Chapter 2 Part 1 : Solubility Part 1 Page 1 SABIS Grade 11 (Level M) Chemistry


Solubility is the same as equilibrium state 

There are two processes that occur in the same rate but in opposite directions 

 The Concentrations are also constant 

 The fixed concentration of the solid dissolved in the liquid is called the solubility 

 So in the solubility of iodine in alcohol The iodine solid particles starts dissolving to make the alcoholic solution and this is reversed as some of the dissolved iodine starts to precipitate again forming the solid particles again. 

 The forward reaction dissolution and the backward reaction precipitation starts happening at the same rate but in opposite directions until equilibrium state is reached. 


 When we apply the equilibrium law to this reaction the equilibrium constant K = [ concentration of iodine in alcohol ]

K = [I2]

So there is a dynamic equilibrium between the rate that iodine molecules leave the crystal and the rate that iodine molecules return to the crystal.





Solubility: A Case Of Equilibrium

The starting point in any quantitative equilibrium calculation is the Equilibrium Law.
For a generalized reaction:

aA + bB  ⇌  eE + f F            (1)

Equilibrium exists when the concentrations satisfy the relation:

K=[E]e[F]f[A]a[B]b         (2)
First, we shall apply expression (2) to the solution system of solid iodine dissolving in liquid ethyl alcohol.


2.1.1 The Solubility Of Iodine In Ethyl Alcohol
As a solid dissolves in a liquid, atoms or molecules leave the solid and become part of the liquid. These atoms or molecules may carry no charge (then they are electrically neutral) or they may be ions. The iodine-alcohol system is of the former kind. As iodine dissolves, neutral molecules of iodine, I2, leave the regular crystal lattice and these molecules become part of the liquid phase. At equilibrium, excess solid must remain and a fixed concentration of iodine is present in solution. This fixed concentration is called the solubility. Solubility is measured as a molar concentration, as grams per dm3 of solution or as grams of solute per 100 grams of solvent.

For this system at equilibrium, the reaction is:
I2(s) ⇌ I2(alcohol solution)          (3)

The Equilibrium Law applied to this reaction gives:
K = a constant = [concentration I2 in alcohol]
K = [I2]                                                       (4)


2.1 Solubility: A Case Of Equilibrium

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2.1.2 The Dynamic Nature Of Solubility Equilibrium


The simple form of the equilibrium expression (4) follows directly from the dynamic 
nature of the solubility equilibrium. There must be a dynamic balance  To understand 
this dynamic balance, we must consider the factors that determine these two rates.

Rate of dissolving

One of the factors that influences the rate of dissolving of solid is the area, A, of the crystal surface that
contacts the liquid. If many crystals (with large A) are dissolving simultaneously, the rate of dissolving is
faster than if there are only a few crystals (with small A) in the solvent. The rate of dissolving is
proportional
 to this liquid-solid surface area, A.
A molecule of iodine is more stable in the crystal than in the solution. The potential energy must rise as a
molecule leaves the crystal and the principles that govern rates of reaction are operative. Presumably
there is an activated complex for the process. The rate at which molecules leave a square centimeter of
surface, passing over the energy barrier, is determined by the height of the barrier and by the temperature.
 We can call this rate kd. Changing the temperature does not affect the activated complex, but the
molecular energy distribution is altered. Hence, kd is a function of temperature. These two factors
determine completely the rate of dissolving.
(rate of dissolving) = (surface area) × (rate molecules leave 

a square centimeter of crystal surface)
= (A) × (kd)
(rate of dissolving) = (A) × (kd)        (5)

Rate of precipitation
The rate of precipitation is the rate at which molecules return to the surface and fit into the crystal lattice.
To do this, the molecules in solution must first strike the crystal surface. Again, the more surface, the more
frequently will dissolved molecules encounter a piece of crystal. The rate of precipitation is proportional to
A.
In addition, the rate that molecules strike the surface depends upon how many molecules there are per unit
 volume of solution. As the concentration rises, more and more molecules strike the surface per unit time.
The rate of precipitation is proportional to the iodine concentration, [I2].
The last factor is, again, the rate that molecules can pass over the energy barrier—the activated complex
for precipitation. Again there is a rate constant, kp, that is determined by temperature and the height of the
energy barrier to precipitation.
We have, then, three factors that determine the rate of precipitation

















=            (A)          ×           [I2]           ×          (kp)
(rate of precipitation) = (kp) × (A) × [I2]           (6)

The dynamic nature of equilibrium
At equilibrium, we can equate (5) and (6):
(rate of dissolving) = (rate of precipitation)
A × kd = A × kp × [I2]             (7)
the area of contact, A, appears both on the left and on both sides of expression (7).
Hence, it cancels out. Dividing both sides of (7) by kp, we obtain:
kdkp=[I2]        (8)

Since kd and kp each depend upon temperature, their ratio depends upon temperature. Otherwise,
however, each is constant. We can write
K = [I2]                    (4)
where:

Thus, by expressing the dynamic balance between the rates of dissolving and precipitation, we obtain (4).
 The concentration of I2 at equilibrium is a constant, fixed by the temperature.
This constant equals the solubility.
Question
Sugar is added to a cup of coffee until no more sugar will dissolve.
Which of the following would happen if another spoonful of sugar is added to the coffee?


