EXOTHERMIC ENDOTHERMIC Why upon mixing do some solutions get cold while other release heat?

Mixture Solutions Pure Solvent Pure Solute

When copper chloride is dissolved in water, the ions are cloaked in water molecules Solvation

5 Heat of Solution:  H soln The amount of heat involved in the process of solution formation

Net energy (  H soln ) is the sum of the parts 1. Break ions apart 2. Hydrate ions  H soln = 1 in + 2 out = 821 – 819 = 2 kJ/mol

 H soln =  H s +  H aq Neg. = out (  T) Positive = in (  T)

9 Heat of Solution If the enthalpy of formation of the solution is more negative that that of the solvent and solute, the enthalpy of solution is negative. The solution process is exothermic !

Energy of a System Depends on the polarity of the solvating molecule, size of the ions and the charge The higher the charge, the stronger the attraction (Mg +2 vs. Li +1 and K +1 ) The smaller the ion, the stronger the attraction (Li +1 vs. K +1 )

11 Heat of Solution:  H soln Determines the solubility of a substance: If the  H soln is very positive, water does not have enough energy to break ions apart and it will NOT dissolve If the  H soln is negative, water will gain energy from ions and it will dissolve

12 Hydration Energy 1. Explain why the enthalpy of hydration of Na+ is more negative than that of Cs+. 2. Explain why the enthalpy of hydration of Mg 2+ is more negative than that of Cs+. 3. Which should have a more negative enthalpy of hydration F - or Cl - ? Explain Na+ is smaller size Mg 2+ has greater charge F- is smaller size

13 MEASRUING Heat of Solution Ba(OH) 2 The solution process is exothermic What is the Molar  H soln ? NH 4 (OH) 2 The solution process is endothermic What is the Molar  H soln ?

14

15 Solutions can be classified as saturated or unsaturated. Definitions A saturated solution contains the maximum quantity of solute that dissolves at that temperature.

Solubility: Maximum amount of solute per 100 g solvent UNSATURATED SOLUTIONSSATURATED SOLUTIONS

Dissolving Effects: c34 Saturated Solution- maximum amount of solute in solution Saturated Solution- maximum amount of solute in solution Any extra solute added just sits on bottom. Any extra solute added just sits on bottom. Unsaturated- less than max. solute than then can be dissolve. Unsaturated- less than max. solute than then can be dissolve. Supersaturated- A solution containing more solute than normal for a given temperature. Supersaturated- A solution containing more solute than normal for a given temperature. Must heat a saturated solution up. Then return it to room temperature. Must heat a saturated solution up. Then return it to room temperature.

For a Supersaturated solution, when more solute is added, the extra solute will settle to the bottom

20 Supersaturated Sodium Acetate Supersaturated Sodium Acetate One application of a supersaturated solution is the sodium acetate “heat pack.” Sodium acetate has an ENDOthermic heat of solution.

Temperature effects the solubility of a substance

Solubility Curves How much LiCl will dissolve into 100 ml of water at a temperature of 20 C? How much KCl will dissolve into 500 ml of water at a temperature of 80 C? How much RbCl will fall out of solution if 200 ml of solution is cooled from a temperature of 80 C to 20 C?

Dissolving Effects Solubility- the quantity of solute that will dissolve in 100 g of solvent at a set temperature Solubility- the quantity of solute that will dissolve in 100 g of solvent at a set temperature If solution is holding the MAX. Amount of solute called saturated If solution is holding the MAX. Amount of solute called saturated Solid ionic compounds: more in with higher temperature Solid ionic compounds: more in with higher temperature Gases: More in at lower temperatures Gases: More in at lower temperatures

Dissolving Gases & Pressure Gas solubility can be changed by both the temperature and PRESSURE. When the can of soda is opened the pressure ( P gas ) drops and causes the solubility to decrease.

25 Henry’s Law Gas solubility (mol/L) = S g S g = k H P gas k H = Henry’s law constants for O 2 = 1.66 x M/mmHg When P gas increases, solubility increases.

Henry’s Law What is the concentration of oxygen in fresh water when the air pressure is 1 atm? S g = k H P gas k H for O 2 = 1.66 x M/mmHg O 2 = 21% so 0.21(760 torr) = P gas S g = 1.66 x M/mmHg 160 torr = 2.66 x M What is the concentration of oxygen in milligrams/ liter? O 2 = 8.5 mg/L