UNIT 6: Chemical Equilibrium Chapter 15

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• Chapter 15

Solutions
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Solution Definitions to Know Use

• Solution homogenous mixture of two or more
  substances in a single physical state.
• Solute the substance being dissolved.
• Solvent the principal component that dissolves
  another component of a solution.
• Solubility a quantifiable measure of the degree
  to which a substance dissolves in another
  substance.
• Soluble a substance that can be dissolved in
  another substance.
• Insoluble a substance that cannot be dissolved
  in another substance.

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Chapter 15 Solutions

• What are solutions?
• Homogeneous mixtures of two or more substances in
  a single physical state.
• A solution consists of a solute dissolved in a
  solvent.
• Many examples exist.
• What are the intrinsic properties of solutions?
• Contain very small particles
• (atoms, ions, molecules)
• Homogeneous throughout. (Particles are evenly
  distributed on a molecular level).
• Particles do not separate with time under
  constant conditions.
• Diverse physical states and chemical compositions.

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Types of Solutions

• Solid Solutions
• Alloys (14 carat gold, stainless steel, brass)
• Gaseous Solutions
• Air, scuba diving gases, vehicle exhaust
• Liquid Solutions
• Vinegar, antifreeze,
• Aqueous Solutions
• Solutions with water as solvent.
• Seawater, soft drinks

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15-2 Concentration of Solutions

• Concentration the amount of solute dissolved
  per unit of solvent.
• There are many ways to describe concentration,
  but they are either qualitative or quantitative.
• Qualitative a representation of the general
  nature of a solution.
• Quantitative a measure of the amount of a
  solute dissolved in the solution.

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Qualitative Descriptions of Solutions

• Dilute a solution containing very little
  solute.
• Concentrated a solution containing a large
  amount of solute.

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Qualitative Descriptions of Solutions

• Saturated a solution containing the maximum
  amount of solute that can be dissolved at the
  current temperature/pressure.
• Unsaturated a solution containing less than the
  maximum amount of solute that can be dissolved at
  the current temperature/pressure.
• Supersaturated an unstable condition in which a
  solution contains more than the maximum amount of
  solute that can normally be dissolved at the
  current temperature/pressure.

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Solution Stability

• Recall that a saturated solution contains the
  maximum amount of solute that can be dissolved at
  given conditions.
• In a saturated solution, the rate of solute
  entering into solution precisely balances the
  rate at which solute comes out of solution
  (forming a solid precipitate).
• The saturated solution is stable and said to be
  in dynamic equilibrium.

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Solution Stability

• In an unsaturated solution more solute can
  dissolve, so it is not yet at equilibrium.
• In a supersaturated solution more than the
  maximum amount of solute that can normally be
  dissolved at the current conditions is present.
  This is an unstable situation that is resolved by
  precipitating solute (solid). The end result is
  an equilibrium condition.

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Saturation
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Quantitative Descriptions of Solutions

• Quantitative methods are much more useful than
  qualitative descriptions because they specify the
  amounts of components in solutions.
• The most common quantitative descriptions
  include
• Molarity, M moles solute/L of solution
• Molality, m moles of solute/kg of solvent

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Molarity, M

• Molarity (M) moles of solute per liter of
  solution. M moles/L
• What is the molarity of a solution made from 145
  g of NaCl in 2.75 L of solution?
• Vinegar is a solution of acetic acid. What is the
  molarity of the solution produced when 125g of
  acetic acid (C2H4O2) is dissolved in sufficient
  water to prepare 1.50L of solution?

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Molality, m

• Molality (m) moles of solute per kilogram of
  solvent.
• m moles/kg
• What is the molality of a solution made from 20.4
  g KBr in 195 g of water?
• What is the molality of a solution containing
  125g of iodine (I2) and 750.g of carbon
  tetrachloride (CCl4)?

