Acids and bases Yr2 and pH Flashcards
1
Q
What are bronsted-lowry acids?
A
substance that donates a proton
2
Q
What is a bronsted-lowry base?
A
substance that can accept a proton
3
Q
What happens when you mix bronsted-lowry acids with water?
A
hydrogen ions are released
4
Q
What ions are formed when acids dissolve in water?
A
H3O+ and acid ion
5
Q
What happens when you mix a base with water?
A
react with H+ ions to form OH-
6
Q
Is H3O+ acidic or basic?
A
acidic
7
Q
What are polyprotic acids?
A
acids that can donate more than one proton
8
Q
How does a base accept a proton explain in terms of bonding?
A
a base uses a lone pair of electrons to form a dative bond to a H+
9
Q
Why is acid and metal reactions not an acid-base reaction?
A
the metal has no lone pairs and is not accepting protons
the H+ are being reduced to H2 where in A-B reactions there is no change of oxidation states
10
Q
What is an example of a monoprotic acid?
A
HNO3
1 mole of HNO3 will produce 1 mol of H+ ions
11
Q
What is an example of a diprotic/dibasic acid?
A
H2SO4
1 mol of H2SO4 = 2 mol of H+ ions
12
Q
What is an example of a triprotic/tribasic acid?
A
H3PO4
1 mol H3PO4 : 3 mol H+
13
Q
What is a conjugate acid?
A
any species that has gained a proton
14
Q
What is a conjugate base?
A
any species that has lost a proton
15
Q
What does the Bronsted-Lowry theory state happens during all acid-base reactions?
A
the transfer of a proton from an acid to a base
16
Q
What is the conjugate acid when water reacts with acid?
A
H3O+ - has gained a proton from acid
17
Q
What is the conjugate base when water reacts with base?
A
OH- = water has lost a proton
18
Q
What is a strong acid?
A
an acid that is totally ionised in aq solution
19
Q
What is an example of a strong acid?
A
HCl
H2SO4
HNO3
20
Q
What is a strong base?
A
a base that is totally ionised in aq solution
21
Q
What is an example of a strong base?
A
NaOH and KOH
22
Q
What is a weak acid?
A
an acid that only partially ionises in aq solution
23
Q
What is an example of a weak acid?
A
carboxylic acids
24
Q
What is a weak base?
A
a base that only partialy ionises in aq solutions
25
What is an example of a weak base?
aq ammonia and amines
26
What dissociation reaction direction is favoured for strong acids/ bases?
forwards = lots of H+/ OH- produced
27
What dissociation reaction direction is favoured for weak acids/ bases?
backwards = less H+/ OH- produced, less dissociates
28
What is the equilibrium equation for the dissociation of water?
2H2O = H3O+ + OH-
29
Why is the concentration of H2O ignored in Kw equation?
assume it has a constant value = water dissociates into its ions very weakly, so there is a lot more H2O than its ions
30
What is the formula equation for Kw and the units for Kw?
Kw = [H+] [OH-]
units = mol2 dm-6
31
What is the standard value of Kw at 25 degrees?
1.00 x 10-14 mol2 dm-6
32
What is the expression for Kw for pure water and explain why it is this?
Kw = [H+]^2
pure water has an equal conc of H+ and OH-
33
What happens when you react H2SO4 and HNO3 together?
H2SO4 is a stronger acid so it will donate a proton to HNO3
forms HSO4- and H2NO3+
34
What is the basicity of an acid?
the number of H+ ions that can be released from 1 molecule of the acid
35
What is pH?
a logarithmic scale that measures the concentration of H+ ions in solution
36
What is assumed when calculating pH of strong acids?
that they dissociate fully
37
What is assumed when you calculate pH for a strong base?
it dissociates fully
38
What is Ka?
the acid dissociation constant
39
When would you use Ka equations?
for working out pH for weak acids
40
What is the units for Ka?
moldm-3
41
What is Kw?
the ionic product of water
42
What are the 2 assumptions of the expression of Ka so it can be simplified?
1. only a small amount of weak acid dissociates so we can assume that the conc of acid at the start of reaction = conc of acid at equilibrium
2. the dissociation of acid leads tp equal numbers of H+ and salt- ions so [H+] = [salt]
43
What is the root of Ka x [acid] equivalent to?
