4.7 Acid/Base Equilibria Flashcards
What are Brønsted-Lowry acids?
Proton donors that realise H+ ions when they’re mixed with water.
What ares Brønsted-Lowry bases?
Proton acceptors that take hydrogen ions from water molecules.
What happens to H+ ions when they are released into water.
They form hydroxonium ions, H3O+.
Give the equation for the dissociation of the general acid HA.
HA(aq) + H2O(l) → H3O+(aq) + A-(aq)
Give the equation of the base B accepting a proton.
B(aq) + H2O(l) → BH+(aq) + OH-(aq)
Give an example of a strong acid and a strong base and explain what makes them “strong”.
Hydrochloric acid and sodium hydroxide. They ionise almost completely in water.
What is a weak acid? Give an example.
Weak acids ionise only very slightly in water, setting up an equilibrium. Citric acid.
Give the equation for the dissociation of ethanoic acid.
CH3COOH(aq) ⇋ CH3COO-(aq) + H+(aq)
What is a weak base? Give an example.
Weak bases only slightly ionise in water. Ammonia is a weak base.
Give the equation for the dissociation of ammonia.
NH3(aq) + H2O(l) ⇋ NH4+(aq) + OH-(aq)
When can acids lose their protons?
When there is a base to accept them.
Give the equation for when the acid, HA, transfers a proton to base, B.
HA(aq) + H2O(l) ⇋ H3O+(aq) + A-(aq)
Name their two conjugate pairs in this equilibrium:
HCl(aq) + H2O(l) ⇋ H3O+(aq) + Cl-(aq)
HCl(aq) and Cl- (acid/base) and H2O(l) and H3O+(aq) (base/acid).
Give the equation for the dissociation of water.
2H2O(l) ⇋ H3O+(aq) + OH-(aq)
Write an expression for the equilibrium constant of the dissociation of water.
Kc = ([H+][OH-]) / [H2O]
What do you get if you multiply the constant Kc by [H2O], which is also constant.
[H+][OH-], the ionic product of water, Kw.
Give the expression for Kw.
Kw = [H+][OH-]
Kw always has the same value for…
An aqueous solution at a given temperature.
What is the value of Kw at the standard temperature?
At 298K, Kw = 1.0 x 10^-14 mol^2 dm^-6
What is pKw equal to?
-logKw
What is the advantage of expressing Kw as pKw?
They’re a reasonable size to work with.
What is pKw at 25ºC?
14
What is the definition of a neutral solution?
A solution in which [H+] = [OH-]
What is a solution where [OH-] is greater than [H+]?
Alkaline.
Give the equation for pH.
pH = -log[H+]
Describe the pH scale.
It goes from 0 (strongly acidic) to 14 (strongly alkaline) and pH 7 is neutral.
What is [H+] for strong monoprotic acids?
[Acid]
What is the pH of 0.1 mol dm^-3
pH 1.0
How can you find [H+] of a strong base if you know its concentration?
[H+] = Kw / [OH-]
Give the original expression for Ka.
Ka = ([H+][A-]) / [HA]
Give the simplified expression for Ka.
Ka = [H+]^2 / [HA]
Give two assumptions made when using the expression for Ka.
Only a small amount of HA dissociates so [HA] start = [HA] equilibrium. All of the H+ ions come from the acid so [H+] = [A-].
What are the units of Ka?
mol dm^-3
How would you use Ka to find the pH of a weak acid?
[H+] = √(Ka [HA]) then use -log[H+]
How can you calculate Ka from pKa?
Ka = 10^(-pKa)
What happens to the pH of a strong acid when you dilute it by a factor of 10?
It increases by 1.
What happens to the pH of a weak acid when you dilute it by a factor of 10?
It increases by 0.5.
Describe the shape of a strong acid, strong alkali titration curve.
S shaped curve starting around pH 1 and finishing around pH 13.
Describe the shape of a strong acid, weak alkali titration curve.
