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Chemistry > Chapter 21 - Buffers and Neutralisation > Flashcards

Chapter 21 - Buffers and Neutralisation Flashcards

1
Q

What is a buffer solution ?

A

A system that minimises changes in pH when small amounts of an acid or a base are added

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2
Q

What are the two components of a buffer solution ?

A

A weak acid and its conjugate base

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3
Q

What is the role of the weak acid ?

A

It removes the added alkali

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4
Q

What is the role of the conjugate base ?

A

It removes the added acid

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5
Q

Does the pH change when a buffer works ?

A
  • The pH does change but only by a small amount
  • Do not assume it stays constant
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6
Q

What are the two methods of preparing a weak acid buffer solution ?

A
  • From a weak acid and its salt
  • From the partial neutralisation of a weak acid
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7
Q

How can a buffer be prepared from a weak acid and a salt ?

A
  • Mixing a solution of a weak acid with one of its salt
  • The acid is the source of the weak acid component
  • The salt of the weak acid dissociates and provides the conjugate base
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8
Q

How can a buffer be prepared from the partial neutralisation of a weak acid ?

A
  • Adding an aqueous solution of an alkali to an excess of the weak acid
  • Weak acid is partially neutralised by the alkali forming the conjugate base
  • Some of the weak acid is left over unreacted providing the weak acid component
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9
Q

What does the conjugate acid base pair in an acid buffer solution control ?

A

It controls the pH

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10
Q

How does the conjugate base remove added acid ?

A
  • H+ conc increases
  • H+ ions react with the conjugate base
  • The eqm position shifts to the left, removing most of the H+ ions
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11
Q

How does the weak acid remove alkali ?

A
  • OH- conc increases
  • The small conc of H+ ions reacts with the OH- ions
  • HA dissociates, shifting the eqm position to the right to restore most of the H+ ions
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12
Q

When is a buffer most effective at removing the added acid or alkali ?

A

When there are equal concentrations of the weak acid and its conjugate base

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13
Q

What are the two assumptions when [HA] = [A-] ?

A
  • Ka = [H+]
  • pKa = pH
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14
Q

What is the pH of the blood plasma like ?

A

Needs to be maintained at a pH between 7.35 and 7.45

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15
Q

What is the pH of the blood plasma controlled by ?

A
  • A mixture of buffers
  • With the carbonic acid - hydrogencarbonate buffer system
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16
Q

What is the pH of normal healthy blood ?

A

7.4

17
Q

What happens if the pH falls below 7.35 ?

A

People can develop a condition called acidosis

18
Q

What is acidosis ?

A

Can cause fatigue, shortness of breath and in extreme conditions, shock or death

19
Q

What happens if the pH rises above 7.45 ?

A

People can develop a condition called alkalosis

20
Q

What is alkalosis ?

A

Can cause muscle spasms, light-headedness and nausea

21
Q

What happens on the addition of an acid in the blood plasma buffer system ?

A
  • H+ concentration increases
  • H+ ions react with the conjugate base, HCO3-
  • The equilibrium position shifts to the left, removing most of the H+ ions
22
Q

What happens on the addition of an alkali in the blood plasma buffer system ?

A
  • OH - concentration increases
  • The small concentration of H+ ions react with OH- ions to create H2O
  • H2CO3 dissociates, shifting the equilibrium position to the right to restore most of the H+ ions
23
Q

How does the body prevent the build up of H2CO3 ?

A

It is converted into CO2 gas, which is then exhaled by the lungs

24
Q

What are the significant points on a pH titration curve ?

A
  • At first, excess of acid, pH increases slowly
  • Vertical section, pH increases rapidly on addition of very small volume of base
    • Equivalence point is at the centre of the vertical section
  • At the end, excess of base, pH increases slowly
25
Q

What is the equivalence point ?

A

The volume of one solution that exactly reacts with the volume of the other solution

26
Q

How can you find the equivalence point from a graph ?

A

The equivalence point is the centre of the vertical section of the pH titration curve

27
Q

What is an acid-base indicator ?

A

A weak acid, HA, that has a distinctively different colour from its conjugate base, A-

28
Q

What is the end point of the titration ?

A
  • Where the indicator contains equal concentrations of HA and A-
  • The colour will be in between the colour of the acid and the base
29
Q

What is equal to what at the end point ?

A
  • [HA] = [A-] and therefore Ka = [H+]
  • Ka = [H+] and pKa = pH
30
Q

How must you choose an indicator, in a titration ?

A

You must use an indicator that has a colour change that coincides with the vertical section of the pH titration curve

31
Q

Is there a suitable indicator for a weak acid and a weak base ?

A

No indicator is suitable

32
Q

Why is no indicator suitable for a weak acid and a weak base ?

A

There is no vertical section

33
Q

What indicator is suitable for a strong acid and a strong base ?

A

Phenolphthalein and methyl orange are both suitable

34
Q

What indicator is suitable for a strong acid and a weak base ?

A
  • Methyl orange is suitable
  • Phenolphthalein is not suitable
35
Q

What indicator is suitable for a weak acid and a strong base ?

A
  • Phenolphthalein is suitable
  • Methyl orange is not suitable

Chemistry (29 decks)

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