21.3 Weak acids and bases Flashcards
What are weak acids and bases
They only slightly Ionise when dissolved in water
Eg ethanoic acid
In equation
CH3COOH <—> H+ + CH3COO-
explain how it dissociates
Most molecules remain bonded, only some dissociate into ions
So reaction stays at equilibrium because it doesn’t fully dissociate
Explain equation
NH3 + H2O <—> NH4+ + OH-
Ammonia acts as a base and only partially dissociates so the NH3 can gain a proton, and water loses one
What is equilibrium for a weak acid dissociating
If weak acid is HA
HA(aq) <—-> H+(aq) + A-(aq)
kc = [H+][A-] / [HA]
However weak acids are usually given symbol Ka and called the acid dissociation constant
ka = [H+(aq)][A-(aq)] / [HA(aq)]
What does the size of Ka represent
The larger the value of ka, the further the equilibrium is to the right, because there are more products than reactants.
So the larger the value of ka, more of the acid is dissociated so the stronger it is.
Ka has units moldm^-3
How do you calculate PH of weak acids
When calculating strong acids, you assume they are fully dissociated
However in weak acids you must use acid dissociation expression to calculate [H+]
Calculate PH of 1moldm-3 ethanoic acid
CH3COOH <—> CH3COO-(aq) + H+(aq)
Value of ka for ethanoic is 1.7 x 10^-5
Ka = [CH3COO-][H+] / CH3COOH
But as each molecule of CH3COOH that dissociates produces one CH3COO- ion and one H+ ion:
[CH3COO-(aq)] = [H+(aq)]
Since the degree of dissociation of ethanoic acid is so small, the H+ concentration is very small so to a good approximation,
1-[H+] = 1
Ka= [H+(aq)]^2 / 1
so 1.7 x 10^-5 = [H+]^2
so [H+] = 4.12 x 10^-3 moldm-3
-log10(4.12 x 10^3) this equals 2.385 is PH
What is pKa of a weak acid referred to as
pKa = -log10Ka
so p is -log10 of
So it is like pH, but for weak acid
Why is [H2O] not shown in Kw expression
Kw = [H+][OH-]
- The concentration of H2O is almost constant so we don’t include it
