buffers and neutralisation Flashcards

1
Q

what is a buffer

A
  • minimises pH changes

- when small amounts of acid/base are added

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2
Q

forming a basic buffer solution

A
  • weak base and its salt
  • made from reacting weak base + strong acid
  • E.g NH3 + NH4+Cl-
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3
Q

forming acidic buffer solutions

A
  • weak acid and its salt
  • Made from weak acid and strong base
  • or mixing weak acid solution with a solution of its salt
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4
Q

forming buffer from a partial neutralisation of the weak acid

A
  • excess WA is partially neutralised by the alkali
  • NaOH
  • forms a conjugate base
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5
Q

adding acid to a buffer solution

A
  • [H+] inc
  • H+ reacts with conjugate base
  • H+ + CB = HA
  • eq shift to left
  • counteracts change
  • restores original [H+]
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6
Q

adding alkali to buffer solution

A
  • [OH-] inc
  • OH - reacts with H+
  • OH- + H+ = H2O
  • HA dissociates
  • eq shifts to right
  • restores [H+]
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7
Q

when [HA] = [A-]

A
  • pH = pKa

- Ka = [H+]

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8
Q

calc pH of a buffer solution

A
[H+] = Ka x [HA] / [A-] 
pH = -log[H+]

pH = pKa + log [A-] / [HA]

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9
Q

pH of blood plasma

A

7.35-7.45
healthy blood = 7.40
controlled by carbonic acid / hydrogen carbonate (H2CO3/HCO3-) buffer system

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10
Q

pH lower/higher than 7.35/7.45

A
  • lower = acidosis = fatigue, shortness of breath

- higher = alkalosis = muscle spasms, nausea

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11
Q

addition of H+/OH- on blood buffer system

A

Acid:

  • [H+] inc
  • H+ react with CB HCO3-
  • eq left
  • decreasing [H+]

Alkali:

  • [OH-] inc
  • [H+] reacts with [OH-]
  • H+ + OH- = H2O
  • H2CO3 dissociates
  • eq right
  • [H+] restored
  • H2CO3 = H+ + HCO3-
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12
Q

what is the equivalence point

A
  • volume of one sol that exactly reacts with volume of another solution
  • centre of the vertical section of pH curve
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13
Q

what is the end point

A
  • indicator contains equal conc of HA and A-
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14
Q

indicator colour changes (basic)

- E.g methyl orange

A
  • indicators are weak acids

basic soltuion:

  • OH- + H+ = H2O
  • HA dissociates
  • eq to right
  • colour change to orange at end point
  • then yellow when eq to right

methyl orange :
acid = red
alkali = yellow
end point = orange

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15
Q

phenolphthalein colours

A
acid = colourless
alkali = pink
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16
Q

indicator for weak acid weak base

A
  • none

- no vertical section