1. Heats of Solution & Heats of Dilution
The integral heat of solution is defined as the enthaply change that occurs when a specified amount of solute is added to a specified amount of solvent at constant temperature and pressure to form a solution.
Consider the following data for dissolving 1 mole of hydrogen chloride gas in varying amounts of liquid water to form aqueous hydrochloric acid solutions:
1 HCl (g) Aq > HCl (10 Aq) DHsoln = kJ/mole
1 HCl (g) Aq > HCl (25 Aq) DHsoln = kJ/mole
1 HCl (g) Aq > HCl (200 Aq ) DHsoln = kJ/mole
Does the exothermicity of this reaction depend linearly on the amount of solvent?
The answer to this question is best seen by plotting the negative of the enthapy change per mole of HCl (g) versus the number of moles of water involved per mole of HCl (g):
16.1

2. 1 HCl (g) + Aq -----> HCl ( Aq)
Note that as the number of moles of solvent increases, the integral heat of solution approaches a limiting value, which, since in the limit of infinite moles of solvent the solution would be infinitely dilute, is known as the heat of solution at infinite dilution:
1 HCl (g) + Aq > HCl ( Aq)
DH soln, HCl (aq) = kJ/mole
Since an infinitely dilute solution is a literal impossibility, for practical purposes an infinitely dilute solution is defined as a solution which is so dilute that the solute molecules do not interact with each other.
As can be seen in the above data, forming aqueous solutions of HCl is an exothermic process. Can you explain why this is from a molecular viewpoint?
16.2

3. DH f, soln, HCl (aq) = - 167.159 kJ / mole
Standard enthalpies of formation of solutions a various concentrations can occasionally be found tabluated, e.g. in The NBS Tables of Chemical Thermodynamic Properties, Wagman. et. al., eds., Journal of Physical and Chemical Reference Data, Vol 11 (Supp. No. 2), More typically only a single standard enthalpy of formation of a solution can be found in the literature, in which case it is almost always a value valid at infinite dilution:
DH f, soln, HCl (aq) = kJ / mole
Suppose we were to try and use standard enthalpies of formation to calculate the integral heat of solution of hydrochloric acid in an infinitely dilute asqueous solution:
DH soln, HCl (aq) = + (1 mole HCl ( Aq)) (DH f, soln, HCl (aq))
- (1 mole HCl (g)) (DHof, HCl (g))
- ( moles Aq) (DHof, Aq)
Do you see where you might have trouble carrying out the caluculation?
What do you think the standard enthalpy of formation of liquid water as symbolized by Aq, DHof, Aq, is?
DH soln, HCl (aq) = + (1 mole HCl ( Aq)) ( kJ / mole)
- (1 mole HCl (g)) ( kJ / mole)
- ( moles Aq) (DHof, Aq  0)
= kJ / mole
16.3

4. KOH2H2O (s) + HCl (12 Aq) -----> ? Some data you may need:
Alkali metal hydroxides readily absorb water from the atmosphere and form hydrates. What is standard enthalpy change when the solid dihydrate of potassium hydroxide reacts with a concentrated hydrochloric acid solution?
KOH2H2O (s) + HCl (12 Aq) > ?
Some data you may need:
DHof, HCl (12 Aq) = kJ / mole
DHof, KOH  2H2O (s) = kJ / mole
DHof, KCl (12 Aq) = kJ / mole
DHof, KCl (13 Aq) = kJ / mole
DHof, KCl (14 Aq) = kJ / mole
DHof, KCl (15 Aq) = kJ / mole
DHof, H2O (l) = kJ / mole
16.4

5. HCl (10 Aq) ------> HCl (g) + 10 Aq - (- 69.0 kJ/mole)
Differences in heats of solution yield heats of dilution, i.e., the heat transferred when a solution is diluted. For example the dilution of a concentrated HCl solution containing 1 mole of HCl for every 10 moles of water with another 190 moles of water can be represented as:
HCl (10 Aq) > HCl (g) Aq ( kJ/mole)
1 HCl (g) Aq > HCl (200 Aq ) ( kJ/mole)
HCl (10 Aq) Aq > HCl (200 Aq ) kJ/mole
An aqueous solution of potassium fluoride containing 2 moles of KF for every 200 moles of water, is mixed with a 2nd aqueous KF solution containing 1 mole of KF for every 700 moles of water. Use the following data:
DHof, KF (100 Aq) = kJ / mole
DHof, KF (200 Aq) = kJ / mole
DHof, KF (300 Aq) = kJ / mole
DHof, KF (700 Aq) = kJ / mole
DHof, KF (800 Aq) = kJ / mole
DHof, KF (900 Aq) = kJ / mole
to calculate the enthalpy change for this mixing.
16.5