…

A

the more the equilibrium lies to the left, the smaller the conc. of the dissociated ions H+ & A- (numerator) and the weaker the acid

34
Q

how do you compare numbers with negative indices in Ka?

A

by converting the Ka into a negative log called Pka

35
Q

How do you work out pKa?

A

pKa = - log10 ka

36
Q

How do you work out ka from pKa?

A

Ka = 10 -pKa

37
Q

what is the strong acid pKa / Ka ?

A

large Ka / small pKa

38
Q

What is the weak acids pKa / Ka ?

A

small Ka / large pKa

39
Q

how can the change in pH be measured in a titration?

A

by using both indicators and pH meters to monitor the change

40
Q

what is a pH meter?

A

a pH meter consists of an electrode that is dipped into a solution, and connected to a meter that displays a pH reading

41
Q

What is pH paper?

A

pH paper is dipped into a solution and the colour compared to a colour chart

42
Q

what is the equivalence point on a pH titration curve?

A

is the volume of one solution that exactly reacts with the volume of the other solution, neutralisation point

43
Q

what does the centre of the pH titration curve mean?

A

an extreme increase in pH rapidly when addition of a small volume of base

44
Q

what does the vertical line on the titration curve determine?

A

the indicator that must be chosen to change colour within this vertical lie region, or we will not accurately and precisely measure the equivalence point in the reaction

45
Q

what are the suitable indicators for a strong - strong titration?

A
  • methyl orange
  • bromophenol blue
  • methyl red
  • bromothymol blue
  • phenolphthalein
46
Q

methyl orange
- the colours at low/high pH
- pH range
- end point

A
  • red (low), yellow (high)
  • 3.2-4.4
  • 3.7
47
Q

bromophenol blue
- the colours at low/high pH
- pH range
- end point

A
  • yellow (low), blue (high)
  • 2.8 - 4.6
  • 4.0
48
Q

methyl red
- the colours at low/high pH
- pH range
- end point

A
  • red (low), yellow (High)
  • 4.2 - 6.3
  • 5.1
49
Q

bromothymol blue
- the colours at low/high pH
- pH range
- end point

A
  • yellow (low), blue (high)
  • 6.0 -7.6
  • 7.0
50
Q

phenolphthalein
- the colours at low/high pH
- pH range
- end point

A
  • colourless (low), pink (high)
  • 8.2 - 10.0
  • 9.3
51
Q

what is the suitable indicator of a weak base titration?

A

methyl orange

52
Q

weak base titration findings?

A

change in pH is more gradual initially, rapid change starting between pH 3-7

53
Q

suitable indictor for strong base titration?

A

phenolphthalein

54
Q

strong base titration findings?

A

start pH of approx 3-4 and the pH rises slowly on addition of the sodium hydroxide, until we reach the equivalence point where there is a rapid rise from 7-11.

55
Q

what is the suitable indicator for a weak acid - weak base titration?

A

none, no vertical section to the pH curve. Use pH meter

56
Q

what is the difference between the weak - weak titration?

A

titration is rarely carried out, as the equivalence point is difficult to spot due to a gradual pH change throughout

57
Q

what are the similarities and differences between the 4 graphs/curves (where they start and finish pH?

A

If you are asked for a sketch – weak acids start at a higher pH than strong acids and
Strong bases have a higher pH than weak bases for the same concentration

58
Q

what are the similarities and differences between the 4 graphs/curves (equivalence point / straight line)?

A

if both acid and alkali are the same concentration AND they are in a 1:1 mole ratio, it will be when they have the same volume .

59
Q

what Is the pH range of the vertical line of:

  • strong acid - strong base
  • strong acid - weak base
  • weak acid - strong base
  • weak acid - weak base
A
  • 4 to 10
  • 4 to 8
  • 6 to 10
  • no line
60
Q

choice of indicators?

A

The working range/ pH range of the indicator must coincide with the vertical region of the curve/graph.’

61
Q

what is a buffer?

A

a system that resists changes in pH where moderate amounts of acid or base are added

62
Q

what does a weak acid buffer contain?

A

a weak acid & the salt of a weak base

63
Q

how of you make a buffer solution: method 1?

A
  • Simply mixing together
  • the weak acid e.g. ethanoic acid
    and one of its salts e.g. sodium ethanoate
64
Q

how of you make a buffer solution: method 2?

A
  • Adding an alkali, e.g. NaOH
  • to an excess of the weak acid e.g. ethanoic acid.
65
Q

how do buffers work?

A

For a buffer to be effective, there needs to be a large excess of the dissociated weak acid HA AND a large excess of its conjugate base, A-.

66
Q

on addition of the acid, the conjugate base removes the acid?

A

H+ from the acid reacts with the conjugate base
The Equilibrium shifts to the left
H+ ions being removed

67
Q

Addition of alkali (OH- ions)
The weak acid removes the added alkali

A

Added OH- reacts with H+
Equilibrium shifts to the right
H+ ions being replaced

68
Q

what is the role of the weak acid in a buffer?

A

The role of the weak acid HA, and its conjugate base, A-, can be summarised by

69
Q

how do you calculate the pH of a buffer?

A

Henderson-Hasselbalch equation derived from rearranged Ka expression

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

Using –log10 ..

pH = pka + log 10 [SALT]/[ACID]

70
Q

What is the equation for making buffer with a different pH?

A

Ka = [H+] [CH3COO-] / [CH3COOH]
[H+] ≠[CH3COO-]

71
Q

what is the rearranged [H+] to make a buffer with a different pH?

A

Ka x ([CH3COOH]/[CH3COO-])
if [HA] = [A-] Then [CH3COOH] = [CH3COO-]

Then [H+] = Ka and using the –log10

Then pH = pKa

72
Q

the pH of the buffer solution is…

A

the same as the pKa value of the weak acid

73
Q

what happens by adjusting the conc of the weak acid and salt?

A

the pH of the buffer can be adjusted by 1 pH value higher or lower.

chemistry sem 2 (10 decks)

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