Equilibrium in SolutionsEquilibrium in Solutions

When discussing whether or not an ionic When discussing whether or not an ionic solid dissolves well into water, we talk solid dissolves well into water, we talk about its Solubility Product Constant, orabout its Solubility Product Constant, or

KKspsp

Which tells us the ratio of dissolved solid Which tells us the ratio of dissolved solid versus undissolved solid at equilibrium.versus undissolved solid at equilibrium.

Consider the following reaction:Consider the following reaction:

BaSOBaSO4(s)4(s) <---------> Ba <---------> Ba2+2+(aq)(aq) + (SO + (SO44))2-2-

(aq)(aq)

This is a reaction where an ionic solid is being This is a reaction where an ionic solid is being redissolved into water….BUT NOT WELL!redissolved into water….BUT NOT WELL!

Most of this solid would be just that at the bottom of Most of this solid would be just that at the bottom of a glass of water….a solid!a glass of water….a solid!

The KThe Kspsp for this reaction would be: for this reaction would be:

[Ba[Ba2+ 2+ ]]11 [SO [SO442-2-]]11

--------------------------------------------

[BaSO[BaSO4(s)4(s)]]11

KKspsp is similar to K is similar to KCC, because it tells us the , because it tells us the ratio of ratio of products to reactants products to reactants AT EQUILIBRIUM!!!AT EQUILIBRIUM!!!

BaSOBaSO4(s)4(s) ---------> Ba ---------> Ba2+2+(aq)(aq) + (SO + (SO44))2-2-

(aq)(aq)

Let’s say that a sample of BaSOLet’s say that a sample of BaSO4(s)4(s) is added to is added to pure water and allowed to come to equilibrium pure water and allowed to come to equilibrium at 25at 2500C. We measure the BaC. We measure the Ba2+2+ concentration concentration and it is 3.87 x 10and it is 3.87 x 10-5-5M. We measure the (SOM. We measure the (SO44))2-2- concentration and it is……?concentration and it is……?

The same! For every BaThe same! For every Ba2+2+ that dissolves, we that dissolves, we get the same number of (SOget the same number of (SO44))2-2- ions! ions!

This tells me that most of the solid still This tells me that most of the solid still remains at the bottom of the container as an remains at the bottom of the container as an undissolved solid!undissolved solid!

BaSOBaSO4(s)4(s) ---------> Ba ---------> Ba2+2+(aq)(aq) + (SO + (SO44))2-2-

(aq)(aq)

The constant for this reaction is:The constant for this reaction is:

KKsp sp = [Ba= [Ba+2+2(aq)(aq)][SO][SO44

-2-2(aq)(aq)]]

------------------------------------

[BaSO[BaSO4(s)4(s)]]

[3.87 x 10[3.87 x 10-5-5M] M] 11[3.87 x 10[3.87 x 10-5-5M]M]11

KKsp sp = ---------------------------------------- = 1.5 x 10= ---------------------------------------- = 1.5 x 10 -9-9

[1][1]11

1.5 x 101.5 x 10-9 -9 = .0000000015 = = .0000000015 = 151510,000,000,00010,000,000,000

This means that only 15 out of 10 billion molecules are going to dissolve This means that only 15 out of 10 billion molecules are going to dissolve in water!in water!

Why is the concentration of the solid equal to 1?Why is the concentration of the solid equal to 1?

BaSOBaSO4(s)4(s) ---------> Ba ---------> Ba2+2+(aq)(aq) + (SO + (SO44))2-2-

(aq)(aq)

Remember, solids and liquids change only in Remember, solids and liquids change only in amount - not in concentration!amount - not in concentration!

If you assume that you start with 1M of the solid, If you assume that you start with 1M of the solid, since so little of it dissolves, you still have 1M of the since so little of it dissolves, you still have 1M of the solid at the end!solid at the end!

When an ionic solid dissolves in water, there are so When an ionic solid dissolves in water, there are so few ions in solution compared to undissolved solid few ions in solution compared to undissolved solid that we assume the concentration of the leftover that we assume the concentration of the leftover solid to be 1!solid to be 1!

We are really measuring how much of the solid We are really measuring how much of the solid dissolved by just looking at the ion concentration!dissolved by just looking at the ion concentration!

But don’t ionic solids dissolve well in water?????But don’t ionic solids dissolve well in water?????

Ionic solids DO dissolve well Ionic solids DO dissolve well in water - at least most of in water - at least most of them!them!

You won’t see KYou won’t see Kspsp values for values for substances like NaCl, MgSOsubstances like NaCl, MgSO44, CaCl, CaCl22, , etc.etc.

Remember the solubility rules? Remember the solubility rules? These substances are REALLY These substances are REALLY soluble in water!soluble in water!

They would have a KThey would have a Kspsp close to 1, close to 1, meaning they dissolve completely!meaning they dissolve completely!

What does KWhat does Kspsp really tell us??? really tell us???

A solid such as CaCrOA solid such as CaCrO44, which has a , which has a

KKspsp = 7.1 x 10 = 7.1 x 10-4-4, would dissolve , would dissolve

comparatively well compared to….comparatively well compared to…. a solid such as HgS, which has a Ka solid such as HgS, which has a Kspsp

= 2 x 10= 2 x 10-53-53, which dissolves poorly in , which dissolves poorly in water!water!

