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Chemistry > Acids and bases > Flashcards

Acids and bases Flashcards

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1
Q

What is a neutralisation reaction?

A

When an acid reacts with a base

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2
Q

Acid + metal —>

A

salt + hydrogen

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3
Q

Acid + metal oxide —>

A

salt + water

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4
Q

Acid + metal hydroxide —>

A

salt + water

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5
Q

Acid + metal carbonate —>

A

salt + CO2 + water

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6
Q

Bronsted Lowry definition of an acid

A

proton donor

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7
Q

Bronsted Lowry definition of a base

A

proton acceptor

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8
Q

Define pH

A

-log10[H+]

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9
Q

How to work out concentration of H+ ions using pH

A

10^-pH

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10
Q

What does a smaller pH indicate about the concentration of H+ ions

A

The smaller the pH, the greater the concentration of H+ ions

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11
Q

What is a strong acid?

A

A strong acid fully dissociates into its ions

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12
Q

Position of the equilibrium of strong acid dissociation

A

Equilibrium lies entirely to the right hand side

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13
Q

What is a weak acid?

A

A weak acid only slightly dissociates into its ions

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14
Q

Position of the equilibrium of weak acid dissociation

A

Equilibrium lies entirely to the left hand side

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15
Q

What is Ka

A

[products] / [reactants]

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16
Q

Ka of a weak acid

A

[H+]2 / [HA]

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17
Q

H+ ions of weak acid

A

square root of Ka x [HA]

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18
Q

pH of a strong acid

A

-log[H+]

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19
Q

Ka conversion into pKa

A

pKa = -log10Ka

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20
Q

pKa conversion into Ka

A

Ka = 10^- pKa

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21
Q

How does a stronger acid affect the value of pKa

A

The stronger the acid, the lower the pH and the lower the pKa value

22
Q

Position of equilibrium of water

A

To the left

23
Q

What does the position of the equilibrium of water indicate about [water]

A

Position of equilibrium is very far to the left so [H2O] is effectively constant

24
Q

What is Kw?

A

Ionic product of water

25
Q

Value of Kw at 298K

A

1 x 10^-14

26
Q

Kw expression for water

A

Kw = [H+][OH-]

27
Q

Why is pure water always neutral

A

[H+] = [OH-]

28
Q

How to work out [H+] of pure water

A

[H+]2 = Kw

29
Q

Position of equilibrium of strong bases

A

Equilibrium lies entirely to the right hand side

30
Q

How to work out [H+] of strong bases

A

[H+] = Kw / [OH-]

31
Q

What is a buffer?

A

A solution which can resist changes in pH when a small amount of acid or base is added

32
Q

What is an acidic buffer made of?

A

A weak acid and a soluble salt of that acid. Maintains a pH below 7

33
Q

What is a basic buffer made of?

A

A weak base and a soluble salt of that base. Maintains a pH above 7

34
Q

How to work out [H+] of a buffer

A

[H+] = (Ka x [HA]) / [A-]

35
Q

How to work out pH of a buffer made by neutralisation?

A

ICE box
use end moles in equation

36
Q

What is half neutralisation?

A

Enough base has been added to neutralise exactly half of the acid present

37
Q

At half neutralisation point what does Ka equal?

A

Ka = [H+]

38
Q

At half neutralisation point what does pKa equal?

A

pH

39
Q

What is a standard solution?

A

Exact concentration is known

40
Q

What are the practical steps you need to take to produce a pH titration curve?

A

Add 25cm3 of acid into a 250ml beaker
Measure the initial pH of the acid solution in the beaker using a pH meter and record
Add 1cm3 of base from the burette
Swirl the mixture
Measure the new pH and record
Continue adding base until it is in excess
Add in smaller intervals when approaching end point

41
Q

Why is it important to calibrate pH meter?

A

After storage it may not give an accurate reading

42
Q

How to calibrate pH meter?

A

Place the pH meter in a solution of known pH and then adjust the meter accordingly

43
Q

What is the equivalence point?

A

When exactly enough acid has been added to neutralise the base

44
Q

What is the end point?

A

Exact volume of acid or base which needs to be added to cause an indicator to change colour

45
Q

How to choose a suitable indicator?

A

Changes colour somewhere on the vertical section of the pH titration curve

46
Q

When does phenolphthalein change colour and what colour is phenolphthalein in acid and alkali? (3)

A

At a pH of about 10
Acid: colourless
Alkali: red

47
Q

When does methyl orange change colour and what colour is methyl orange in acid and alkali? (3)

A

At a pH of about 4.5
Acids: red
Alkali: yellow

48
Q

What is the half-neutralisation point?

A

Enough base has been added to neutralise exactly half of the acid

49
Q

A student has an unknown weak acid of unknown concentration and a bottle of 0.5 mol dm-3 NaOH.
Explain, using brief practical details, how you would determine the Ka of the weak acid. [6]

A

Measure 25 cm3 of acid into a beaker/conical flask using volumetric pipette.
Use a pH probe to measure the initial pH
Add 1cm3 NaOH from a burette swirl and record the pH
Repeat until the NaOH is in excess
Plot a graph of pH against vol NaOH added in cm3
Use the vertical section to find the volume of NaOH needed for neutralisation
The half neutralisation point is half the volume of neutralisation.
Read off the pH at the half neutralisation point.
At half neutralisation ka = [H+]/pKa=pH
Ka = 10-pH

50
Q

Why is a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration? (1)

A

Concentration of hydrogen ions in aqueous solution covers a very wide range

51
Q

What are the properties of a good indicator? (3)

A

Sharp colour change - no more than one drop of acid/alkali needed for colour change
End point must be the same as the equivalence point, or titration gives wrong answer
Distinct colour change so it is obvious when end point has been reached

52
Q

What products are buffers found in? (2)

A

Shampoos
Detergents

Chemistry (28 decks)

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