hlanganani tutu, c403 school of chemistry email: [email protected]
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Hlanganani Tutu, C403 School of Chemistry Email: [email protected]. The Solution Process. Solution - homogeneous mixture of solute and solvent. In solutions, intermolecular forces become rearranged. Examples of solutions. gas in gas – e.g. air gas in liquid -- e.g. soda - PowerPoint PPT PresentationTRANSCRIPT
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Prentice Hall © 2003 Chapter 13
• Solution - homogeneous mixture of solute and solvent.
• In solutions, intermolecular forces become rearranged.
The Solution ProcessThe Solution Process
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Prentice Hall © 2003 Chapter 13
• gas in gas – e.g. air• gas in liquid -- e.g. soda• gas in solid -- e.g. gas on solid, catalyst• liquid in liquid• liquid in solid -- e.g. mercury amalgam• solid in liquid• solid in solid -- e.g. 14-karat gold, brass
Examples of solutions
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Prentice Hall © 2003 Chapter 13
• Consider NaCl (solute) dissolving in water (solvent):– Interruption of water H-bonds,
– NaCl→Na+ + Cl-,
– ion-dipole forces form: Na+ … -OH2 and Cl- … +H2O.
– If water is the solvent, we say the ions are hydrated.
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Prentice Hall © 2003 Chapter 13
Energy Changes and Solution Formation3 energy steps in forming a solution:
• separation of solute molecules (H1),• separation of solvent molecules (H2), and• formation of solute-solvent interactions (H3).
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Prentice Hall © 2003 Chapter 13
Hsoln = H1 + H2 + H3.
Hsoln can be +ve or -ve depending on the intermolecular forces.
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Prentice Hall © 2003 Chapter 13
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Prentice Hall © 2003 Chapter 13
• “Rule”: polar solvents dissolve……………………….?
Non-polar solvents dissolve..............................................?
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Prentice Hall © 2003 Chapter 13
Exercise: Why doesn’t gasoline dissolve NaCl?
Exercise: Why doesn’t water and octane mix well (immiscible)?
Remember: the resultant solution’s interactions must be stronger than the interactions in the original substance
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Prentice Hall © 2003 Chapter 13
Solution Formation, Spontaneity, and Disorder
• When the energy of the system decreases (e.g. dropping a book and allowing it to fall to a lower potential energy), the process is spontaneous.
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Prentice Hall © 2003 Chapter 13
Example: a mixture of CCl4 and C6H14 is less ordered than the two separate liquids. Therefore, they spontaneously mix
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Prentice Hall © 2003 Chapter 13
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Prentice Hall © 2003 Chapter 13
• There are solutions that form by physical processes and those by chemical processes.
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Prentice Hall © 2003 Chapter 13
• Consider:
Ni(s) + 2HCl(aq) NiCl2(aq) + H2(g).
• When all the water is removed from the solution, no Ni is found only NiCl2·6H2O. Therefore, Ni dissolution in HCl is a chemical process.
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Prentice Hall © 2003 Chapter 13
• Consider:
NaCl(s) + H2O (l) Na+(aq) + Cl-(aq).
• When the water is removed from the solution, NaCl is found. Therefore, NaCl dissolution is a physical process.
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Prentice Hall © 2003 Chapter 13
• Dissolve: solute + solvent solution.• Crystallization: solution solute + solvent.• Saturation: crystallization and dissolution are in
equilibrium.
Saturated Solutions and Saturated Solutions and SolubilitySolubility
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Prentice Hall © 2003 Chapter 13
Solubility: amount of solute required to form a saturated solution.Supersaturation: reached when more solute is dissolved than in a saturated solution.
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Prentice Hall © 2003 Chapter 13
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Prentice Hall © 2003 Chapter 13
Solute-Solvent Interaction
• Miscible liquids: mix in any proportions.• Immiscible liquids: do not mix.• Intermolecular forces are important• The more C atoms, the less the solubility in water.
Factors Affecting Factors Affecting SolubilitySolubility
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Prentice Hall © 2003 Chapter 13
• The -OH groups in a molecule increase solubility in water….“like dissolves like”
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Prentice Hall © 2003 Chapter 13
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Prentice Hall © 2003 Chapter 13
Which of these two would be more soluble in water?
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Prentice Hall © 2003 Chapter 13
• Network solids do not dissolve. Why?
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Prentice Hall © 2003 Chapter 13
Pressure Effects• Solubility of a gas in a liquid is a function of the pressure
of the gas.
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Prentice Hall © 2003 Chapter 13
Pressure Effects
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Prentice Hall © 2003 Chapter 13
• The higher the pressure, the more molecules of gas are close to the solvent
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Prentice Hall © 2003 Chapter 13
where: Sg - solubility of a gas, k is a constant, and Pg is the partial pressure of a gas
gg kPS Henry’s Law gives:
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Prentice Hall © 2003 Chapter 13
Example
27g of acetylene, C2H2, dissolves in 1L of acetone at 1.0 atm pressure. If the partial pressure of acetylene is increased to 12 atm, what is the solubility in acetone?
Solution:
S1 = kP1…………(1)
S2 = kP2…………(2)
Ans: 3.2 x 102 g
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Prentice Hall © 2003 Chapter 13
• Carbonated beverages are bottled with a partial pressure of CO2 > 1 atm.
• What happens when a bottle is opened?
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Prentice Hall © 2003 Chapter 13
Temperature Effects• As temperature increases, solubility of solids generally
increases, e.g. sugar in warm water• Sometimes, solubility decreases as temperature increases
(e.g. Ce2(SO4)3).
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Prentice Hall © 2003 Chapter 13
Temperature Effects• Gases - less soluble at high temperature
• Thermal pollution in dams and rivers – loss of O2
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Prentice Hall © 2003 Chapter 13
Mass Percentage, ppm, and ppb• Definitions:
Ways of Expressing Ways of Expressing ConcentrationConcentration
100solution of mass total
solutionin component of masscomponent of % mass
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Prentice Hall © 2003 Chapter 13
Example: How would you prepare 425 g of an aqueous solution containing 2.40% by mass of sodium acetate, NaC2H3O3?
Ans:Mass of NaC2H3O3 = 10.2 gMass of H2O = mass of solution - mass of NaC2H3O3 = 415 g
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Prentice Hall © 2003 Chapter 13
610solution of mass total
solutionin component of masscomponent of ppm
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Prentice Hall © 2003 Chapter 13
910solution of mass total
solutionin component of masscomponent of ppb
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Prentice Hall © 2003 Chapter 13
solution of moles totalsolutionin component of moles
component offraction Mole
solution of literssolute moles
Molarity
Mole Fraction, Molarity, and Molality
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Prentice Hall © 2003 Chapter 13
• Converting between molarity (M) and molality (m) requires density.
solvent of kgsolute moles
Molality, m
Exercise: 0.2 mol of ethylene glycol is dissolved in 2000 g of water. Calculate the molality
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Prentice Hall © 2003 Chapter 13
Example: What is the molality of a solution containing 5.67 g of glucose, C6H12O6 (Mr = 180.2 g), dissolved in 25.2 g of water? (Calc. the mole fractions of the components as well).
Solution: Think about the solute!................glucose (express in moles) Think about the solvent!...............water (express in kilograms)
Ans: 1.25 m
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Prentice Hall © 2003 Chapter 13
Example: Converting molarity to molality
An aqueous solution is 0.907M Pb(NO3)2. What is the molality of lead nitrate, Pb(NO3)2, in this solution? The density of the solution is 1.252 g/mL. (Molar mass of Pb(NO3)2 = 331.2 g)
Ans: 0.953 m Pb(NO3)2