CHAPTER 2. Solutions By Dr. Hisham Ezzat . http://www.staff.zu.edu.eg/ezzat_hisham/browseMyFiles.asp?path=./userdownloads/physical%20chemistry%20for%20clinical%20pharmacy/. The Dissolution Process. Solutions are homogeneous mixtures of two or more substances.
Dr. Hisham Ezzat
Seven Homogeneous Possibilities
Two Heterogeneous Possibilities
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The Dissolution Process
Colligative Properties of Solutions
IonRadius(Å)Charge/radiusHeat of Hydration
K+ 1.33 0.75 -351 kJ/mol
Ca2+ 0.99 2.02 -1650 kJ/mol
Cu2+0.72 2.78 -2160 kJ/mol
Al3+ 0.50 6.00 -4750 kJ/mol
Because gases have very weak solute-solute interactions, gases dissolve in liquids in exothermic processes.Dissolution of Gases in Liquids
Compare the dissolution of granulated sugar and sugar cubes in cold water.
The reason is simple, look at a single cube of NaCl.
The enormous increase in surface area helps the solid to dissolve faster.
many smaller crystals
NaClRates of Dissolution and Saturation
Since saturated solutions are at equilibrium, LeChatelier’s principle applies to them.
Possible stresses to chemical systems include:
Heating or cooling the system.
Changing the pressure of the system.
Changing the concentrations of reactants or products.Effect of Temperature on Solubility
It depends on whether the dissolution is exo- or endothermic.
For an exothermic dissolution, heat can be considered as a product.
Warming the water will decrease solubility and cooling the water will increase the solubility.
Predict the effect on an endothermic dissolution like this one.Effect of Temperature on Solubility
The vapor pressure of water is 17.5 torr at 20°C. Imagine holding the temperature constant while adding glucose, C6H12O6, to the water so that the resulting solution has XH2O = 0.80 and XGlu = 0.20. What is , the vapor pressure of water over the solution
= 14 torr
Glycerin, C3H8O3, is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25°C. Calculate the vapor pressure at 25°C of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure of pure water at 25°C is 23.8 torr
The vapor pressure of pure water at 110°C is 1070 torr. A solution of ethylene glycol and water has a vapor pressure of 1.00 atm at 110°C. Assuming that Raoult's law is obeyed, what is the mole fraction of ethylene glycol in the solution? Answer: 0.290
P°H2O =1070 torr
PH2O = 1 Atm = 760 torr
XH2O = ---------
XH2O + XEG = 1
0.7103 + XEG = 1
1- 0.7103 = XEG
Many solutions do not obey Raoult's law exactly: They are not ideal solutions.
If the intermolecular forces between solvent and solute are weaker than those between solvent and solvent and between solute and solute, then the solvent vapor pressure tends to be greater than predicted by Raoult's law.
Conversely, when the interactions between solute and solvent are exceptionally strong, as might be the case when hydrogen bonding exists, the solvent vapor pressure is lower than Raoult's law predicts.
Although you should be aware that these departures from ideal solution occur, we will ignore them for the remainder of this chapter.
Sucrose is a nonvolatile, nonionizing solute in water. Determine the vapor pressure lowering, at 27°C, of a solution of 75.0 grams of sucrose, C12H22O11, dissolved in 180. g of water. The vapor pressure of pure water at 27°C is 26.7 torr. Assume the solution is ideal.
Vapor Pressure Lowered = 26.7-26.1= 0.6
solution is made by mixing 52.1 g of propyl chloride, C3H8Cl, and 38.4 g of propyl bromide, C3H8Br. What is the vapor pressure of propyl chloride in the solution at 25°C? The vapor pressure of pure propyl chloride is 347 torr at 25°C and that of pure propyl bromide is 133 torr at 25°C. Assume that the solution is an ideal solution.
. At 25°C a solution consists of 0.450 mole of pentane, C5H12, and 0.250 mole of cyclopentane, C5H10. What is the mole fraction of cyclopentane in the vapor that is in equilibrium with this solution? The vapor pressure of the pure liquids at 25°C are 451 torr for pentane and 321 torr for cyclopentane. Assume that the solution is an ideal solution.
The addition of a nonvolatile solute lowers the vapor pressure of the solution.
At any given temperature,
the vapor pressure of the solution is lower than that of the pure liquid
The increase in boiling point relative to that of the pure solvent, DTb, is directly proportional to the number of solute particles per mole of solvent molecules.
Molality expresses the number of moles of solute per 1000 g of solvent, which represents a fixed number of moles of solvent
Automotive antifreeze consists of ethylene glycol, C2H6O2, a nonvolatile nonelectrolyte. Calculate the boiling point of a 25.0 mass percent solution of ethylene glycol in water.
Calculate the boiling point of a solution of 2.0 molal of NaCl. Kb, water= 0.52 °C /mola.
Dt = Kbm
NaCl(aq) Na+ + Cl-
2.0 m 2.0 m 2.0 m
2.0 m + 2.0 m = 4.0m
Dt = (0.52 °C/molal)(4.0 molal) =2.08 °C
BP = NBP +Dt = 100.00°C +2.08 °C = 102.08° C
The only differences are the size of the effect which is reflected in the sizes of the constants, Kf & Kb.
This is easily seen on a phase diagram for a solution.Freezing Point Depression
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i has an ideal value of 3 for 2:1 electrolytes like K2SO4, CaCl2, SrI2, etc.Colligative Properties and Dissociation of Electrolytes
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net solvent flow
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Water Purification by Reverse Osmosis