Let’s take two compounds that do Let’s take two compounds that do not dissolve well in water….not dissolve well in water….

Why is KWhy is Kspsp important to us? important to us? Ionic solids with small KIonic solids with small Kspssps do do

not dissolve well in water, and not dissolve well in water, and fall out as….fall out as….

PRECIPITATES!!!PRECIPITATES!!!

These solids are used as paint These solids are used as paint colorings, chalks, and for colorings, chalks, and for medical procedures, like X-medical procedures, like X-rays, upper and lower GI’srays, upper and lower GI’s

5. The K5. The Kspsp for barium iodate at 25 for barium iodate at 2500C is 6.00 x 10C is 6.00 x 10-10-10. What is the molar . What is the molar

solubility of each ion at 25solubility of each ion at 2500C in solution? Barium iodate is used to C in solution? Barium iodate is used to purify edible oils.purify edible oils.

What does the term molar solubility mean?What does the term molar solubility mean? It is the amount of each ion dissolved in water at equilibrium!It is the amount of each ion dissolved in water at equilibrium! Let’s write the reaction for barium iodate dissolving in water, and set up an ICE Let’s write the reaction for barium iodate dissolving in water, and set up an ICE

equation – equation – 1 Ba(IO1 Ba(IO33))2(s)2(s) Ba Ba+2+2

(aq)(aq) + 2 IO + 2 IO33-1-1

(aq)(aq)

I 1M 0 0I 1M 0 0

C -xC -x +x +2x +x +2x

E 1M x 2xE 1M x 2x

KKspsp = 6.00 x 10 = 6.00 x 10-10-10 = [Ba = [Ba+2+2(aq)(aq)]]11[IO[IO33

-1-1(aq)(aq)]]22

[1][1] 6.00 x 106.00 x 10-10-10 = [x] = [x]11[2x][2x]22 = 4x = 4x33

1.5 x 101.5 x 10-10 -10 = x= x33

5.31 x 105.31 x 10-4-4M = x, so BaM = x, so Ba+2+2 = 5.31 x 10 = 5.31 x 10-4-4M, and IOM, and IO33-1-1 = .00106M = .00106M

Why does the concentration of the solid stay 1M?Why does the concentration of the solid stay 1M?

The Common Ion Effect….The Common Ion Effect….

What is the common ion effect?What is the common ion effect? It is when there is already one of the ions in solution, making it harder for the solid to dissolve!It is when there is already one of the ions in solution, making it harder for the solid to dissolve! 14. What is the molar solubility of calcium oxalate solid in .15M calcium chloride solution? Compare 14. What is the molar solubility of calcium oxalate solid in .15M calcium chloride solution? Compare

this molar solubility to that for calcium oxalate in pure water, which is 4.8 x 10this molar solubility to that for calcium oxalate in pure water, which is 4.8 x 10-5-5. The K. The Kspsp for calcium for calcium

oxalate is 2.3 x 10oxalate is 2.3 x 10-9-9.. A .15M CaClA .15M CaCl22 solution breaks up 100% into Ca solution breaks up 100% into Ca+2+2

(aq)(aq) and C and C22OO44-2-2

(aq)(aq), because it follows the solubility rules!, because it follows the solubility rules!

This, in effect, adds CaThis, in effect, adds Ca+2+2(aq)(aq) ions into the solution! ions into the solution!

1 CaC1 CaC22OO4(s)4(s) Ca Ca+2+2(aq)(aq) + C + C22OO44 (aq)(aq)

I 1M .15M 0I 1M .15M 0

C -xC -x +x +x +x +x

E 1M .15+x x E 1M .15+x x

KKspsp = 2.3 x 10 = 2.3 x 10-9-9 = [Ca = [Ca+2+2(aq)(aq)]]11[C[C22OO44

-2-2 (aq)(aq)]]11

[1][1] 2.3 x 102.3 x 10-9-9 = [.15 + x] = [.15 + x]11[x][x]11 = x = x22 + .15x + .15x 0= x0= x22 + .15x - 2.3 x 10 + .15x - 2.3 x 10-9-9 1.53 x 101.53 x 10-8-8M = x, so CaM = x, so Ca+2+2 = .15M, and C = .15M, and C22OO44

-2-2 = 1.53 x 10 = 1.53 x 10-8-8M M

Why does the concentration of the CaWhy does the concentration of the Ca+2+2 stay .15M? stay .15M?

Ca+2 ions already added

Ca+2 ions already added

Solid CaC2O4Solid CaC2O4

Does water dissolve itself???Does water dissolve itself???

We know that water has a partial charge - We know that water has a partial charge - does it pull itself apart?does it pull itself apart?

Water actually has a KWater actually has a Kspsp, which is also , which is also

called:called:

KKww

Water naturally dissociates into HWater naturally dissociates into H+ + and and (OH)(OH)-- ions. Only one in ten million water ions. Only one in ten million water molecules actually dissociates, thoughmolecules actually dissociates, though