4.7 Acid/Base Equilibria
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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.