AQA A-Level Chemistry: acids and bases
What is pKa, why is it used?
-log(Ka)
pH
-log[H+]
Assumptions for Ka
1.)Weak bases and acids dissociate only slightly in aqueous solution 2.) [H+] from dissociation of HA not water 3.)[A-] comes from HA unless salt present
What is the value of Kw @ r.t.p.
1x10^-14
Basic buffer equation
B + H+ <-> BH+
Diprotic base example
Ba(OH)2
Example of a weak acid and weak base
Ethanoic acid is weak, ammonia is a weak base.
Dibasic acid example
H2SO4,
Acidic buffer equation
HA <-> H+ + A-
Example of a strong acid and strong base
HCL strong acid, NaOH strong base.
Monobasic acid example
HCl, HNO3, HBr
Kw
Kw = [H+] [OH-]
How can you manipulate the Kw equation if using pure water? Why does this work?
Kw = [H+]^2 Because OH- and H+ concentrations are equal.
Indicator for Strong acid - Weak base
Methyl orange
Indicator for Strong acid - Strong base
Methyl orange/phenolphthalein
What do you have to do when calculating pH of a dibasic acid?
Multiply the concentration of the acid by 2
Monoprotic base example
NaOH
Indicator Weak Acid - Weak Base
No suitable indicator
What is a monobasic acid?
One mole of acid reacts with one mole of base
What is a dibasic acid?
One mole of acid reacts with two moles of base
What is a monoprotic base?
One mole of base reacts with one mole of acid
What is a diprotic base?
One mole of base reacts with two moles of acid
Indicator Strong base- Weak acid
Phenolphthalein
What is a Bronsted-Lowry base?
Proton acceptor
What is a Bronsted-Lowry acid?
Proton donor
Explain why the value of Kw increases as temperature increases
Reaction is endothermic, Therefore reaction shifts to right to decrese temperature change
What does an acidic buffer solution contain? + Example
Solution of weak acid and its salt e.g. ethanoic acid + sodium ethanoate
What does a basic buffer solution contain? + Example
Solution of weak base and its salt e.g. ammonia + ammonium chloride
How are strong and weak acids and bases different?
Strong acids/ bases completely dissociate into ions when in solution. Weak acids/ bases only partially dissociate.
Why is a logarithmic scale used for pH?
The concentration of hydrogen ionsin aqueous solution covers a very wide range
Why doesn't Kw include concentration of water?
[H2O] is constant [H2O] is very high compared to [OH-] and [H+]
What is a buffer?
a solution that resists changes in pH on addition of small amounts of acid or base
What does Kw stand for?
The ionic product of water