Chapter 21 - Buffers and Neutralisation Flashcards

1
Q

What is a buffer solution?

A

System that minimises pH change when a small amount of acid or base is added

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2
Q

What are the two components of a buffer?

A

1) weak acid (HA) which removes added alkali
2) conjugate base (A-) which removes added acid

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3
Q

How is a buffer solution prepared from a weak acid and it’s salt?

A

Mix weak acid with it’s salt
- Weak acid added to water: partially dissociates so source of weak acid ONLY
- Salt added to water: fully dissociates so source of conjugate base ions

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4
Q

How is a buffer solution prepared from partial neutralisation of weak acid?

A
  • Add aqueous solution of alkali to excess of weak acid (excess will allow some weak acid to remain)
  • Reaction will form a salt as weak acid is partially dissociated to it’s conjugate base
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5
Q

Explain buffer action when acid is added

A
  • [H+] increases
  • H+ ions react with conjugate base A-
  • Equilibrium shifts to left to remove H+ ions
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6
Q

Explain buffer action when alkali is added

A
  • [OH-] increases
  • Small concentration of H+ ions from acid react with OH- ions to form water so H+ used up
  • HA dissociates
  • Equilibrium shifts to right to restore H+ ions
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7
Q

When is a buffer solution most effective?

A

When there is equal concentrations of weak acid and conjugate base [HA] = [A-]

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8
Q

What is the pH of a buffer solution when [HA] = [A-]?

A

pKa of HA because Ka = [H+]

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9
Q

What assumption for weak acids is no longer valid for buffer solutions?

A

[H+] ≠ [A-]

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10
Q

What is the buffer system in blood?

A

H2CO3 <-> HCO3- + H+

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11
Q

What is the ratio of HCO3- : H2CO3?

A

[HCO3-] = Ka
[H2CO3] [H+]

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12
Q

What is the equivalence point?

A

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

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13
Q

What is the end point?

A

Indicator contains equal concentrations of HA and A- and the colour is in between the two extremes

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14
Q

What does the pH equal at the end point?

A

pKa value of HA

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15
Q

Why does the pH titration curve plateau slightly at the beginning of a weak acid - strong/weak base reaction?

A

Acts as a buffer to minimise pH change

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16
Q

When is an indicator suitable?

A

When the pH range of the indicator passes through the vertical line of the pH titration curve