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4.3 Precipitation Reactions
insoluble product(precipitate
) after
mixing of twoelectrolyte solutions– The driving force of
precipitation reactionsis the elimination ofions from the solutionby formation of aninsoluble product
Example:
When mercury(I) nitrate and potassiumphosphate solutions are mixed, mercury(I)phosphate
precipitates
. Write the net ionic
equation.
mercury(I)
→
Hg
2+ 2
→
Hg
(NO 2
) 3
2
⇒
Skeletal eq: Hg
(NO 2
(aq) 2
+ K
PO 3
(aq) 4
(Hg
(PO 3
(s) 2
+ KNO
(aq) 3
⇒
Overall balanced eq: 3Hg
(NO 2
(aq) 2
+ 2K
PO 3
(aq) 4
(Hg
(PO 3
(s) 2
+ 6KNO
(aq) 3
⇒
Complete ionic eq: 3Hg
2+ 2
+ 6NO
+ 6K
+^
+ 2PO
3- 4
(Hg
(PO 3
(s) 2
+ 6K
+^
+ 6NO
⇒
Net ionic eq: 3Hg
2+ 2
+ 2PO
3- 4
(Hg
(PO 3
(s) 2
Predicting the outcome of precipitation
- Precipitation reactions are classified as
double
replacement
(metathesis) reactions – exchange of
ions leads to an insoluble combination of ions
Example:
Predict the outcome of the mixing of silvernitrate and potassium carbonate solutions. ⇒
Ions present in the solution: Ag
+ , NO
, K
+ , CO
2- 3
⇒
consider all possible combinations of ions to findif an insoluble product can form: Ag
+^
and CO
2- 3
form insoluble Ag
CO 2
3
⇒
Net ionic eq:
2Ag
+^
+ CO
2- 3
Ag
CO 2
(s) 3
Note:
The net ionic equation can be predicted
directly from the formula of the precipitate.
4.4 Acid-Base Reactions
- Acids - sharp, sour taste; Bases - soapy, bitter taste -^
Arrhenius acids
H
+^ (aq)
[or
H
O 3
+^ (aq)]
, in water solutions
- Acidic hydrogen atoms in molecules
+^ ions
- formulas normally begin with the acidic Hs Examples: ⇒
HCl, H
SO 2
, HCN, …. 4
HCl(g)
H
+^
HCl(g) + H
O(l) 2
H
O 3
+^
H
O 2
•^
Arrhenius bases
OH
Examples: ⇒
NaOH
dissolves in water and dissociates to
Na
+^
and
OH
NaOH(s)
Na
+^
+ OH
Ammonia gas,
NH
, dissolves in water and 3
produces
NH
OH
NH
(g) + H 3
O(l) 2
NH
+ OH
H
O 2
•^
Strong acids
- almost completely ionized in
aqueous solutions ⇒
HBr
(g)
+ H
O 2
(l)
H
O 3
+^
- (~100% ionized)
- The strong acids in aqueous solution are:
HCl
(aq)
, HBr
(aq)
, HI
(aq)
, HNO
, H 3
SO 2
HClO
, and HClO 4
3
•^
Weak acids
- only partially ionized in
aqueous solutions (HF, H
S, organic acids ... ) 2
CH
COOH 3
(aq)
+ H
O 2
(l)
H
O 3
+^
+ CH
COO 3
-^
(~1% ionized)
- Net ionic equations for reactions between
strong acids and weak bases Example: HCl
(aq)
+ NH
(aq) 3
NH
Cl 4
(aq)
NH
(aq) 3
→
weak base (only partially ionized)
H
+^
-^
+ NH
(aq) 3
NH
H
+^
+ NH
(aq) 3
NH
+^
is present in the form of
H
O 3
H
O 3
+^
+ NH
(aq) 3
NH
+ H
O 2
(l)
⇒
transfer of a proton from H
O 3
+^ to NH
3
Gas Formation Reactions • Reactions of salts of weak or volatile acids
with strong acids Example:
ZnS
(s)
(aq)
ZnCl
(aq) 2
+ H
S 2
(g)
ZnS
(s)
+ 2H
+^
-^ →
Zn
2+
-^
+ H
S 2
(g)
ZnS
(s)
+ 2H
+^
Zn
2+
+ H
S 2
(g)
-^
H
+^
is present in the form of H
O 3
ZnS
(s)
+ 2H
O 3
+^
Zn
2+
+ H
S 2
(g)
+ 2H
O 2
(l)
⇒
transfer of a proton from H
O 3
+^
to S
2-
