acid-baseequilibria involve the transfer of protons between substances. therefore substances can be classified as acids or bases depending on their interaction with protons
a bronsted-lowry acid is a proton donor (e.g NH4+)
a bronsted-lowry base is a proton acceptor (e.g OH-)
a strong acid is an acid that completely dissociates to ions when in soltuion with a pH 3-5
a weak acid is an acid that only slightly dissociates when in a solution with pH 0-1
strong bases have pH 12-14
weak bases have a pH 9-11
pH = -log 10 [H+]
[H+] = 10^-pH
As temperature changes, the value of Kw changes
The forward reaction in the equilibrium of water
Is endothermic and is therefore favoured when temperature of the water is increased
As temperature increases
More H+ions are produced meaning the water becomes more acidic
on a pH titration curve, the equivalence point/ neutralisation point is the large vertical section on a graph
in strong acid-strong base reactions, the equivalence point is around pH7
in a strong acid-weak base, the equivalence point is <pH7 so it is more acidic
in a weak acid-strong base, the equivalence point is >pH7 (more basic)
Methyl Orange
Used for reactions with a more acidic neutralisation point, orange in acids and turns yellow at the neutralisation point
Phenolphthalein
Used for reactions with a more basic neutralisation point, pink in alkalis and turns colourless at the neutralisation point
Indicator colours in acidic and basic solutions
MethylOrange: red in acid,yellow in basic
Litmus: red in acid,blue in basic
Phenolphthalein: colourless in acid, pink in basic
a buffer solution is
a solution that is able to resist changes in pH when small volumes of acid or base are added
calculate buffer in acid+base
find number of moles in each species
calculate their concentration when at equilibrium using the total volume
use Ka to find [H+] and pH
calculate buffer in acid+salt
find the moles of the salt
use Ka to find pH
adding small amounts of H+ to a buffer solution will increase the concentration of the acid and the overall solution will get more acidic
adding small amounts of OH- to a buffer solution decreases the concentration of acid in the solution and makes the overall solution more basic
buffer solutions are common to keep systems regulated
strong acids and bases completely dissociate in water whereas weak acids and bases don’t; they only partly dissociate into their own ions
strong acid examples
HCl,H2SO4, H3PO4
strong bases examples
NaOH,CaCO3,Na2CO3
weak acid examples
CH3COOH (ethanoic), any organic acid
weak base examples
NH3
Ka expression
acid HA: HA<->H+ + A-
Ka = [H+][A-]
[HA]
a titration is the addition of an acid/base of known titration to base/acid of unknown titration to determine the concentration. an indicator/pHmeter is used to show that neutralisation has occured
around the equivalence point there is a large and rapid change in pH except in the weak-weak titration
the end point is the volume of acid or alkali added when the indicator just changes colour. if the correct indicator is used then the equivalence point=end point
properties of a good indicator for a reaction
sharp colour change (not gradual) - no more than 1 drop needed for a colour change