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A - Chemistry > Unit 1 - Section 9: Acids, bases and pH CDS * > Flashcards

Unit 1 - Section 9: Acids, bases and pH CDS * Flashcards

Acids, bases and Kw pH calculations more pH calculations pH curves and indicators titration calculations buffer action

1
Q

what are Bronsted-Lowry acids?

A

proton donors - they release hydrogen ions (H+) when they’re mixed with water.
HA –> H+ + A-

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2
Q

How are H+ ions present in water?

A

they’re always combined with H2O to form hydroxonium ions H3O+
HA +H2O –> H3O+ + A-

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3
Q

what are Bronsted-Lowry bases?

A

proton acceptors - when in solution, they take hydrogen ions from water molecules
B +H2O –> BH+ +OH-

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4
Q

what are strong acids?

A

acids that fully dissociate in water - all the H+ ions are released.
You use a 1 way arrow in a reaction equation

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5
Q

what are examples of strong acids?

A

HCl
H2SO4
HNO3

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6
Q

what are strong bases?

A

bases that ionise completely in water

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7
Q

what are examples of strong bases?

A

NaOH

KOH

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8
Q

what are weak acids?

A

acids that dissociate slightly in water - only small numbers of H+ ions are formed.
the reaction is in equilibrium
2 way arrows

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9
Q

what are examples of weak acids?

A 
ethanoic acid (CH3COOH)
citric acid
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10
Q

what are weak bases?

A

bases that only slightly ionise in water.

2 way arrows in an equation

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11
Q

what are examples of weak bases?

A

ammonia (NH3)

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12
Q

when can acids give up their protons?

A

only when a base is present to accept them, this can be water

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13
Q

what equilibrium exists in water?

A

H2O H+ + OH-

water dissociates very slightly

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14
Q

what is Kw?

A

Kc for water dissociating is [H+][OH-]/[H2O]
concentration of water is considered to have a constant value
you can multiply the 2 constants of Kc and [H2O] together together to get the ionic product of water constant Kw

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15
Q

what is the equation for Kw?

A

Kw=[H+][OH-]

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16
Q

how does Kw change?

A

Kw is the same for an aqueous solution at a given temperature. at 298K (25*C) Kw = 10^-14 mol^2dm^-6

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17
Q

what is Kw in pure water?

A

in pure water the concentrations of [H+] and [OH-] are equal, so the water is neutral
Kw =[H+]^2

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18
Q

what is the equation for pH?

A

pH= -log [H+]

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19
Q

what is the equation to find [H+] from pH?

A

[H+] = 10^-pH

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20
Q

what are monoprotic acids?

A

each molecule of strong acid will release 1 proton when it dissociates.
HA —> H+ + A-
it is a 1 to 1 ratio
[HA] = [H+]

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21
Q

what are diprotic acids?

A

each molecule of strong acid releases 2 protons when it dissociates.
H2A —> 2H+ + A-
it is a 1 to 2 ratio
2[HA] = [H+]

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22
Q

How to find the pH of a strong base?

A 
fully ionises
AOH --> A+ + OH-
[AOH] = [OH-]
use equation for Kw
[H+] = Kw/[OH-]
use equation for pH
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23
Q

why is it difficult to find the pH of weak acids and bases?

A

they only slightly dissociate so [HA] is not equal to [H+]

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24
Q

what is Ka?

A

for weak acids only
HA H+ + A-
a small amount of HA dissociates so the [HA] at the beginning = [HA] at equilibrium
Ka = [H+][A-]/[HA]

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25
Q

how can Ka be written if H+ and OH- are the only products?

A

HA H+ + A-
it is a 1 to 1 ratio
[H+] = [A-]
Ka = [H+]^2/[HA]

26
Q

How can you find the pH of a weak acid?

A 
[H+] = √Ka[HA]
pH = -log [H+]
27
Q

what are the equations of pKa and Ka?

A 
pKa = -log Ka
Ka = 10^-pKa
28
Q

what happens in an acid-base titration?

A

to find conc. of base
a standard solution of acid is added to a measured quantity of base
indicator is added to the base so you know when it is neutralised by the acid
you find amount of acid to neutralise base and then the concentration of the base.

29
Q

what are pH curves?

A

they plot pH against volume of acid or base added
if it starts with a strong acid, the pH will start low
if it starts at a weak acid, the pH will start higher
if you start with a base, pH will start high
ends high if neutralised by strong base and lower for weak base

30
Q

What can pH curves be used to show?

A

there is a vertical part of the curve except for weak-weak
this is the equivalence point or end point, a small increase in base can cause a big increase in pH.
The midpoint of this line is where the solution is neutral

31
Q

why are pH curves useful?

A

they can help you decide which indicator to use

32
Q

how can pH curves help you decide which indicator to use?

A

indicator needs to change colour at end point.

33
Q

what 2 indicators can be used for an acid base reaction?

A

methyl orange

phenolphthalein

34
Q

what is methyl orange?

A

acidic - red
alkaline - yellow
colour change at pH 3.1 - 4.4

35
Q

what is phenolphthalein?

A

acidic - colourless
alkaline - pink
colour change at pH 8.3 - 10

36
Q

when should each pH indicator be used?

