20 - Acids / Bases And PH Flashcards
Bronsted Lowry acid definition
Proton donor
Bronsted Lowry base
Proton acceptor
Dibasic acid
Releases 2 protons per molecule
Conjugate acid base pair
Contains 2 species inter converted by transfer of a proton
How to calculate pH of strong acid
Use
pH= -log(H+)
Fully dissociates
How to find (H+)
Use 10^-pH
What is a strong acid
Completely dissociates in aqueous solutions
Strong acid examples
HCl
H2SO4
HNo3
HBr
Weak acid examples
CH3COOH
HNO2
H3PO4
Strong base examples
NaOH
KOH
Ca(OH)2
Weak bases examples
NH3
Why is pKa used
Too difficult to compare numbers with negative indices
Relationship between ka and pKa for stronger acids
Stronger acids
Larger Ka
Smaller pKa
Relationship between ka and pKa for weaker acids
Weaker acids
Lower Ka
Larger pKa
Equilibrium of weak acids
HA H+ + A-
Approximations used for weak acids
Only a bit of HA dissociates
- (HA)> (H)+ So … (HA)start = (HA)equilibrium
Assume dissociation of acid is much greater that dissociation of water - assume all H+ ions come from acid
- (H+) = (A-)
Why can’t the approximations of weak acids be used for strong
Strong acids dissociate more
The difference between
(HA)start = (HA)equilibrium Becomes more significant
Assumptions no longer valid
Determination of Ka for a weak acid experimentally
Preparing standard solution of weak acid
Measuring pH using pH meter
When are the approximations not true
Don’t work for very weak acids ph>6
- dissociation or water will be significant compared to dissociation of acid
Not justified for stronger weak acids with Ka > 10^-2
- doesn’t work when (H+) becomes significant
What is kw
The ionic product of water
Ions H+ and OH- multiplied together
What is the value of KW
1 x 10^-14 mol2dm-6
