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LC Chemistry > pH > Flashcards

pH Flashcards

1
Q

How does water ionise

A

It self ionises

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2
Q

Write the balanced equation which represents the self ionisation of water

A

H2O(equilibrium)H+ + OH-
Or
2H2O( equilibrium) H3O+ + OH-

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3
Q

What is the concentration of H+ and OH- ions in pure water

A

Equal concentration of both

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4
Q

Where does equilibrium lie on the balanced equations of the self ionisation of water

A

Left due to way more water molecules than H+

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5
Q

Write the expression for the ionic product of water (Kw)

A

Kw= [H3O+][OH-]

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6
Q

How does the temperature of water affect Kw

A

As the temperature of water increases Kw increases
A greater degree of self ionisation occurs as higher temperatures
Therefor more H+ and OH- ions are present
Therefore, self ionisation of water is endothermic

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7
Q

Is the self ionisation of water endo or exothermic

A

Endothermic

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8
Q

Define pH

A

pH = -log10[H+]

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9
Q

Limitations of the pH scale

A

Doesn’t work with highly concentrated solutions
Limited to aqueous solutions

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10
Q

How do we measure pH

A

Electronic pH meter/sensor
Indicators

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11
Q

Where is an electronic pH Meter used

A

In hospitals to measure the pH of blood and other fluids

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12
Q

Which method of measuring pH is most accurate

A

Electronic pH meter

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13
Q

Describe how to use Electronic pH meter

A

Probe is calibrated in a solution to give an accurate reading for pH

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14
Q

Name the 4 indicators

A

Litmus
Universal indicator
Phenolphthalein
Methyl orange

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15
Q

Describe the colour of litmus in an acid and base

A

Acid: red
Base: blue

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16
Q

Describe the colour of universal indicator in an acid and base

A

Match to the colour chart to get pH

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17
Q

Describe the colour of phenolphthalein in an acid and base

A

Acid: colourless
Base: pink

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18
Q

What is important to remember when writing Ka

A

Do not include water
RHS on top
LHS on bottom

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19
Q

what does Ka tell you

A

how strong an acid is, gives an indication to how dissociated an acid is

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20
Q

what does a higher value of Ka mean

A

the substances on the right side of the balanced equation are in greater amount ie equilibrium lies to the right

the greater the dissociation of acid in water, then there are more ions of the rhs of the equation ie the stronger the acid

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21
Q

what does a lower value of Ka mean

A

greater amount of substance on the LHS ie equilibrium lies to the left

thus less dissociation in water, acid is weaker

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22
Q

what must be included when writing acid base dissociation equations

A

WATER
EQUILIBRIUM SIGN

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23
Q

give examples of strong bases

A

NaOH and KOH

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24
Q

why is there no equilibrium sign in the dissociation balanced equation of a strong base

A

the reaction goes completely from left to right

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25
Q

give an example of a weak base

A

ammonia, NH3

26
Q

what is not included in a Kb expression

A

water

27
Q

what formulas can be used to find the concentration of OH- ions

A

pH + pOH=14
pOH= 14- pH
pOH= -log10[OH-]

28
Q

how do we calculate the pH of weak acids

A

[h+]=square root of Ka+ Macid

29
Q

how do we calculate the pH of weak bases

A

[OH-]=square root of Kb+ Mbase

30
Q

what does mAcid/base mean

A

concentration of base/ acid in mol/L

31
Q

are most indicators acidic or basic

A

weak bases

32
Q

explain how indicators work

A

litmus:
H+ ions are added to blue litmus, equilibrium shifts to the left to remove H+ ions to reduce stress. colour change from blue to red is observed

if a base is added to the reaction above then OH- ions combine with H+ ions to form water. this removes H+ ions from the right hand side. equilibrium will shift to the right to replace the H+ ions to reduce stress. a colour change from red to blue is observed

33
Q

how do we ensure an indicator has negligible contribution to the H+ and OH-

A

only adding a few drops

34
Q

what is the range of an indicator

A

the pH interval over which there is a clear change in colour for that indicator

35
Q

what is the range for methyl orange

A

3-5

36
Q

what is the colour in an acid for methyl orange

A

red

37
Q

what is the colour in a base for methyl orange

A

yellow

38
Q

range for litmus

A

5-8

39
Q

colour of litmus in acid

A

red

40
Q

colour of litmus in base

A

blue

41
Q

colour of phenolphthalein in acid

A

colourless

42
Q

colour of phenolphthalein in base

A

pink

43
Q

range for phenolphthalein

A

8-10

44
Q

why are pH ranges of indicators different

A

each indicator has its own dissociation constant

45
Q

what can be used to continuously stir a solution in a titration

A

a magnetic stirrer bar

46
Q

what is the purpose of a magnetic stirrer bar

A

it ensures adequate mixing which gives a more accurate reading of pH

47
Q

what is used in the titration to measure ph

A

a pH sensor is places into the acid in the beaker and connected to a data logger

48
Q

what does a data logger record

A

change in pH

49
Q

why is no indicator used in the pH by titration experiment

A

it is not needed at pH sensor is monitoring the pH directly

50
Q

how is the endpoint of the pH by titration experiment shown

A

by the dramatic increase in pH caused by 1 drop of base

51
Q

what indicator is used for a strong acid- weak base titration

A

methyl orange

52
Q

what is the vertical pH range for a strong acid- weak base titration

A

3-7

53
Q

why can you not use litmus or phenolphthalein in a strong acid- weak base titration

A

they do not change colour completely within the vertical range of 3-7

54
Q

how do we know methyl orange is a suitable indicator for a strong acid- weak base titration

A

colour change coincides with the sharp rise in the graph, which is in the range of pH 3-7. methyl oranges range is 3-5 which is within this range

55
Q

why is this a strong acid? (based off of a graph)

A

steep part of the curve is mainly below pH 7
initial pH value is low (pH1

56
Q

what is a suitable indicator for a weak acid - strong base titration

A

phenolphthalein

57
Q

what is the vertical range for a weak acid - strong base titration

A

pH: 7-10

58
Q

why is phenolphthalein a suitable indicator for a weak acid - strong base titration

A

the colour change coincides with the sharp rise on the graph

59
Q

how do we know that this is a strong base (according to a graph)

A

initial pH is at pH 3
steep part of curve is mainly above pH 7

60
Q

how does a weak acid - weak base titration appear on a graph?

A

no sudden jump in pH.

61
Q

why can you not use an indicator in a weak acid - weak base titration

A

cannot use an indicator to detect the end point of this titration as there is no sudden jump in pH so there is no interval range

62
Q

what type of acid- base titration cannot be used to determine the concentration of an acid or a base

A

a weak acid - weak base titration

LC Chemistry (21 decks)

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