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4.3 Precipitation Reactions, Study notes of Chemistry

– Precipitation reactions are classified as double. replacement (metathesis) reactions – exchange of. ions leads to an insoluble combination of ions. Page 2. ...

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4.3 Precipitation Reactions
Formation of an
insoluble product
(precipitate) after
mixing of two
electrolyte solutions
The driving force of
precipitation reactions
is the elimination of
ions from the solution
by formation of an
insoluble product
Example:
When mercury(I) nitrate and potassium
phosphate solutions are mixed, mercury(I)
phosphate precipitates. Write the net ionic
equation.
mercury(I) Hg22+ Hg2(NO3)2
Skeletal eq:
Hg2(NO3)2(aq) + K3PO4(aq)
(Hg2)3(PO4)2(s) + KNO3(aq)
Overall balanced eq:
3Hg2(NO3)2(aq) + 2K3PO4(aq)
(Hg2)3(PO4)2(s) + 6KNO3(aq)
Complete ionic eq:
3Hg22+ + 6NO3-+ 6K++ 2PO43-
(Hg2)3(PO4)2(s) + 6K++ 6NO3-
Net ionic eq:
3Hg22+ + 2PO43- (Hg2)3(PO4)2(s)
Predicting the outcome of precipitation
Precipitation reactions are classified as double
replacement (metathesis) reactions – exchange of
ions leads to an insoluble combination of ions
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4.3 Precipitation Reactions

  • Formation of an

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

  • 3

+ 6K

+^

+ 2PO

3- 4

(Hg

(PO 3

(s) 2

+ 6K

+^

+ 6NO

  • 3

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

  • 3

, 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

  • release hydrogen ions,

H

+^ (aq)

[or

H

O 3

+^ (aq)]

, in water solutions

  • Acidic hydrogen atoms in molecules
    • can be released as H

+^ ions

  • formulas normally begin with the acidic Hs Examples: ⇒

HCl, H

SO 2

, HCN, …. 4

HCl(g)

H

+^

  • Cl

HCl(g) + H

O(l) 2

H

O 3

+^

  • Cl

H

O 2

•^

Arrhenius bases

  • release hydroxide ions,

OH

  • , in water solutions

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

  • 4 and

OH

NH

(g) + H 3

O(l) 2

NH

  • 4

+ OH

H

O 2

•^

Strong acids

  • almost completely ionized in

aqueous solutions ⇒

HBr

(g)

+ H

O 2

(l)

H

O 3

+^

  • Br
  • (~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

+^

  • Cl

-^

+ NH

(aq) 3

NH

  • 4
  • Cl

H

+^

+ NH

(aq) 3

NH

  • 4
  • H

+^

is present in the form of

H

O 3

H

O 3

+^

+ NH

(aq) 3

NH

  • 4

+ 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)

  • 2HCl

(aq)

ZnCl

(aq) 2

+ H

S 2

(g)

ZnS

(s)

+ 2H

+^

  • 2Cl

-^ →

Zn

2+

  • 2Cl

-^

+ 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-