Altering Solubility - BC Learning Network

Report
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.
It is known that barium sulphate
(BaSO4) dissolves endothermically.
Barium sulphate (BaSO4), is known to dissolve endothermically in water.
It is known that barium sulphate
(BaSO4) dissolves endothermically.
Its solubility equilibrium equation
Its solubility equilibrium equation…
It is known that barium sulphate
(BaSO4) dissolves endothermically.
Its solubility equilibrium equation
can be written like this:
B aS O 4 ( s )  h eat
2
Ba ( aq )
can be written like this, with the solid BaSO4 on the left…

2
SO 4 ( aq )
It is known that barium sulphate
(BaSO4) dissolves endothermically.
Its solubility equilibrium equation
can be written like this:
B aS O 4 ( s )  h eat
the dissolved ions on the right…
2
Ba ( aq )

2
SO 4 ( aq )
It is known that barium sulphate
(BaSO4) dissolves endothermically.
Its solubility equilibrium equation
can be written like this:
B aS O 4 ( s )  h eat
2
Ba ( aq )

2
SO 4 ( aq )
And because it’s endothermic, and heat is being consumed, the heat term is written on the Left
side of the arrow.
B aS O 4 ( s )  h eat
2
Ba ( aq )

2
SO 4 ( aq )
When the temperature
is increased, heat is
added to the system
When the temperature of a system is increased, it means heat is added to this endothermic
system.
B aS O 4 ( s )  h eat
2
Ba ( aq )

2
SO 4 ( aq )
When the temperature
is increased, heat is
added to the system
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.
This will cause a shift to the RIGHT
B aS O 4 ( s )  h eat
2
Ba ( aq )
When the temperature
is increased, heat is
added to the system
In this case the heat term is on the left, so it causes a shift to the right.

2
SO 4 ( aq )
Shift right
B aS O 4 ( s )  h eat
Which occurs like this
2
B a ( aq )

2
S O 4 ( aq )
Shifted to the RIGHT
B aSO 4 ( s )
 h eat
2
Ba ( aq )

2
SO 4 ( aq )
Concentrations of the
dissolved ions have increased
We see that as a result of shifting to the right, the concentrations of the dissolved ions have increased.
Shifted to the RIGHT
B aSO 4 ( s )
 h eat
2
Ba ( aq )
The mass of solid
BaSO4 has decreased
And the mass of the solid BaSO4 present has decreased.

2
SO 4 ( aq )
Shifted to the RIGHT
B aSO 4 ( s )
 h eat
The mass of solid
BaSO4 has decreased
2
Ba ( aq )

2
SO 4 ( aq )
Concentrations of the
dissolved ions have increased
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..
Shifted to the RIGHT
The solubility of BaSO4(s) has INCREASED.
B aSO 4 ( s )
 h eat
The mass of solid
BaSO4 has decreased
2
Ba ( aq )

2
SO 4 ( aq )
Concentrations of the
dissolved ions have increased
As a result of shifting to the right the solubility of BaSO4 has increased.
Increasing the temperature of a solution of a
compound that dissolves endothermically…
B aSO 4 ( s )  heat
2
Ba ( aq )

2
SO 4 ( aq )
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…
Causes a shift to the RIGHT…
B aSO 4 ( s )
 h eat
2
Ba ( aq )
Causes the solubility equilibrium to shift to the right…

2
SO 4 ( aq )
Increasing the temperature of a solution of a
compound that dissolves endothermically…
Causes a shift to the RIGHT…
Which INCREASES the solubility of the solid compound
B aSO 4 ( s )
 h eat
2
Ba ( aq )
Which increases the solubility of the solid compound.

2
SO 4 ( aq )
B aS O 4 ( s )
 h eat
2
B a ( aq )
 S O 42 ( aq )
When the temperature is
DECREASED, heat is
removed from the system
It should be easy to see now, that if we were to decrease the temperature of this endothermic
system…
B aS O 4 ( s )
 h eat
2
B a ( aq )
 S O 42 ( aq )
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…
This will cause a shift to the LEFT
B aS O 4 ( s )
 h eat
2
B a ( aq )
 S O 42 ( aq )
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…
A shift to the LEFT
B aS O 4 ( s ) 
h eat
Would increase the mass of undissolved solid…
2
B a ( aq )

2
S O 4 ( aq )
A shift to the LEFT
B aS O 4 ( s ) 
h eat
2
B a ( aq )
And decrease the concentrations of the dissolved ions.

2
S O 4 ( aq )
A shift to the LEFT
Causes the solubility to DECREASE
B aS O 4 ( s ) 
h eat
2
B a ( aq )

In other words, a shift to the left causes the solubility to decrease.
2
S O 4 ( aq )
Decreasing the temperature of a solution of a
compound that dissolves endothermically…
B aSO 4 ( s )
 heat
2
Ba ( aq )
 SO 42 ( aq )
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…
Causes a shift to the LEFT…
B aS O 4 ( s ) 
Causes a shift to the left
h eat
2
B a ( aq )

2
S O 4 ( aq )
Decreasing the temperature of a solution of a
compound that dissolves endothermically…
Causes a shift to the LEFT…
Which DECREASES the solubility of the solid compound
B aS O 4 ( s ) 
h eat
2
B a ( aq )
Which decreases the solubility of the solid compound.

