12: Acid-base Equilibria Flashcards
1
Q
Define an acid (Bronsted-Lowry)
A
A proton donor
2
Q
Define an base (Bronsted-Lowry)
A
A proton acceptor
3
Q
Define an alkali
A
A soluble base
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
5
Q
What does the acidity of an aqueous solution depend upon
A
The number of H+ ions in solution
6
Q
Define pH
A
7
Q
Rearrange the equation of pH to find the concentration of H+
A
8
Q
What is a strong acid
A
An acid that dissociates almost completely in aqueous solutions to form H+ ions
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
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
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
12
Q
What is the acid dissociation constant (Ka)
A
- Equilibrium constant for weak acids
- Units of mol dm-3
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
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
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)
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
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
18
Q
How is the ionic product for water derived from the dissociation of water
A
19
Q
What is the relationship between Kw and pKw
A
pKw = -logKw
20
Q
What is the relationship between Ka and pKa
A
pKa = -logKa
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
What are examples of weak bases (lower pH)
- NH3
- CH3NH2
26
Why do some salts have pH 7.00
They are made from a strong acid and a strong base (e.g. NaCl)
27
Why are some salts alkaline
They are made of a weak acid and a strong base (e.g. CHCOONa)
28
Why are some salts acidic
They are made of a strong acid and weak base (e.g. NH4Cl)
29
Why are some salts neutral
They are made of a weak acid and weak base
30
What is the effect of dilution on the pH of strong acids
- 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
What is the effect of dilution on the pH of weak acids
- As concentration increases by a factor of 10 the pH increases by a factor of around 0.5
32
What does a pH curve show
How the pH of a solution changes as the acid/base is added
33
What can be determined by pH graphs
- 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
What is the equivalence point of a pH curve
The point in the titration at which the amount of titrant added is enough to completely neutralise the analyse solution
35
Draw the strong acid + strong base titration curve
(Inverse for acid against base titration)
36
Draw the weak acid + strong base titration curve
(Inverse for acid against base)
37
Draw the strong acid + weak base titration curve
(Inverse for acid against base)
38
Draw the weak acid + weak base titration curve
(Inverse for acid against base titration)
39
What is an acid-based indicator and how does it work
- 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
How is a suitable indicator chosen for acid-base titrations
- 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
What indicator is picked for a weak acid + weak base titration
- There is no sudden pH change at the end point of a weak acid + weak base titration, so there is no suitable indicator
42
What is a buffer solution
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
How does ethanoic acid and sodium ethanoate work as a buffer
- 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
What happens when H+ ions are added to a buffer solution (e.g. ethanoic acid and sodium ethanoate)
- 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
What happens when OH- ions are added to buffer solutions (e.g. ethanoic acid and sodium ethanoate)
- 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
How is a buffer with a pH lower than 7 made
From a weak acid and it’s salt, or by partial neutralisation of a weak acid
47
How is the pH of a buffer solution calculated
- 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
How to make a buffer solution with a required pH
- 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
How are buffer solutions applied to control the pH of blood
- 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
Using the weak acid + strong base titration, describe what is happening on the graph
- 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
