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OCR A A-Level Inorganic Chemistry > Chapter 21 > Flashcards

Chapter 21 Flashcards

1
Q

What is a buffer solution?

A
  • A system that minimises pH changes on addition of small amounts of an acid or a base
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2
Q

What do buffer solutions consist of?

A
  • Some undissociated acid (which is why WEAK acids must be used)
  • Its conjugate base
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3
Q

How can buffer solutions be formed?

A
  • By mixing a weak acid and a salt of the weak acid (e.g. ethanoic acid and sodium ethanoate)
  • By reacting excess weak acid with an alkali (e.g. ethanoic acid and sodium hydroxide)
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4
Q

How do the 2 methods of forming buffer solutions provide weak acid and conjugate base?

A
  • Method 1: the salt of the weak acid supplies the conjugate base (as it would break up in solution)
  • Method 2:
  • the acid and the alkali react to form the conjugate base (the salt formed would break up in solution)
  • an EXCESS of acid is used to supply the weak acid (otherwise it would all be neutralised)
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5
Q

How do buffer solutions control pH when acid is added?

A
  • The weak acid dissociates, creating an equilibrium
  • If H+ is added, it reacts with the conjugate base to form the acid
  • This shifts equilibrium to the left, which removes H+ ions
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6
Q

How do buffer solutions control pH when alkali is added?

A
  • If alkali is added, OH- is added
  • The OH- ions react with H+ ions to form water
  • This decreases [H+], which shifts equilibrium to the right, and restores [H+]
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7
Q

Why do buffers only work when a small amount of acid or alkali is added?

A
  • If too much alkali or acid is added, buffers cannot remove or replenish enough H+ ions
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8
Q

How do you calculate the pH of a buffer solution? What shortcut is there to this method?

A
  • [H+]= Ka × [HA] / [A-]
  • You can use the moles of HA and A- if it’s faster, since they would have the same volume
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9
Q

What special quality do certain buffer solutions have, and why?

A
  • For some, [HA] = [A-], so they would cancel out in the Ka expression, and [H+] = Ka, and pH = pKa
  • This is because they contain equal concentrations of a weak acid and its conjugate base (in the reaction method of making a buffer, you would add half as much as alkali as weak acid)
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10
Q

What approximation is used for weak acids, but not for buffers?

A
  • [H+] = [A-]
  • (A- was added in the buffer, so it’s no longer true)
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11
Q

What range of values should the pH of blood plasma be between?

A
  • 7.35-7.45
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12
Q

How is the pH of blood maintained? Give the equation to demonstrate this.

A
  • Using the carbonic acid-hydrogencarbonate buffer system
  • H2CO3 <-> H+ + HCO3-
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13
Q

How do pH titration curves work? (Use the example of acid in the conical flask, and you’re adding base.)

A
  • You plot volume of base added against pH
  • Twice as much base as needed is used
  • This causes the graph to be flat at first, have a steep rise, and then go flat again (s-shaped)
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14
Q

List 4 main features of pH titration curves (going from acidic to alkaline).

A
  • At first, there is an excess of acid, so the pH slowly increases
  • Then there is vertical line called the vertical section, where pH increases rapidly on addition of a very small amount of base
  • The equivalence point is at the centre of the vertical section
  • At the end, there is an excess of base, so the pH slowly increases
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15
Q

What is the equivalence point of a reaction?

A
  • The point in a titration where the volume of one solution has reacted exactly with the volume of the other solution
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16
Q

What does the pH titration curve of a reaction between a strong acid and a strong base look like?

A
  • The graph starts at roughly pH 1
  • It ends at pH 13
  • The vertical section is from about pH 4 to 10
  • The pH steadily increases at first
17
Q

What does the pH titration curve of a reaction between a strong acid and a weak base look like?

A
  • The graph starts at roughly pH 1
  • It ends at pH 11
  • The vertical section is from about pH 4 to 7
  • The pH steadily increases at first
18
Q

What does the pH titration curve of a reaction between a weak acid and a strong base look like?

A
  • The graph starts at roughly pH 3
  • It ends at pH 13
  • The vertical section is from about pH 7 to 10
  • The pH rapidly increases at first
19
Q

What does the pH titration curve of a reaction between a weak acid and a weak base look like?

A
  • The graph starts at roughly pH 3
  • It ends at pH 11
  • There is no vertical section
  • The pH rapidly increases at first
20
Q

How do pH indicators work?

A
  • They’re weak acids where the acid and conjugate base have different colours
  • When hydroxide ions are added, they react with H+ ions, which shifts equilibrium to the right, making the solution the colour of the conjugate base
  • If H+ ions are added, equilibrium shifts to the left, so the solution becomes the colour of the acid
21
Q

How are appropriate indicators chosen for titrations?

A
  • Each indicator has a pH range that it works for
  • Its range has to be within the vertical section for it to be able to show the end point of a titration
22
Q

What is the end point of a titration?

A
  • When the indicator contains equal concentrations of acid and conjugate base; the point where the indicator changes colour
23
Q

What are the pH ranges for methyl orange and phenolphthalein? Therefore, which types of titrations can they be used for?

A
  • Methyl orange: 3 to 4
  • Any with a strong acid
  • Phenolphthalein: 9 to 10
  • Any with a strong base
24
Q

What colours does phenolphthalein change between, and in what conditions?

A
  • Colourless in acids
  • Pink in alkalis
25
Q

What colours does methyl orange change between, and in what conditions?

A
  • Red in acids
  • Yellow in alkalis
26
Q

How is the end point found for weak acid-weak base titrations? Why?

A
  • No indicator can be used for weak acid-weak base reactions, as there is no vertical section
  • A pH meter is used instead
27
Q

How are pH meters used?

A
  • You start by measuring your acid/ alkali into a conical flask with a pipette
  • You put the electrode of the pH meter into the solution to measure its pH
  • You then add the alkali/ acid from the burette, swirling after each addition, and recording the volume added and the pH after every each addition
  • Continue doing this until the pH starts to change more rapidly
  • After the substance has been neutralised, keep adding alkali/ acid so you can draw the entire pH titration curve

OCR A A-Level Inorganic Chemistry (17 decks)

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