Precipitation Reactions

Precipitation Reactions

A precipitation reaction occurs when water solutions of two different ionic compounds are mixed and an

insoluble solid

insoluble solid separates out of solution.

KCl + AgNO

AgCl + KNO

The precipitate is itself ionic; the cation comes from one

solution and the anion from another.

• Solubility of a compound = concentrations of a concentrations of a soluble species at equilibrium with its insoluble soluble species at equilibrium with its insoluble

form.

form.

• If the compound is sparingly soluble, it will produce cation & anion.

• AgCl slightly dissolved in water. So AgCl has a specific solubility, s = solid phase aq = aqueous phase

AgCl (s) Ag

(aq)+ Cl

(aq)

• The equilibrium constant for the reaction is known as equilibrium constant solubility product constant.

K

(AgCl) = [Ag

][Cl

]

• Concentration of any solid (AgCl) is constant and is combined in the equilibrium constant to give K

• Solubility product constants are used to describe Solubility product constants

saturated solutions of ionic compounds of relatively low solubility.

• A saturated solution is in a state of dynamic

equilibrium between the dissolved, dissociated, ionic

compound and the undissolved solid.

What is the solubility of AgCl?

AgCl  Ag

+ Cl

K

= [Ag

][Cl

] = 1.2 10

dur.

Solubility of AgCl:

K

= [Ag

][Cl

] = 1.2 10

den;

[Ag

] = [Cl

] = = 1.1 10

mol/L

that a saturated solution contains only about 1.1 10

moles

of AgCl per liter of water.

Solubility of Ag

CrO

Ag

CrO

 2Ag

+ CrO

K

= [Ag

]

[CrO

] = 1.1 10

dir.

(2x)

. x = 1.1 10

4x

= 1.1 10

x = solubility (s) = 6.5 10

mol/L

Solubility of Ba(IO₃)₂ is 4.5 10^–4 mol/L. What is the Ksp value?

Ba(IO3)2  Ba²⁺ + 2IO3–

K_sp = [Ba²⁺][IO₃^–]² = (4.5 10^–4 )(2 x 4.5 10^–4)² = 3.65 10^–10

Which halide precipitates out first?

•K

AgCl = 1.2 10

,

•K

AgBr = 5.0 10

,

•K

AgI = 4.5 10

•

A mixture of 0.01M Sr

ve 0.1M Ca

is titrated with (NH

)

C

O

Which oxalate precipitates out first?

• K_sp SrC₂O₄ = 5.6 10^–10 K_sp CaC₂O₄ = 1.3 10^–9

[Sr²⁺] [C₂O₄^2–] = 5.6 10^–10 [Ca²⁺] [C₂O₄^2–] = 1.3 10^–9 (0.01) [C₂O₄^2–] = 5.6 10–10 (0.1) [C₂O₄^2–] = 1.3 10^–9 [C₂O₄^2–] = 5.6 10–8 [C₂O₄^2–] = 1.3 10^–8 Answer: CaC2O4

A mixture of 0.01M Cl

ve 0.01M

CrO

is titrated with Ag

Which one precipitates out first?

• Ksp AgCl = 1.2 10^–10 ve Ksp Ag2CrO4 = 1.1 10^–12

Ksp AgCl = [Ag⁺] (0.01) = 1.2 10^–10 den [Ag⁺] = 1.2 10^–8 M Ksp Ag2CrO4 = [Ag⁺]² (0.01) = 1.1 10^–12 den [Ag⁺] = 1.0 10^–5 M

Answer: AgCl

10

SnS(s) Sn

(aq)

+ S

(aq)

26 2

2

sp

 [Sn

][S

]  1.0 x 10

K

s

s

s 



 ( )( ) 10

x

1.0

M s  1.0 x 10

Calculate the solubility of SnS at 25°C.

11

Pb

(aq)

+ CrO

(aq) PbCrO

(s)

M

s

5

10 x g 1.4

323.2 x mol

L

g 10

x

4.5





] ][CrO

[Pb

2 sp

 



 s K

14 7

7

sp

 [1.4 x 10

M ][1.4 x 10

M ]  2.0 x 10

K

The solubility of lead(II) chromate (PbCrO4) is 4.5 x 10^5 g/L. Calculate the solubility product (Ksp) of lead(II) chromate.

12

• Precipitation can be predicted

• Compare the reaction quotient (Q) to the K_sp

where “i” designates the initial concentration

• Q < Ksp no precipitate will form

• Q > Ksp precipitate will form until Q = Ksp

n i i

Q  [M

] [X

]

MX

(s) M ⇄

(aq) + nX

(aq)

13

BaSO

(s) Ba

(aq) + SO

(aq)

2 10 4 2

sp

 [Ba

][SO

]  1.1 x 10

K

10

x 1.1 )

(0.00100 )

( )

)(0.00100

( s M  s  s M 

M s  1.1 x 10

Calculate the molar solubility of BaSO4 in 0.0010 M Na2SO4.