altering solubility #2 effect of temperature here we’ll have a look at how changes in temperature...

Altering Solubility #2

Effect of Temperature

Here we’ll have a look at how changes in temperature affect the solubility of a compound.

How temperature affects the solubility of a compound depends on whether dissolving the compound is:

How temperature affects the solubility of a compound depends on whether dissolving the compound is:

How temperature affects the solubility of a compound depends on whether dissolving the compound is:• Endothermic or

Endothermic, or

How temperature affects the solubility of a compound depends on whether dissolving the compound is:• Endothermic or• Exothermic

Exothermic. We’ll do a couple of examples to illustrate this.

Barium sulphate (BaSO4), is known to dissolve endothermically in water.

It is known that barium sulphate (BaSO4) dissolves endothermically.

Its solubility equilibrium equation…

It is known that barium sulphate (BaSO4) dissolves endothermically.Its solubility equilibrium equation

can be written like this, with the solid BaSO4 on the left…

It is known that barium sulphate (BaSO4) dissolves endothermically.Its solubility equilibrium equation can be written like this:

4(s)BaSO heat 2(aq)Ba 2

4(aq)SO

the dissolved ions on the right…



4(aq)SO

And because it’s endothermic, and heat is being consumed, the heat term is written on the Left side of the arrow.



4(aq)SO

When the temperature of a system is increased, it means heat is added to this endothermic system.

4(s)BaSO heat

When the temperature is

increased, heat is added to the system

2(aq)Ba 2

4(aq)SO

According to Le Chatelier’s Principle, the system can compensate for the added heat by shifting away from the heat term in order to use up the excess heat.

4(s)BaSO heat



2(aq)Ba 2

4(aq)SO

In this case the heat term is on the left, so it causes a shift to the right.

4(s)BaSO heat



This will cause a shift to the RIGHT

2(aq)Ba 2

4(aq)SO

Which occurs like this


4(aq)SO

Shift right

24(aq)SO 2

(aq)Ba

We see that as a result of shifting to the right, the concentrations of the dissolved ions have increased.

heat 4(s)BaSO

Concentrations of the dissolved ions have

increased

Shifted to the RIGHT

24(aq)SO 2

(aq)Ba

And the mass of the solid BaSO4 present has decreased.

heat 4(s)BaSO

The mass of solid BaSO4 has decreased


24(aq)SO 2

(aq)Ba

So because a shift to the right caused the mass of the solid to decrease and the concentration of dissolved ions to increase, we can say that..

heat 4(s)BaSO




increased

24(aq)SO 2

(aq)Ba

As a result of shifting to the right the solubility of BaSO4 has increased.

heat 4(s)BaSO



The solubility of BaSO4(s) has INCREASED.


increased

So we can summarize by saying that increasing the temperature of a solution of a compound that dissolves endothermically…

Increasing the temperature of a solution of a compound that dissolves endothermically…


4(aq)SO

24(aq)SO 2

(aq)Ba

Causes the solubility equilibrium to shift to the right…

heat 4(s)BaSO

Causes a shift to the RIGHT…


24(aq)SO 2

(aq)Ba

Which increases the solubility of the solid compound.

heat 4(s)BaSO

Causes a shift to the RIGHT…

Which INCREASES the solubility of the solid compound


It should be easy to see now, that if we were to decrease the temperature of this endothermic system…


4(aq)SO

When the temperature is DECREASED, heat is removed from the system

It would mean that we are removing heat from the system. In order to compensate for the loss of heat…


4(aq)SO

When the temperature is DECREASED, heat is removed from the system

Le Chatelier’s Principle would predict that this equilibrium would shift to the left in order to produce more heat. A shift to the left…


4(aq)SO

When the temperature is DECREASED, heat is

removed from the system

This will cause a shift to the LEFT

Would increase the mass of undissolved solid…


4(aq)SO

A shift to the LEFT

And decrease the concentrations of the dissolved ions.


4(aq)SO

A shift to the LEFT

In other words, a shift to the left causes the solubility to decrease.


4(aq)SO

A shift to the LEFT

Causes the solubility to DECREASE

So we can summarize by saying that decreasing the temperature of a solution of a compound that dissolved endothermically…

Decreasing the temperature of a solution of a compound that dissolves endothermically…


4(aq)SO

Causes a shift to the left


4(aq)SO

Causes a shift to the LEFT…


Which decreases the solubility of the solid compound.


4(aq)SO

Causes a shift to the LEFT…

Which DECREASES the solubility of the solid compound


Now let’s consider a compound that dissolves exothermically. It is known that calcium acetate [Ca(CH3COO)2] dissolves exothermically.

