Chemistry’s contribution to the Olympic games … Acid base properties of salt solutions
Bizarro by Dan Piraro Salt ionic solid
Cation is not H , anion is neither OH nor O .
Examples: NaCl, K 2CO 3 Al(NO )3 3
NaCl (s) Na (aq)+ Cl (aq)
Salt solution acidic, basic or neutral ?
Read 19.1,9.2 1. Cation effect on pH
Practice 2. Anion effect on pH
PS 4 is posted online. You can solve Q 1–4
3. Combined effect of the cation and the anion
Solutions to term exam 1 will be posted when I get a
chance to type them.
Cation effect Anion effect
Cations derived from strong bases (group 1 + heavier Anions derived from strong acids (Cl , Br , I , NO ,
group 2 cations) no effect ClO 4 SO 4 ) no effect
Why ? Why ?
+ + Na +
Na + H 2 NaOH + H + H2O Cl + H 2 HCl + OH Cl + H 2
NaOH is a strong base so it immediately dissociates to HCl is a strong acid so it immediately dissociates to H
Na +and OH and the OH and H recombine to form and Cl and the H and OH recombine to form H 2O.
H 2. So no reaction for Na and water.
So no reaction for Cl and water.
All other cations react with water to yield weakly acidic
All other anions react with water to yield weakly basic
NH ,4Transition metals, Al all weakly acidic F– + H O HF + OH basic
(aq) 2 (aq) (aq)
NH 4 (aq) H 2 H 3 + NH 3 (aq) weakly acidic 5 6
Combined effect of cations and anions
l l a r eno i nAno i t aC Find the pH of a 1M NH 4I solution
NH I acidic ------ acidic
4 Complete dissociation NH 4 NH 4
Zn(NO )3 2 acidic ------ acidic
I is derived from a very strong acid.
KClO 4 ------ ------ neutral
Na 3O 4 ------ basic basic I + H O HI + OH –
NH F acidic basic ???
HI is a very strong acid and it immediately dissociates,
Rarely would the two effects exactly cancel each out
HI H + I –
When K a> K b acidic
H + OH H O 2
K a< K b basic
K a= K b neutral Net reaction: I + H2O I + H 2 neffect
K aNH )4= 5.5 x 10 Remember: ions derived from strong acids or strong
– Kw 1 x 10–14 –11
K bF ) = = –4 = 1.4 x 10 bases have no effect on the pH of the solution.
K aHF) 7 x 10 +
NH 4s a weak acid and we need to consider its
K aNH )4> K (Fb) – acidic
dissociation in water.
+ + +
NH 4 + H2O NH 3 H O 3 Ka(NH )4