[H+]
44
What is the limitation of assuming that [H+] = [salt] ?
it ignored H+ formed by the ionisation of H2O
it is a reasonable assumption because the no of H+ from ionisation of acid is greater than the number of H+ from H2O
45
What is the limitation of assuming that the conc of undissociated acid is equal to the tot conc of acid?
it ignores that some of the acid is broken up into H+ and salt-, so the actual conc of undisociated [acid] is less than the overall acid conc
46
What is pKa?
a way of measuring the strength of an acid
47
What value of pKa suggests a stronger acid?
a lower value
48
What is a buffer?
a chemical that resists the change in pH when small amounts of acid or base are added
49
What are the 2 types of buffers?
acidic
basic
50
What are acidic buffers?
resist the change in pH in order to keep the solution below pH 7
51
What are acidic buffers made of?
a weak acid and its salt (the conjugate base)
52
What 2 equilibrium reactions are taking place within an acidic buffer solution? and where do their equilibriums lie?
the dissociation of the acid - equilib lies far to the left
the dissociation of the salt - equilib lies far to the right
53
What happens when you add H+ (acid) to an acidic buffer solution?
the H+ ions will react with the negative salt ions
there is a high conc of negative salt ions from the dissociation of the salt
more acid from buffer solution is produced and equilib shifts to the left
54
What happens when you add OH- (base) to an acidic buffer solution?
the OH- react with the H+ ions from the dissociation of the acid in solution
there is a low conc of H+ but they can be reproduced from a high conc of acid to counter the change
equilibrium shifts to the right to replace the reacted H+ ions
55
What happens to the moles of buffer acid when you add a strong acid to solution?
the moles of buffer acid will increase by the number of moles of strong acid added
56
What happens to the moles of buffer acid SALT when you add a strong acid to solution?
the moles of buffer salt will decrease by the number of moles of strong acid added
57
What is an example of an acidic buffer solution?
ethanoic acid and sodium ethanoate
58
Under what condition is a buffer solution most effective?
when pH = pKa of the acid
the [acid] = [salt]
59
What happens to the value of Kw when you increase temperature?
value of Kw increases = equilib shifts to the right in endo direction
H2O = H+ + OH-
60
What is the relationship of the pH of a buffer and the pKa of the acid?
pH is within +/- 1 of pKa
when a buffer is most effective - equal amounts of acid and salt
61
What makes up a basic buffer?
a weak base and its salt (conjugate acid)
62
What is an example of a basic buffer solution?
NH3 and NH4+
63
Why is buffering important in biological systems?
enzymes are highly sensitive to pH
64
What is an example of a buffered biological system?
blood
65
What is the overall equation for blood buffering?
H2CO3 + H2O --> HCO3- + H3O+
66
What is H2CO3?
carbonic acid
67
How is carbonic acid formed?
CO2 dissolved in water or in the blood
68
How can you continuously measure the pH?
using a pH meter
69
How does the pH meter measure pH?
measures voltage and converts it into a pH reading
pH meter is calibrate using solutions of accurately known pHs
70
What is the equivalence point of a titration curve?
the end point of a titration where the exact volume of base has been added to neutralise the acid or vice-versa - there is equal moles of H+ and OH-
71
What are indicators?
weak acids which have different colours in dissociated and undissociated form
72
What happens when you add more acid to an indicator in terms of colour and equilibria?
HA == H+ + A-
HA = red
A- = yellow
(methyl orange)
the equilibrium shifts to the left to oppose the increasing H+ so there will be more HA than A- and the colour turns red
73
What happens when you add more alkali to an indicator in terms of colour and equilibria?
HA == H+ + A-
HA = red
A- = yellow
(methyl orange)
H+ decreases so equilibrium shifts to the right to oppose this
more A- is produced so solution turns yellow
74
In terms of indicators explain what pH = pKa means?
the concentrations of the ionised and unionised forms are equal
75
What is the usual range that an indicator will work?
1 pH unit either side of the pKa value
76
How can you determine which indicator is most appropriate from a titration curve?
the indicator pH range should lie within the vertical part of the titration curve - at the equivalence point
77
What is the end point in a titration?
the point in a titration at which pH changes rapidly and where you observe the indicator change colour
78
What is the pH of the equivalence point of a strong acid and strong base titration?
pH 7
79
Why is the equivalence point of titrating a weak base --> a strong acid less than pH 7?
the salt of the weak base is a proton donor - conjugate acid
80
Why is the equivalence point of titrating a strong base --> a weak acid greater than pH 7?
the salt produced from the weak acid is a proton acceptor = conjugate base
81
What are 3 properties of a good indicator for a titration?
1. sharp colour change
2. end point must be the same as the equivalence point
3. distinct colour change so that the end point is obvious
82
Which indicator would you use for a strong acid-strong base titration?
phenolphthalein
83
Which indicator would you use for a strong acid-weak base titration?
methyl orange
84
Which indicator would you use for a strong base-weak acid titration?
phenolphthalein
85
Why is there not many indicators suitable for a weak acid-weak base titration?
no indicators can give a sharp change at the end point as there is no vertical region in this titration
86
What colour is phenolphthalein in acid and alkali?
acid = colourless
alkali = dark pink
87
Is a neutralisation reaction exothermic or endothermic?
exothermic
88
What are the 3 things you need to calculate to predict a titration curve?
1. starting pH
2. the equivalence point
3. end pH
89
How do you calculate the equivalence point?
1. find moles of acid/base in beaker
2. find the volume of solution in the buretted needed to have the equal amount of moles of the solution in the beaker
90
91
what does a larger value of Ka tell you?
the more the acid dissociates
92
Is HCN a strong or weak acid?
weak