It is known that calcium acetate [Ca(CH3COO)2] dissolves exothermically.

So its solubility equilibrium equation can be written like this, with the solid on the left

It is known that calcium acetate [Ca(CH3COO)2] dissolves exothermically.Its solubility equilibrium equation can be written like this:

3 2(s)Ca(CH COO) 2(aq)Ca

3 (aq)2CH COO heat

The dissolved ions on the right



3 (aq)2CH COO heat

And the heat term on the right.



3 (aq)2CH COO heat

Let’s see what happens when we increase the temperature of a saturated solution of calcium acetate.


3 (aq)2CH COO heat

An increase in temperature

means that heat is added.

Increasing the temperature means we’re adding heat to the system.




3 (aq)2CH COO heat

According to Le Chatelier’s Principle, this will cause the equilibrium to shift to the left in order to use up some of the excess heat.

This will cause a shift to the LEFT


3 (aq)2CH COO heat



A shift to the left occurs like this…

3 2(s)Ca(CH COO) heat 2(aq)Ca

3 (aq)2CH COO

Shift Left

3 2(s)Ca(CH COO)

We can see that as a result of the shift to the left, the concentrations of the dissolved ions have decreased.

heat 2(aq)Ca

3 (aq)2CH COO


decreased

Shifted to the LEFT

3 2(s)Ca(CH COO)

And the mass of solid calcium acetate in the container has increased.

heat 2(aq)Ca

3 (aq)2CH COO

The mass of solid has increased


decreased

Shifted to the LEFT

3 2(s)Ca(CH COO)

Because there is more solid and less dissolved ions, we can say that the solubility of calcium acetate has decreased.

heat 2(aq)Ca

3 (aq)2CH COO

The mass of solid has increased


decreased

Shifted to the LEFT

The Solubility of calcium acetate has DECREASED

So we can summarize by saying that increasing the temperature of a solution of a compound that dissolves exothermically, which means adding heat to it…

Increasing the temperature of a solution of a compound that dissolves exothermically


3 (aq)2CH COO heat



3 2(s)Ca(CH COO)

Causes the solubility equilibrium to shift to the left

heat 2(aq)Ca

3 (aq)2CH COO

Causes a shift to the LEFT


3 2(s)Ca(CH COO)

Which decreases the solubility

heat 2(aq)Ca

3 (aq)2CH COO

Causes a shift to the LEFT

Which DECREASES the solubility


Now if we take a saturated solution of calcium acetate, a compound which dissolves exothermically and we Decrease the temperature…

If we decrease the temperature of a solution of a compound that dissolves exothermically


3 (aq)2CH COO heat

It means we are removing heat from the system

If we decrease the temperature of a solution of a compound that dissolves exothermically


3 (aq)2CH COO heat

We are removing

heat

This will cause this equilibrium to shift to the right.


3 (aq)2CH COO heat


We are removing

heat

Giving us this


3 (aq)2CH COO heat


As a result of shifting to the right, we can see that the concentrations of the dissolved ions have increased.


3 (aq)2CH COO heat



increased

And the mass of the solid has decreased, so more of it has dissolved.


3 (aq)2CH COO heat


The mass of solid has decreased


increased

Because there is less solid and more dissolved ions, the solubility of calcium acetate has Increased


3 (aq)2CH COO heat


The mass of solid has decreased


increased

The solubility of calcium acetate has INCREASED

So to summarize, if a compound dissolves exothermically and the temperature is Increased…

So if a compound dissolves exothermically and the temperature is Increased…


3 (aq)2CH COO heat

The equilibrium will shift to the left



3 (aq)2CH COO heat

The equilibrium will shift LEFT

And the solubility will Decrease



3 (aq)2CH COO heat

The equilibrium will shift LEFT

And the Solubility will DECREASE.

And if a compound dissolved exothermically and the temperature is Decreased

So if a compound dissolves exothermically and the temperature is Decreased…


3 (aq)2CH COO heat

The solubility equilibrium will shift to the Right


3 2(s)Ca(CH COO) 2(aq)Ca 3 (aq)2CH COO heat

The equilibrium will shift RIGHT

And the solubility will Increase.


3 2(s)Ca(CH COO) 2(aq)Ca 3 (aq)2CH COO heat

The equilibrium will shift RIGHT

And the Solubility will INCREASE.

altering solubility #2 effect of temperature here we’ll have a look at how changes in temperature...

Documents