topic 12 - acid base equilibria Flashcards
acid
proton donor
positive charge/becomes more negative
higher pH weaker the acid
carboxylic - weak
others strong
base
proton acceptor
lone pair/becomes more positive
higher pH stronger base
oh - strong base
NH3 - weak base
conjugate base pair
strong reactactant forms weak opposite product
monoprotic/monobasic
donates/accepts 1 proton
diprotic/dibasic
donates/accepts 2 protons
e.g sulfuric acid
amphoteric
acts as either a base or an acid
strong acid
completely dissociates in an aqueous solution
conc = [H+]
pH = -lg[H+]
[H+] = 10^-pH
pH increases by a factor of one for each 10x decrease of conc
weak acid
partly dissociated <10%
HA ⇌ H+ + A-
[H+] = square root(Ka x [acid])
pH=-lg[H+]
pH increases 0.5 for each 10x decrease in conc
dissociation of water
H2O ⇌ H+ + OH-
Kw = [H+][OH-]
pKw = -lgKw
equivalence point
when an acid and base have reacted together in the exact proportions as dictatef by the stoichiometric equation
strong acid strong base
equivalence 7
weak acid strong base
equivalence 8-9
conjugate base makes it alkaline
strong acid weak base
equivalence <7
- conjugate acid makes it acidic
weak acid weak base
point of inflexion
indicators
either weak acids or weak bases
The best indicator for a particular titration is the one with pKIn close to pH at equivalence
buffer solution
A buffer solution’s pH is almost unchanged on the addition of small amounts of acid or base
Buffer solutions can be made by mixing:
A buffer solution can be made from a weak acid and a salt of the acid.
The acid is partially dissociated and the salt fully dissociated
The resulting solution has a high concentration of undissociated acid molecules, CH3COOH(aq), and a high concentration of the conjugate base, CH3COO-(aq)
calculating the ph of buffer sol
use Ka and conc of acid and salt
Assume the dissociation of the acid is negligible
When mixed the concentrations of each solution are halved as the volume doubles.
pH = pKa (-lgKa) +log [salt]/ [acid]
[H+]=Ka x [acid]/[salt]
pH=-lg[H+]
making buffer
To make a buffer with a pH < 7 - use a mixture of a weak acid and its conjugate base
e.g - sodium hydroxide and ethanoic acid
To make a buffer with a pH > 7 - use a mixture of a weak base and its conjugate acid
eg strong acids
HCl
strong base
NaOH
eg weak acids
CH3COOH
-carboxylic
eg weak bases
NH3
phenolphthalein indicator
acid-colourless
neutral - pale pink
base-pink
methyl orange
acid - red
neutral - orange
base - yellow
only with strong acids
when can you calculatte Kc using equilibrium amounts
when moles are equal on both sides as volume cancels
enthlapy change of reaction of diff conc of acid
strong acids have high values
dibasic acids have 2x enthlapy change for 1 mol as 2 mols are formed
weak acids are lower as some energy is required to break o-h bonds to realse h+ ions as includes dissociation
good indicator
ph at equivilence point is very close to pka of indicator
ph region of indication lies completely in vertical region on curve
buffer action
carbonate ions react with h+ ions
hydrogencarbonate ions react with oh- ions
strong base
Kw = [H+][OH-]=1x10^-14
[H+