The sweetness of the liquid will decrease.

The sweetness of the liquid will increase.

The rate at which the sugar molecules leave the crystal phase and enter the liquid phase is not affected.

The rate at which the sugar molecules leave the crystal phase and enter the liquid phase increases.

The sweetness of the liquid will not be affected.
Question

Choose the methods that can be used to increase the rate at which salt dissolves in water.

Cooling

Using larger salt crystals

Using smaller salt crystals

Heating



Choose the methods that can be used to increase the rate at which salt dissolves in water.

Heating

Using larger salt crystals

Cooling

Using smaller salt crystals



2.1.3 Factors That Fix The Solubility Of A Solid


All of the discussion we have just applied to the dissolution of iodine in ethyl alcohol applies equally well to
 the dissolution of iodine in carbon tetrachloride, CCl4.
Iodine at room temperature dissolves in carbon tetrachloride at a certain rate, that at equilibrium, exactly 
equals the rate of precipitation. Again we reach the simple equilibrium expression:
            (4)


 Despite this qualitative similarity, the solubility of iodine in CCl4 is very different from its solubility in
alcohol. One liter of alcohol dissolves 0.84 mole of iodine, whereas one liter of CCl4 dissolves
only 0.12 mole:
Kalcohol = 0.84 mole/liter       (9)
KCCl4 = 0.12 mole/liter        (10)

Why are these constants so different? To see why, we must turn to the two factors that control every
equilibrium, tendency toward minimum energy and tendency toward maximum randomness.
















Effect of tendency towards maximum randomness
In either solvent, alcohol or carbon tetrachloride, the dissolution process destroys the regular crystal
 lattice of iodine and forms the disordered solution. It also increases randomness.
The tendency toward maximum randomness tends to cause solids to dissolve.

Effect of tendency towards minimum potential energy
Experiments have shown that heat is absorbed as iodine dissolves. The regular, ideally packed iodine
crystal gives an iodine molecule a lower potential energy than does the random and loosely packed
solvent environment. We see that the second factor, tendency toward minimum energy, favors precipitation
 and growth of the crystal. Now we see the opposing factors at equilibrium:
• Tendency towards more randomness pushes solid to dissolve.
• Tendency towards lower P.E. pushes solid to precipitate.
• Equilibrium is reached when the concentration is such that these two tendencies are equal.

The effect of raising the temperature
Raising the temperature always tends to favor the more random state. Generally, this means when, say,
iodine is placed in alcohol, more solid will dissolve, since the liquid solution is more random than the solid.
The solubility of iodine increases as temperature is raised, both in alcohol and in carbon tetrachloride.
Heating increases molecular motions and the tendency towards randomness. This is why in general the
solubility of solids increases with temperature.

Effect of the heat of solution on dissolving

Effect of the heat of solution on dissolving How much the energy factor favors the crystal depends upon the
 change in heat content as a mole of solid dissolves. This change is called the heat of solution. The heats
 of solution of iodine in these two solvents have been measured; they are as follows:

I2(s) + 6.7 kJ ⇌ I2(in alcohol)      (11)
I2(s) + 23.8 kJ ⇌ I2(in CCl4)        (12)


























We see that there is a much greater energy rise when a mole of I2 dissolves in CCl4 than when a mole
of I2
dissolves in alcohol. Thus the energy factor (favoring the crystal) that opposes the randomness factor

(favoring solution) is much larger for CCl4 than for alcohol. The solubility of iodine in CCl4 is not as high
as
it is in alcohol. This energy rise establishes, in this case, the “hostility” toward mixing referred to in the

quotation at the beginning of the chapter. The larger the “hostility” as measured by heat absorbed on
mixing,

 the lower the solubility will be.
Question

 The heat of solution of iodine in benzene is +17.5 kJ/mol (heat is absorbed).
Given that the heat of solutions
 of iodine in alcohol and CCl4 are + 6.7 kJ and + 23.8 kJ respectively, and assuming the increase in
randomness is the same when iodine dissolves in liquid benzene as it is in ethyl alcohol and in CCl4,
which of the below represents the increasing order of solubility of iodine in different solvents?
Let solubility of I2 in benzene be S1, solubility of I2 in alcohol be S2 and the solubility of I2 in CCl4 be S3.



S2 < S1 < S3

S3 < S2 < S1

S3 < S1 < S2

S2 < S3 < S1

S1 < S2 < S3
answer
S3 < S1 < S2
Response:
That's the correct answer

Question
When a solid evaporates directly (without melting), the process is called sublimation.
Evaporation of "dry ice" (solid CO2) is a familiar example. Two other substances that sublime are
FCN and ICN:

                                FCN(s)  ⇌   FCN(g)            ΔH = +24 kJ
                                ICN(s)   ⇌   ICN(g)             ΔH = +60 kJ

In sublimation, does the tendency toward maximum randomness favor solid or gas?

In sublimation, does the tendency toward minimum energy favor solid or gas?