1.12 Acids and Bases Flashcards
Brønsted-Lowry acid
Proton (H+) donor
Brønsted-Lowry base
Proton (H+) acceptor
Brønsted-Lowry acid-base reaction
Reaction involving the transfer of a proton
Monoprotic acid
Acid that releases one H+ ion per molecule e.g. HCl
Diprotic acid
Acid that releases two H+ ions per molecule e.g. H2SO4
Definition of pH, and useful rearrangement
pH = – log [H+]
[H+] = 10^-pH
Ionic Product of Water (Kw)
(Kw) H2O ⇌ H+ + OH-
ΔH = endothermic
Kc = [H+] [OH-]/[H2O]
So Kc [H2O] = [H+] [OH-]
so Kc [H2O] = a constant = Kw
Kw = [H+][OH-] where Kw = 1x10^-14
The effect of temperature on the pH of water and the neutrality of water
As the temperature increases, the equilibrium moves right to oppose the increase in temperature
Therefore [H+] and [OH-] increase
So Kw increases and therefore pH decreases
However, the water is still neutral as [H+] = [OH-]
Calculating the pH of water
In pure water, [H+] = [OH-]
So Kw = [H+]^2
Therefore [H+] = √Kw
pH of a Strong Base
Find [OH-] and then substitute into: [H+] = Kw/[OH-]
Mixtures of Strong Acids and Strong Bases
1) Calculate moles H+
2) Calculate moles OH-
3) Calculate moles XS H+ or OH-
4) Calculate XS [H+] or XS [OH-]
5) Calculate pH
Weak Acids
HA ⇌ H+ + A-
The acid dissociation constant, Ka
Ka = [H+] [A-] / [HA]
pKa = -log Ka
Ka = 10^-pKa
These expressions hold for weak acids at all times
The acid dissociation constant, Ka, notes
• Ka – has units mol dm-3
• Ka – the bigger the value, the stronger the acid
• pKa – the smaller the value, the stronger the acid
In a solution of a weak acid in water, with nothing else added:
a) [H+] = [A-]
b) [HA] ~ [HA]initial
Ka = [H+]^2 / [HA]
This expression ONLY holds for weak acids in aqueous solution with nothing else added
Reactions between weak acids and strong bases
When a weak acid reacts with a strong base, for every mole of OH- added, one mole of HA is used up and one mole of A- is formed.
Finding the pH in reactions between weak acids and strong bases
1) Calculate moles HA (it is still HA and not H+ as it is a weak acid)
2) Calculate moles OH-
3) Calculate moles XS HA or OH-
If XS HA
4) Calculate moles HA left and A- formed
5) Calculate [HA] leftover and [A-] formed
6) Use Ka to find [H+]
7) Find pH
OR
If XS OH-
4) Calculate [OH-]
5) Use Kw to find [H+]
6) Find pH
OR
If mol HA = OH-
4) pH = pKa of weak acid
Titration Calculations
These ionic equations which help a great deal in titration calculations. H+ + OH- ➔ H2O
2H+ + CO3^2- ➔ H2O + CO2
H+ + HCO3^- ➔H2O + CO2
H+ + NH3 ➔ NH4^+
pH Curves and Indicators
What are indicators and how do they work?
• Indicators are weak acids where HA and A- are different colours. HA ⇌ H+ + A-
• At low pH, HA is the main species present. At high pH, A- is the main species present.
• The pH at which the colour changes varies from one indicator to another.
• Note that universal indicator is a mixture of indicators and so shows many colours at different pHs.
What is a buffer solution?
• Buffer solution = solution that resists changes in pH when small amounts of acid or alkali are added.
• Note – the pH does change, just not by much.
• Acidic buffer solutions have a pH less than 7.
• Basic buffer solutions have a pH less than 7.
Acidic buffers
• Acidic buffer solutions are made from a mixture of a weak acid and one of its salts (i.e. HA and A-) (e.g. ethanoic acid & sodium ethanoate).
• An acidic buffer solution can also be made by mixing an excess of a weak acid with a strong alkali as it results in a mixture of HA and A-.
• The key in an acidic buffer solution is that the [acid] and [salt] are much higher than [H+].
Basic buffers
• Basic buffer solutions are made from a mixture of a weak alkali and one of its salts (e.g. ammonia & ammonium chloride).
• A basic buffer solution can also be made by mixing an excess of a weak alkali with a strong acid
• The key in a basic buffer solution is that the [base] and [salt] are much higher than [OH-].
To work out the pH of an acidic buffer
Use: [H+] = Ka x [HA] / [A-]
To work out the change in pH of an acidic buffer when acid is added
Use: HA (aq) ⇋ A- (aq) + H+ (aq)
Where: H+ added shifts the equilibrium to the left so A- decreases and HA increases
Calculate the original moles and then adjust these values by the moles of the acid added as above.
To work out the change in pH of an acidic buffer when alkali is added
Use: HA (aq) + OH- ⇋ A- (aq) + H2O(aq)
Where: OH- added reacts with HA which will decrease and produces more A- which will increase.
Calculate the original moles and then adjust these values by the moles of alkali added as above.