S shaped curve starting around pH 1 and finishing around pH 8.
Describe the shape of a weak acid, strong alkali titration curve.
S shaped curve starting around pH 4 and finishing around pH 13, with a small kink at the beginning.
Describe the shape of a weak acid, weak alkali titration curve.
S shaped curve starting around pH 4 and finishing around pH 9, with no vertical region.
Give the colours of the indicator methyl orange and state the pH range where the colour changes.
Red at low pH, changes around 3.1 - 4.4 to yellow at a high pH.
Give the colours of the indicator Phenolphthalein and state the pH range where the colour changes.
Colourless at low pH, changes around 8.3 - 10 to pink at a high pH.
Which indicator would you use for strong acid, strong alkali titration?
Methyl orange or phenolphthalein.
Which indicator would you use for strong acid, weak alkali titration?
Methyl orange.
Which indicator would you use for weak acid, strong alkali titration?
Phenolphthalein.
Which indicator would you use for weak acid, weak alkali titration?
Neither methyl orange nor phenolphthalein. A pH meter would be more suitable.
What is the half-equivalence point?
Where half of a weak acid has been neutralised by a strong alkali.
What happens to [A-] at the half-equiclance point?
[A-] = [H+]
What is Ka equal to at the half-equivalence point?
Ka = [H+]
How can you find pKa at the half-equivalence point?
pKa = pH
What is a buffer solution?
A solution that resists changes in pH when small amounts of acid of alkali are added.
What are acidic buffers made from?
A weak acid and one of its salts.
Give an example of an acidic buffer.
Ethanoic acid and sodium ethanoate.
What happens to the acid salt of a buffer when it dissolves?
It fully dissociates into its ions.
What is in an ethanoic acid buffer solution?
A large amount of ethanoate ions and a large amount of undissociated ethanoic acid molecules.
What happens to an ethanoic acid buffer solution when you add small amounts of acid?
Most of the extra H+ ions combine with the ethanoate ions to form undissociated acid, shifting the equilibrium and reducing the h+ concentration to near its original value.
What happens to an ethanoic acid buffer solution when you add small amounts of alkali?
The OH- ions react with H+ to form water, removing h+ ions form the solution. This causes more acid to dissociate, replacing most of the H+ ions lost.
What are alkaline buffers made form?
A weak base and one of its salts.
Give an example of an alkaline buffer.
Ammonia solution and ammonium chloride.
What happens when a small amount of acid is added to an ammonia buffer.
The acid adds H+ ions, but most of them react with the NH3, shifting the equilibrium and reducing the H+ concentration to near its original value.
What happens when a small amount of alkali is added to an ammonia buffer?
The OH- concentration increases and the OH- ions react with H+ ions to form water. The NH4+ molecules dissociate to generate the lost H+ ions, resisting the change in pH.
What causes the distinctive shape of a weak acid, strong base titration curve?
Buffer action.
In terms of buffer action, what happens when sodium hydroxide is added to ethanoic acid?
The pH initially starts to change because NaOH is a strong alkali but then sodium ethanoate is formed and the ethanoic acid gradually dissociates to replenish the H+ ions, causing the curve to level off.
Give two examples of buffer action in biological environments.
Constant pH in cells. Blood needs to be kept at pH 7.4.
Give the equation for the buffer equilibrium occurring in cells to maintain a constant pH.
H2PO4- ⇋ H+ + HPO4`2-
Dihydrogen phosphate and hydrogen phosphate.
Give the equations for the buffer equilibriums occurring in the blood to maintain pH 7.4.
H2CO3(aq) ⇋ H+(aq) + HCO3-(aq)
H2CO3(aq) ⇋ H2O(l) + CO2(g)
Give three examples of buffers used in foods.
Sodium citrate, phosphate ions and benzoate ions.
Why are buffers used in foods?
Changes in pH can be caused by bacteria and fungi and this causes the food to deteriorate.
