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Transcript of Chapter 13 Solubility as An Equilibrium Phenomena - · PDF fileTemperature on Ionic Salts The...
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SOLUBILITY AS AN EQUILIBRIUM PHENOMENA
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Equilibrium in Solution
solute (undissolved) solute (dissolved)
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Solubility
A saturated solution contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature
An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature
A supersatured solution contains more solute than is present in a saturated solution at a specific temperature
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Crystallization
Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate
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Ways of Expressing Concentration
The concentration is the amount of solute present in a given quantity of solution or solvent.
Parts by mass Percent by mass (parts per hundred) Parts per million (ppm) Parts per billion (ppb)
Mole fraction Molarity Molality
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Ways of Expressing Concentration
Mole Fraction (X)
XA = moles of A
sum of moles of all components
M = moles of solute
liters of solution
Molarity (M)
Molality (m)
m = moles of solute
mass of solvent (kg) Converting between
molarity (M) and molality (m) requires density.
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Ways of Expressing Concentration
Percent by Mass
% by mass = x 100% mass of solute mass of solute + mass of solvent
= x 100% mass of solute mass of solution
Parts per milion
ppm = x 106 mass of solute mass of solution
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Ways of Expressing Concentration
Parts per milion
For dilute solutions (in the ppm and ppb level), density is almost equal to 1 g/mL
ppm = x 106 mass of solute mass of solution
ppm = gsolute gsolution
1 gsolution mLsolution
1000 mLsolution
1 Lsolution
1000 mgsolute
1 gsolute
x x x
ppm = mgsolute L solution
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Exercise
1) A solution is made by dissolving 13.5 g of glucose, C6H12O6, in 0.100 kg of water. What is the mass percentage of solute in this solution?
2) A 2.5 g sample of ground water was found to contain 5.4 μg of Zn2+. What is the concentration of Zn2+ in parts per million?
3) The solubility of MnSO4∙H2O in water at 20°C is 70 g per 100 mL of water. Is a 1.22 M solution of MnSO4∙H2O in water at 20°C saturated, supersaturated or unsaturated?
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Exercise
4) What is the mass percentage of NaCl in a solution containing 1.50 g of NaCl in 50.0 g of water?
5) What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL?
6) What is the mole fraction of HCl in a solution of hydrochloric acid that has 36% HCl by mass? What is its molality? What is its molarity? The density of the acid solution is 1.305 g/mL
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Exercise
7) Calculate the molality, molarity and mole fraction of FeCl3 in a 28.8 mass % aqueous solution (with density of 1.280 g/mL)
8) An automobile antifreeze mixture is made by mixing equal volumes of ethylene glycol (d=1.114 g/mL; MW = 62.07 g/mol) and water (d=1.00 g/mL) at 20°C. The density of the resulting mixture is 1.070 g/mL. a) % volume c) molarity e) mole fraction b) % mass d) molality
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Intermolecular Forces Temperature Pressure
Factors Affecting Solubility
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Intermolecular Forces
Substances with similar intermolecular forces tend to be soluble in one another.
The stronger the attractions between solute and solvent, the greater the solubility.
Liquid-Liquid Interactions: “Miscible” – when two liquids mix in all proportions, e.g. H2O, C2H5OH “Immiscible” – when two liquids do not dissolve significantly in one another, e.g. H2O, C6H14
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Temperature on Ionic Salts
The change in enthalpy of salt solutions (ΔHsol‘n) can either be positive or negative
solute + solvent + heat solution
solute + solvent solution + heat
Temperature is a measure of internal energy in a system. It is a state…
Heat is the transfer of energy due to differences in energy of interacting systems. It is a path taken… (which for a path that is constant in pressure is equal to the ENTHALPY, ΔH)
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Temperature on Ionic Salts
Solubility is the amount of solute needed to form a saturated solution in a given quantity of solvent
Solids:
↑T → ↑Solubility
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Thermodynamics of different processes
For the any process,
Gibbs free energy(G) = the energy that’s free to do work (other than expansion work!)
The higher the G, the more work a system can do
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Thermodynamics of different processes
When Gfinal is higher than Ginitial, the energy to do work of a system increase, then you need an input of energy (from the surrounding!) this process does not happen spontaneously
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Thermodynamics of different processes
When Gfinal is less than Ginitial, the energy to do work of a system decreases… the system loses energy, it has less energy to do further work this process happens spontaneously
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Thermodynamics of different processes
When Gfinal is equal to Ginitial, there is no change in energy in the system, and we call it a state of equilibrium.
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Thermodynamics of solution process
For solutions to occur, ∆Gsol’n<0 How is ∆G related to enthalpy (∆H), entropy (∆S)
and Temperature?
Temp
(+) (+/-) (+)
Negative x positive x positive = negative
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Temperature on Gases
Why don’t carbonated beverages taste as good when they’re no longer cold?
Gases:
↑T → ↓Solubility
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Thermodynamics of solution process
For solutions to occur, ∆Gsol’n<0
(-) (-)
Negative x positive x negative = positive
(+)
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Pressure
Sg = kHPg
The solubility of a gas in a liquid is proportional to its partial pressure over the solution (Henry’s law).
Carbonated beverages are bottled under PCO2 > 1 atm. As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.
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Crystallization – playing with Solubility
Lower temp low solubility Higher temp high solubility
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Exercise
For a saturated aqueous solution of each of the following at 20°C and 1 atm, will be the solubility increase, decrease or the same? He (g), decrease T RbI (s), increase P
Caffeine is about 10 times soluble in hot water as in cold water. A chemist puts a hot-water extract of caffeine into an ice bath and some caffeine crystallizes. Is the remaining solution saturated, unsaturated, or supersaturated?