2
S O 4 ( aq )
It is known that calcium acetate
[Ca(CH3COO)2] dissolves exothermically.
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.
Its solubility equilibrium equation can be
written like this:
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
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:
C a(C H 3 C O O ) 2 ( s )
The dissolved ions on the right
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
It is known that calcium acetate
[Ca(CH3COO)2] dissolves exothermically.
Its solubility equilibrium equation can be
written like this:
C a(C H 3 C O O ) 2 ( s )
And the heat term on the right.
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
An increase in
temperature means
that heat is added.
Let’s see what happens when we increase the temperature of a saturated solution of calcium
acetate.
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
An increase in
temperature means
that heat is added.
Increasing the temperature means we’re adding heat to the system.
This will cause a shift to the LEFT
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
An increase in
temperature means
that heat is added.
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.
Shift Left
C a ( C H 3C O O ) 2 ( s )
A shift to the left occurs like this…
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
Shifted to the LEFT
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq )


2C H 3 C O O ( aq )
 heat
Concentrations of the
dissolved ions have decreased
We can see that as a result of the shift to the left, the concentrations of the dissolved ions have
decreased.
Shifted to the LEFT
C a(C H 3 C O O ) 2 ( s )
The mass of solid
has increased
2
C a ( aq )


2C H 3 C O O ( aq )
Concentrations of the
dissolved ions have decreased
And the mass of solid calcium acetate in the container has increased.
 heat
The Solubility of calcium acetate has DECREASED
Shifted to the LEFT
C a(C H 3 C O O ) 2 ( s )
The mass of solid
has increased
2
C a ( aq )


2C H 3 C O O ( aq )
 heat
Concentrations of the
dissolved ions have decreased
Because there is more solid and less dissolved ions, we can say that the solubility of calcium
acetate has decreased.
Increasing the temperature of a solution of a
compound that dissolves exothermically
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
An increase in
temperature means
that heat is added.
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
Causes a shift to the LEFT
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq )
Causes the solubility equilibrium to shift to the left


2C H 3 C O O ( aq )
 heat
Increasing the temperature of a solution of a
compound that dissolves exothermically
Causes a shift to the LEFT
Which DECREASES the solubility
C a(C H 3 C O O ) 2 ( s )
Which decreases the solubility
2
C a ( aq )


2C H 3 C O O ( aq )
 heat
If we decrease the temperature of a solution
of a compound that dissolves exothermically
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )
 heat
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
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )
We are
removing
heat
It means we are removing heat from the system
 heat
This will cause a shift to the RIGHT
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )
We are
removing
heat
This will cause this equilibrium to shift to the right.
 heat
This will cause a shift to the RIGHT
C a(C H 3 C O O ) 2 ( s )
Giving us this
2
C a ( aq ) 

2C H 3 C O O ( aq ) heat
This will cause a shift to the RIGHT
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq ) heat
Concentrations of the
dissolved ions have increased
As a result of shifting to the right, we can see that the concentrations of the dissolved ions
have increased.
This will cause a shift to the RIGHT
C a(C H 3 C O O ) 2 ( s )
The mass of solid
has decreased
2
C a ( aq ) 

2C H 3 C O O ( aq ) heat
Concentrations of the
dissolved ions have increased
And the mass of the solid has decreased, so more of it has dissolved.
The solubility of calcium acetate has INCREASED
This will cause a shift to the RIGHT
C a(C H 3 C O O ) 2 ( s )
The mass of solid
has decreased
2
C a ( aq ) 

2C H 3 C O O ( aq ) heat
Concentrations of the
dissolved ions have increased
Because there is less solid and more dissolved ions, the solubility of calcium acetate has
Increased
So if a compound dissolves exothermically and
the temperature is Increased…
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
So to summarize, if a compound dissolves exothermically and the temperature is Increased…
So if a compound dissolves exothermically and
the temperature is Increased…
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq )

The equilibrium will shift LEFT
The equilibrium will shift to the left

2C H 3 C O O ( aq )
 heat
So if a compound dissolves exothermically and
the temperature is Increased…
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq )

The equilibrium will shift LEFT
And the Solubility will DECREASE.
And the solubility will Decrease

2C H 3 C O O ( aq )
 heat
So if a compound dissolves exothermically and
the temperature is Decreased…
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 

2C H 3 C O O ( aq )  heat
And if a compound dissolved exothermically and the temperature is Decreased
So if a compound dissolves exothermically and
the temperature is Decreased…
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 
The equilibrium will shift RIGHT
The solubility equilibrium will shift to the Right

2C H 3 C O O ( aq) heat
So if a compound dissolves exothermically and
the temperature is Decreased…
C a(C H 3 C O O ) 2 ( s )
2
C a ( aq ) 
The equilibrium will shift RIGHT
And the Solubility will INCREASE.
And the solubility will Increase.

2C H 3 C O O ( aq) heat

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