Acids and Bases A2 Flashcards
What is a Bronsted-Lowry acid?
A proton/H+ donor
What is a Bronsted-Lowry base?
A proton/H+ acceptor
What is the acid dissociation constant?
Ka= [H+(aq)][A-(aq)]
[HA(aq)]
How does the acid dissociation constant tell us the strength of an acid?
A large Ka value shows that the extent of the dissociation is large so the acid is strong
what are the units for the acid dissociation constant?
mol dm-3
What is pH defined as?
pH=-log10[H+(aq)]
How can H+(aq) be calculated from pH?
H+=10^-pH
How do you calculate the pH of a weak acid and why?
Ka = [H+(aq)]^2
[H20(l)]
Due to the negligible amount of ionisation of water the concentration of H+ is approx = to the concentration of OH-
How do you calculate pKa and what is it?
pKa = -log10Ka
It is basically the same thing as pH, it’s just another way to find out the strength of an acid/alkali. So a low value of Ka matches to a high value of pKa
How do you find the ionic product of water?
Kw= [H+(aq)][OH-(aq)]
What are the units for Kw?
mol^2dm-6
At 25 degrees Celsius, what is the value of Kw?
Kw = 1.0 x 10^-14 mol^2dm-6
In water what are the concentrations of [H+(aq)] and [OH-(aq)] ions?
They are the same: 10^-7 moldm-3
What can we refer to pure water as and why?
Kw = [H+(aq)]^2
Since in pure water the conc of H+ ions = OH- ions
How to calculate the pH of strong bases/alkalis?
We need to use Kw to get the [H+(aq)] value to put into the pH equation ( Kw = [H+(aq)] [OH-(aq)] )
also, we need to know that the concentration of the base = the concentration of the OH- ions
In strong Alkalis/bases why does the conc of base = the conc of OH- ions?
Most strong bases disassociate to produce 1 OH- ion for every base molecule
What does a titration show us?
The endpoint of a titration, as well as the change in pH across a titration
What is the endpoint of a reaction?
When neutralisation occurs
How do you know how to pick a suitable indicator for your titration?
You need to make sure that it changes colour within the end point of your titration
What indicator(s) can be used for the titration between a strong acid and alkali?
Phenolphthalein and Methyl Orange
What indicator(s) can be used for the titration between a strong acid and a weak alkali?
Methyl Orange
What indicator(s) can be used for the titration between a weak acid and a strong alkali?
Phenolphthalein
What indicator(s) can be used for the titration between a weak acid and alkali?
Neither Phenolphthalein nor Methyl Orange
What is Methyl Orange like at low and high pH values?
RED: Low pH values
YELLOW: High pH values
What is Phenolphthalein like at low and high pH values?
COLOURLESS: Low pH values
PINK: High pH values
Between what pHs does Methyl Orange change colour?
pH 3 - 4.5
Between what pHs does Phenolphthalein change colour?
pH 8.2 - 10
What do buffer solutions do?
A buffer solution maintains an approximately constant pH, despite the dilution or addition of small amounts of acid or base
What are there in any buffer solution?
2 equilibrium equations; they co-exist together
Acidic buffer solutions contain a weak acid and the salt of that weak acid.
Basic buffer solutions contain a weak base and the salt of that weak base.
What happens to salts in equilibrium?
They dissociate fully