A

strong/strong - either can be used as the end point occurs over a wide range of pH
strong acid/weak base -methyl orange
weak acid/strong base - phenolphthalein
weak/weak - no sharp pH change, indicator can’t be used. pH meter

37
Q

how to make results of a titration as accurate as possible?

A

measure the neutralisation volume as precisely as you can
repeat the titration at least 3 times, take a mean titre. results are reliable
don’t use anomalous results, they should be within 0.1 cm^3 of each other

38
Q

how can you use a pH meter for a titration?

A

draw a pH curve of pH by volume of acid/ base added. Then find the volume added at the point where the curve is vertical to find the volume needed to neutralise then concentration can be found

39
Q

what does the pH curve of a diprotic acid show?

A

it releases 2 protons, so there are 2 equivalence points

40
Q

what is a buffer?

A

a solution that resists changes in pH when small amounts of acid or base are added, or when it is diluted

41
Q

what are acidic buffers?

A

they have a pH of 7 or less

made by mixing a weak acid with one of its salts

42
Q

how do acidic buffers work?

A

weak acid slightly dissociates
salt fully dissociates into its ions when dissolved.
e.g CH3COOH and CH3COONa
there are lots of undissociated acid molecules and lots of CH3COO- ions from salt
when [H+] and [OH-] are altered equilibrium shifts to oppose change

43
Q

what happens when acid is added to an ethanoic acid sodium ethanoate buffer solution?

A

CH3COOH H+ + Ch3COO-
when small amount of acid is added [H+] increases.
equilibrium shifts to left, reducing [H+] so pH doesn’t change

44
Q

what happens when base is added to an ethanoic acid sodium ethanoate buffer solution?

A

CH3COOH H+ + Ch3COO-
when small amount of base is added [OH-] increases
this reacts with H+ to form water removing H+
shift to right to replenish H+
pH doesn’t change

45
Q

what are basic buffers?

A

they have a pH greater than 7

made by mixing weak base with one of its salts

46
Q

how do basic buffers work?

A

salt fully dissociates
base slightly reacts with water
e.g. ammonia and ammonium chloride
solution will contain lots of ammonium ions and ammonia molecules

47
Q

what happens when base is added to a basic buffer?

A

NH3 + H2O NH4+ + OH-
small amount of base is added
[OH-] increases these react with NH4+ to cause shift to the left removing [OH-] and pH doesn’t change

48
Q

what happens when acid is added to a basic buffer?

A

NH3 + H2O NH4+ + OH-
small amount of acid added
[H+] increases, reacts with OH- so [OH-] decreases
shift to right to replace OH-
some H+ reacts with NH3 to make NH4+ to reduce H+
pH doesn’t change

49
Q

how are pH buffers useful?

A

high pH makes human hair rougher so buffer is used

biological washing powders have buffers to keep enzymes at optimum pH

50
Q

what is the pH of water at 298K?

A 
pH = 7
[OH-] = [H+] = 10^-7
Kw = 10^-14
51
Q

how does the pH of water change with temperature?

A

H2O H+ + OH-
breaking bonds is endothermic
so if temperature increases equilibrium shifts to the right
[H+] and [OH-] increase, so pH decreases so more acidic

52
Q

How to find the concentration of something after water has been added to the solution?

A

old volume/ new volume x old concentration = new concentration
or
find moles divide by new volume

53
Q

Hw to find the pH of a solution when an acid and a base have reacted?

A

find the moles of H+
find moles of OH-
find number of moles in excess of H+ or OH-
divide by the new volume of the 2 solutions added together
divide moles by volume to find conc. of H+ or OH-
find pH

54
Q

How to calculate the neutralisation point on a pH curve?

A

if starting with an acid find the moles of acid
this means the same number of moles of alkali is needed to neutralise it
divide moles by the concentration of he alkali to find volume needed.
when the volume added (x-axis) is this value, that’s when the neutral point will be

55
Q

what are the 2 ways a buffer can be made?

A

mixing a weak acid/base and its salt
mixing excess weak acid with a strong base so a salt is made to be part of the mixture
mixing excess weak base and strong acid to form a salt

56
Q

how to find the pH of a buffer solution?

A

CH3COOH CH3COO- + H+
CH3COONa —> CH3COO- + Na+
weak acid slightly dissociates so assume start conc. is equilibrium conc.
find moles of acid by multiplying conc. and vol
salt fully dissociates so find moles at start conc. x vol then use ICE to find the moles of e.g. CH3COO- at equilibrium (same as moles of CH3COONa at start)
Ka = [H+][salt ion-]/[acid]
[H+] = kacid/salt (leave acid and salt in moles)
find [H+] and then pH

57
Q

How to find the pH of a buffer solution formed from an acid base reaction?

A

moles of base will all be turned into product so will be the same as moles of base in 1:1
there will be excess acid
excess acid x Ka / salt = [H+]

58
Q

what is a back titration?

A

where the reaction between acid and base is slow
react an excess of acid with a base
react leftover acid to see how much is left over

59
Q

What to do if a solution is made up to another one?

A

if you have excess moles and the solution is made up to 250cm^3
there is the same number of moles in the solution, but if 25cm^3 of that solution is then used then only 1/10 of the original moles will be in it

60
Q

what is a conjugate base?

A

the other product formed in the dissociation of an acid which is not a proton

A - Chemistry (26 decks)

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