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Chemistry > 12: Acid-base Equilibria > Flashcards

12: Acid-base Equilibria Flashcards

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1
Q

Define an acid (Bronsted-Lowry)

A

A proton donor

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2
Q

Define an base (Bronsted-Lowry)

A

A proton acceptor

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3
Q

Define an alkali

A

A soluble base

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4
Q

What are conjugate acid-base pairs

A
  • A pair of reactants and products that are linked to each other by the transfer of a proton
  • Reactant CH3COOH is linked to product CH3COO- by transfer of a proton from the acid to the base
  • Reactant H2O is linked to product H3O+ by transfer of a proton from the acid to the base
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5
Q

What does the acidity of an aqueous solution depend upon

A

The number of H+ ions in solution

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6
Q

Define pH

A
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7
Q

Rearrange the equation of pH to find the concentration of H+

A
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8
Q

What is a strong acid

A

An acid that dissociates almost completely in aqueous solutions to form H+ ions

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9
Q

What is a weak acid

A

An acid that only partially dissociates in aqueous solutions, and can be represented as an equilibrium equation

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10
Q

Why is the Enthalpy change of neutralisation of strong acids and strong bases similar

A
  • Because the acids and alkalis are fully ionised and the neutralisation reaction between H+ + OH- occurs to produce water
  • H+(aq) + OH-(aq) ➡️ H2O(l)
  • Any other ions involved are spectator ions, and don’t affect neutralisation
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11
Q

Why is the Enthalpy change of neutralisation between weak acids and weak bases less exothermic

A

As weak acids and weak bases are only partially ionised, so energy has to be used to fully ionise them

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12
Q

What is the acid dissociation constant (Ka)

A
  • Equilibrium constant for weak acids
  • Units of mol dm-3
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13
Q

What assumptions are made with the Ka expression for weak acids

A
  • The concentration of H+ ions due to the ionisation of water is negligible
  • Initial concentration of HA is the same as concentration of HA at equilibrium
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14
Q

What can be assumed about strong acids to calculate their pH

A
  • Strong acids are completely ionised, and the number of H+ ions formed from the ionisation of water is negligible
  • So the total concentration of H+ is the same as the concentration of HA
  • pH calculation can then be used
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15
Q

What is a dibasic/diprotic acid

A

Two replaceable protons and with react in a 1:2 ratio with bases (e.g. H2SO4)

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16
Q

What is assumed about dibasic acids to calculate their pH

A
  • Although they are strong acids, so should produce an H+ concentration of double the [HA], it isn’t
  • The pH is lower than expected, indicating the acid isn’t fully ionised
  • Ionisation of dibasic acids happens in two steps, so the second step is suppressed by the abundance of H+ ions, creating an equilibrium
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17
Q

What assumptions have to be made to calculate the pH of weak acids

A
  • The concentration of acid and Ka value of acid must be known
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18
Q

How is the ionic product for water derived from the dissociation of water

A
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19
Q

What is the relationship between Kw and pKw

A

pKw = -logKw

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20
Q

What is the relationship between Ka and pKa

A

pKa = -logKa

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21
Q

What assumptions are made to calculate the pH of strong bases

A
  • Strong bases are completely ionised in solution
  • Therefore the concentration of OH- ions is equal to the concentration of base
  • Concentration of OH- in solution and ionic product of water can be used to calculate pH
22
Q

What are examples of strong acids (low pH)

A
  • H2SO4
  • HCl
  • H3PO4
  • HNO3
23
Q

What are examples of weak acids (higher pH)

A
  • Ethanoic acid
  • Methanoic acid
24
Q

What are examples of strong bases (high pH)

A
  • NaOH
  • Ba(OH)2
25
Q

What are examples of weak bases (lower pH)

A
  • NH3
  • CH3NH2
26
Q

Why do some salts have pH 7.00

A

They are made from a strong acid and a strong base (e.g. NaCl)

27
Q

Why are some salts alkaline

A

They are made of a weak acid and a strong base (e.g. CHCOONa)

28
Q

Why are some salts acidic

A

They are made of a strong acid and weak base (e.g. NH4Cl)

29
Q

Why are some salts neutral

A

They are made of a weak acid and weak base

30
Q

What is the effect of dilution on the pH of strong acids

A
  • As concentration increases by a factor of 10, pH decreases by one unit
  • However acids to a concentration factor of more than 1.00x10^-6 are so dilute that the contribution of H+ ions from the water is no longer ignored
31
Q

What is the effect of dilution on the pH of weak acids

A
  • As concentration increases by a factor of 10 the pH increases by a factor of around 0.5
32
Q

