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chemistry > acids, bases and buffers > Flashcards

acids, bases and buffers Flashcards

1
Q

define a bronsted-lowry acid

A

a proton donor

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2
Q

define a bronsted-lowry base

A

a proton acceptor

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3
Q

define a lewis acid

A

electron pair acceptor

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4
Q

define a lewis base

A

electron pair donor

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5
Q

what ion causes a solution to become acidic? give their names and formulas

A

H+ (hydrogen ion)

H3O+ (oxonium ion)

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6
Q

what ion causes a solution to be alkaline?

A

OH- (hydroxide ion)

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7
Q

write an equation for the ionisation of water (2)

A

2H2O (l) ⇌ H3O (aq) + OH- (aq)

H2O (l) ⇌ H+ (aq) + OH- (aq)

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8
Q

give an example of a monobasic acid

A

HCl

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9
Q

give an example of a dibasic acid

A

H2SO4

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10
Q

what is a monobasic acid?

A

a monobasic acid releases 1 mole of H+ ions for every 1 mole of acid

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11
Q

what is dibasic acid?

A

a dibasic acid releases 2 moles of H+ ions for every 1 mole of acid

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12
Q

what is a tribasic acid?

A

a tribasic acid releases 3 moles of H+ ions for every 1 mole of acid

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13
Q

give an example of a tribasic acid

A

H3PO4

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14
Q

name examples of strong acids

A
  1. HCl (hydrochloric acid)
  2. HNO3 (nitric acid)
  3. H2SO4 (sulfuric acid)
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15
Q

name examples of weak acids

A
  1. H3PO4 (phosphoric acid)
  2. HF (hydrofluoric acid)
  3. CH3COOH (methanoic acid)
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16
Q

identify the acid-base pairs for the reaction below:

CH3COOH + H2O ⇌ CH3COO- + H3O+

A

CH3COOH = acid 1

H2O = base 2

CH3COO- = base 1

H3O+ = acid 2

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17
Q

define a strong acid

A

an acid that fully dissociates in solution to produce H+ ions

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18
Q

what is the difference between concentrated and strong?

A

concentrated means many moles per dm3, strong refers to the amount of dissociation

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19
Q

define weak acids

A

acids that only partially dissociate in solution to H+ ions, they remain in equilibrium

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20
Q

what is the constant that is used to measure the extent of acid dissociation called?

A

acid dissociation constant

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21
Q

what is the symbol of acid dissociation constant

A

Ka

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22
Q

write the acid dissociation constant expression for:

Ha ⇌ H+ + A-

A

Ka = [H+][A-] / [HA]

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23
Q

what does a large Ka value mean

A

the larger the Ka the greater the extent of dissociation

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24
Q

write the equation used to convert Ka into pKa

A

pKa = -log10Ka

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25
Q

write the equation that is used to convert pKa into Ka

A

Ka - 10^-pka

26
Q

what is the relationship between pKa and the strength of the acid

A

smaller the pKa the stronger the acid

27
Q

write the equation used to convert concentration of H+ into pH

A

pH = -log[H+]

28
Q

write the equation used to convert pH into concentration of H+

A

[H+] = 10^-pH

29
Q

why is a pH scale useful compared to the concentration of H+

A

pH scale allows a wide range of H+ concentration to be expressed as simple positive values

30
Q

what is the relationship between pH and [H+]

A

high pH value means small [H+]

31
Q

if two solutions have a pH difference of 1, what is the difference in [H+]?

A

a factor of 10

32
Q

[H+] of a strong acid is equal to what?

A

[H+] = [HA]

33
Q

write the equation used to calculate [H+] of weak acids

A

[H+] = √Ka x [HA]

34
Q

what is the assumption made when calculating pH of a weak acid?

A
  • the concentration of acid at equilibrium is equal to the concentration of acid after dissociation
  • this is because very little of the acid dissociates
35
Q

write the expression for ionic product of water, Kw

A

Kw = [H+][OH-]

36
Q

what are the units for Kw

A

mol2dm-6

37
Q

what is the value of Kw at 298K?

A

1x10^-14

38
Q

what physical factors affect the value of Kw? how do they affect it?

A

temperature only

if temperature is increased the equilibrium moves to the right so the Kw increases and the pH of pure water decreases

39
Q

indices of [H+] and [OH-] always add up to what value?

A

-14

40
Q

define the term strong base

A

bases that dissociate completely in water to release OH-

41
Q

give examples of some strong bases

A
  1. NaOH (sodium hydroxide)
  2. KOH (potassium hydroxide)
  3. Ca(OH)2 (calcium hydroxide)
42
Q

give examples of some weak bases

A

NH3 (ammonia)

43
Q

write the equation used to calculate the [H+] of some strong bases

A

[H+] = Kw / [OH-]

44
Q

define a buffer solution

A

a mixture that minimises pH change on addition of small amounts of an acid or base

45
Q

what are the 2 ways in which buffers can be made

A
  • weak acid and its conjugate base
  • weak acid and a strong alkali
46
Q

in which direction does the equilibrium shift when an acid is added to a buffer solution? why?

A
  • equilibrium shifts to the left
  • because [H+] increases
  • conjugate base reacts with [H+] to remove most of the [H+]
47
Q

in which direction does the equilibrium shift when an alkali is added to a buffer solution? why?

A
  • equilibrium shifts to the right
  • because [OH-] increases
  • small concentration of H+ reacts with OH- to restore the H+ ions, HA dissociates shifting the equilibrium
48
Q

write the equation used to calculate [H+] of buffer solution

A

[H+] = Ka x [Ha]/[A-]

49
Q

which buffer system maintains blood at 7.4? what happens when acid/alkali is added?

A

H+ + HCO3 ⇌ CO2 + H2O

  • Add OH- = reacts with H+ to form H2O, then shifts equilibrium left to restore the H+ lost
  • Add H+ = equilibrium shifts to the right, removing excess H+
50
Q

what is a titration?

A

the addition of an acid/base of known concentration to base/acid to determine the concentration, an indicator is used to show that neutralisation has occurred

51
Q

list the equipment that can be used in a titration experiment

A
  1. clamp stand
  2. white tile
  3. burette
  4. pipette and filler
  5. conical flask
  6. acid/alkali of known concentration (goes into burette)
  7. acid/alkali of unknown concentration (goes into conical flask)
52
Q

define the term equivalence point

A

the point at which the exact volume of base has been added to just neutralise the acid

53
Q

what is the end point?

A

the point at which pH changes rapidly

54
Q

what are the properties of a good indicator for a reaction?

A
  • sharp colour change = no more than 1 drop of alkali/acid needed for colour change
  • end point must be the same as the equivalence point
  • distinct colour change so it is obvious when end point is reached
55
Q

what indicator would you use for a strong acid - strong base titration

A

phenolphthalein or methyl orange

56
Q

what indicator would you use for a strong acid - weak base titration

A

methyl orange

57
Q

what indicator would you sue for a strong base - weak acid titration

A

phenophalein

58
Q

what indicator would you sue for a weak base - weak acid titration

A
  • neither methyl orange or phenophalein is suitable
  • neither give a sharp change at the endpoint
59
Q

what colour is methyl orange in acid, alkali and the end point?

A

acid = red

alkali = yellow

end point = yellow

60
Q

what colour is phenophalein in acid, alkali and the end point?

A

acid = colourless

alkali = pink

end point = colourless

61
Q

describe how you would use a ph metre

A
  • remove pH probe from storage solution and rinse with distilled water
  • dry probe and place into solution of unknown pH
  • let pH probe stay in solution until it gives a settled reading

chemistry (37 decks)

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