T12 Acid-base equilibria Flashcards
acids
proton donors, releasing H + ions in water to form hydroxonium ions (H3O+)
bases
proton acceptors
take H+ ions from water molecules in solution to form OH- ions
strong acids /bases
dissociate completely in water
e.g HCl/ NaOH
weak acids/bases
dissociate partially in water
forms an equilibrium lying to the left
conjugate pairs
species linked by the transfer of a proton
conjugate base
species that has lost a proton
conjugate acid
species that has gained a proton
neutralisation reactions
when acids and base react together to form water and a salt.
neutral solution
equal conc of H+ and OH- ions
standard enthalpy change of neutralisation
enthalpy change when solutions of an acid and base react under standard conditions to produce one mole of water.
always negative as is exothermic
weak acid base neutralisation enthalpy
vary as only dissociate partially/reversibly and ions get used up quickly
strong acid base neutralisation enthalpy
no dissociation enthalpy just the reaction of H+ and OH- ions
pH equation
-log10(H+)
pH define
measure of hydrogen ion concentration
monoprotic acids
each mole of acid produces one mole of hydrogen ions
acid conc for strong, monoprotic acids
concentration of H+ ions
how do you find H+ conc from the pH
10 to the negative pH
polyprotic acids
acids with more than one proton available to release into solution.
how do you find pH of a weak acid
Ka
Ka
acid dissociation constant
Ka equation
(H+ conc * acid conc at eq)/conc of acid at start
Kc
equilibrium constant
Kw
ionic product of water
Kw equation
concentration of OH- * concentration of H+
what’s Kw units
mol^2dm^-6
Kw of pure water
conc of H+ squared
Kw at 25 degrees celsius or 298 K
1*10^-14
pKw
-log10(Kw)
Kw from pKw
10^-pKw
pKa
-log10(Ka)
how to calibrate a pH meter
place in deionised water and set to 7 and repeat w standard pH solutions, washing with deionised water after every one.
effects of diluting by a factor of 10 in strong acids
increases pH by 1
effects of diluting by a factor of 10 in weak acids
increases pH of 0.5
basics of how to titrate
measure out base with a pipette and put in a flask with an indicator.
rinse burette w acid standard solution before filling
carry out rough titration and note end point
repeat to receive concordant results (within 0.1 cm cubed of each other)
equivalence line
almost vertical line on a titration curve upon which the end point lies in which all the acid has been neutralised
methyl orange
color at low/high pH
pH of colour change
red at low, yellow at high
3.1-4.4
phenolpthalein
color at low/high pH
pH of colour change
colourless at low
pink at high
8.3-10
half-equivalence
the stage of the titration when all of the acid has been neutralised
pH at half-equivalence is actually the pKa for the acid
buffer
solution that minimises change in pH when small amounts of acid or base are added.
acidic buffer composition
a weak acid and a conjugate base
how do you make acidic buffers
mix an excess of weak acid with a strong base
mix a weak acid with the salt of its conjugate base
what happens when you mix excess weak acid with a strong base
all of the base reacts w the acid
leftover weak acid slightly dissociates
what happens when you mix a weak acid with the salt of its conjugate base
salt will fully dissociate into its ions when it dissolves
ethanoic acid partially dissociates