What does a pH curve show

A

How the pH of a solution changes as the acid/base is added

33
Q

What can be determined by pH graphs

A
  • Determine the pH of acid/base by looking where the curve starts on the y-axis
  • Find the pH at the equivalence point
  • Find the volume of base/acid at the equivalence point
  • Obtain the range of pH from the vertical section of the curve
34
Q

What is the equivalence point of a pH curve

A

The point in the titration at which the amount of titrant added is enough to completely neutralise the analyse solution

35
Q

Draw the strong acid + strong base titration curve

A

(Inverse for acid against base titration)

36
Q

Draw the weak acid + strong base titration curve

A

(Inverse for acid against base)

37
Q

Draw the strong acid + weak base titration curve

A

(Inverse for acid against base)

38
Q

Draw the weak acid + weak base titration curve

A

(Inverse for acid against base titration)

39
Q

What is an acid-based indicator and how does it work

A
  • A weak acid which dissociates to give an anion of a different colour (e.g. Hln)
  • Hln and its conjugate base ln- are different colours
  • If the solution is acidic the POE will move to the left and more Hln will be present (its colour is visible)
  • If the solution is alkaline the POE will move to the right and more ln- will be present (its colour is visible)
  • The colour of the indicator will change gradually
  • The pH the indicator changes at depends on the Ka of the indictor
  • At the endpoint of the reaction, there is a balance between Hln and ln- concentrations, so pKa of indicators = the pH of its endpoint
40
Q

How is a suitable indicator chosen for acid-base titrations

A
  • An indicator will be appropriate is the pH range of the indicator falls within the rapid pH change for the titration
  • The indicator should suit the pH change represented by the equivalence point
41
Q

What indicator is picked for a weak acid + weak base titration

A
  • There is no sudden pH change at the end point of a weak acid + weak base titration, so there is no suitable indicator
42
Q

What is a buffer solution

A

A solution which resists changes in pH when a small amount of acid/alkali is added. They can consist of a weak acid + conjugate base, or weak base + conjugate acid and are used to keep the pH constant

43
Q

How does ethanoic acid and sodium ethanoate work as a buffer

A
  • Ethanoic acid is a weak acid, so partially dissociates in solution to form a low concentration of ethanoate ions
  • Sodium ethanoate is a salt which fully dissociates in solution to form a high concentration of ethanoate ions
  • The buffer contains the high concentrations of ethanoic acid, and high concentrations of ethanoate ions, and the ethanoic acid is in equilibrium with the H+ and ethanoate ions
44
Q

What happens when H+ ions are added to a buffer solution (e.g. ethanoic acid and sodium ethanoate)

A
  • POE shifts to the left as H+ ions react with ethanoate ions to form more ethanoic acid until equilibrium is reestablished
  • As there is a large reserve of ethanoate ions, its concentration doesn’t change much when reacting with H+ ions
  • As there is a large reserve of ethanoic acid, its concentration doesn’t change much when more is formed
  • This results in the pH staying constant
45
Q

What happens when OH- ions are added to buffer solutions (e.g. ethanoic acid and sodium ethanoate)

A
  • The OH- and H+ ions react to form water, and the H+ concentration decreases
  • POE shifts to the right and more ethanoic acid molecules dissociate to form more H+ and ethanoate ions until equilibrium is reestablished
  • Due to large reserves of ethanoate ions and ethanoic acid, the concentration of both don’t change much
  • pH stays constant
46
Q

How is a buffer with a pH lower than 7 made

A

From a weak acid and it’s salt, or by partial neutralisation of a weak acid

47
Q

How is the pH of a buffer solution calculated

A
  • Using the Ka of the weak acid
  • Using the equilibrium constant of the weak acid and its conjugate base (salt)
  • Therefore the [H+] is needed, and can be found using the equilibrium expression
48
Q

How to make a buffer solution with a required pH

A
  • To make a buffer with pH<7 use a mixture of weak acid and its conjugate base
  • To make a buffer with pH>7 use a mixture of weak base and its conjugate acid
49
Q

How are buffer solutions applied to control the pH of blood

A
  • HCO3- ions act as a buffer to keep blood pH between 7.35-7.45
  • Respiration produces CO2, which combines with H2O in blood to form an equilibrium between CO2 and HCO3-
  • If [H+] isn’t regulated blood pH drops (acidosis) and the body malfunctions
  • If [H+] increases POE shifts to the left until equilibrium is restored
  • If [H+] decreases POE shifts to the right until equilibrium is restored
50
Q

Using the weak acid + strong base titration, describe what is happening on the graph

A
  • pH of acid starts at roughly 3, and initial rise in pH is steep as the neutralisation of the weak acid by strong base is rapid
  • Ethanoate ions are formed which creates a buffer, which resists changes to pH so the pH rises gradually
  • The half equivalence point is the stage where exactly half the amount of weak acid havens been neutralised
  • pKa = pH at the half equivalence

Chemistry (19 decks)

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