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Mole Fraction, xsolute

• Mole Fraction moles of component per total
  moles of solution.
• Xsolute moles solute
• total moles
• Example What is the mole fraction of sulfur
  dioxide in an industrial exhaust gas containing
  128.0 g of SO2 dissolved in every 1500. g of CO2?
• Answer
• XSO2 (mole fraction SO2)/(total moles of
  solution)
• Moles SO2 128.0g (1 mol SO2 /64.04g SO2)
  1.999 mol SO2
• Moles CO2 1500.g CO2 1 mol CO2/44.01g CO2
  34.08 mol CO2
• XSO2 1.999 mol SO2 ___________ 1.999
  0.05540
• 1.999 mol SO2 34.08 mol CO2 36.08

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Molarity and Dilution Factors

• When diluting a solution into a less concentrated
  one, the total number of moles of solute does not
  change.
• (The compound didnt go anywhere it is still in
  the container!)
• Because Molarity moles/volume,
• the moles Molarity Volume, or M V.
• Therefore we may write the following for the each
  of the two solutions
• moles1 M1V1 and moles2
  M2V2
• Since there are the same number of moles in the
  first solution as in the second, we may let
  moles1 moles2, or also
• McVc MdVd
• Can use this simple equation to calculate the new
  molarity.

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More Solution Definitions to Know

• Miscible liquids that may be mixed together in
  any amount.
• Oil and gasoline.
• Immiscible liquids that cannot be mixed.
• Oil and water.
• Aqueous Solution liquid solutions for which the
  solvent is water.
• Acetic acid and water (vinegar).
• Electrolyte a substance that forms ions in
  solution, enabling the solution to conduct
  electricity.
• NaCl in sea water, Gatorade.
• Non-electrolyte a substance that does not form
  ions in solution, thus giving a non-conducting
  solution.
• Sugar in tea.

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15-3 Formation of Solutions

• Dissolution - the complex interaction of two or
  more separate substances (the solute and the
  solvent) to form a single system (the solution).
• Solvation the process whereby solvent particles
  pull the solute particles into solution and
  surround them the interaction between solute
  and solvent particles to form a solution.
• Hydration - the process whereby water particles
  pull the solute particles into solution and
  surround them to form a solution.
• Solubility a quantifiable measure of the degree
  to which a substance dissolves in another
  substance it is the amount of a solute that
  will dissolve in a specific solvent under given
  conditions. Expressed in gram of solute per 100
  grams of solvent.

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Saturation
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Factors Affecting Solubility

• Nature of Solute and Solvent
• Similar substances dissolve in one another.
  (Likes dissolve likes.)
• Polar substances dissolve in polar substances
• Water dissolves sugar salt.
• Water dissolves rubbing alcohol.
• Nonpolar substances dissolve in nonpolar
  substances.
• Gasoline dissolves oil.
• Dry cleaning fluids dissolve grease and oils.
• Temperature see next slides
• Pressure
• Gas solubility increases with pressure.

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Solubility and Temperature
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Solubility and Temperature
Temperature Solubility of solids and liquids
generally increases with temperature. Solubility
of gases decreases with temperature.
Solubility of salts
Solubility of gases
http//www.elmhurst.edu/chm/vchembook/174temppres
.html
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Energy Changes and the Formation of Solutions

• Solvation/Hydration may be
• exothermic or endothermic.
• ?H depends on the balance of energy released by
  the attraction of solute particles to the solvent
  versus the energy consumed in breaking the
  attractions of solute particles for each other
  (crystal lattice energy).
• Dissolving CaCl2 is very exothermic, but an
  ammonium nitrate cold pack works because the
  solvation is endothermic.

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Factors That Affect Dissolution Rates

• Surface area
• Increasing surface area (making smaller
  particles) increases the rate of dissolution.
• Stirring
• Stirring the solution increases the rate of
  dissolution.
• Temperature
• Increasing the temperature increases the rate of
  dissolution.

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15-4 Colligative Properties

• These are properties that depend on solution
  concentration rather than the nature or type of
  solute.
• They are dependent on molality (molsolute/kgsolven
  t)
• Examples include
• Vapor Pressure Reduction
• Boiling Point Elevation
• Freezing Point Depression

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Vapor Pressure Reduction

• Raoults Law the magnitude of the vapor
  pressure reduction is proportional to the solute
  concentration, regardless of the solute.
• Why? Nonvolatile solute molecules interfere with
  the solvent molecules, preventing them from
  leaving the surface of the solution, and thus
  decreasing the vapor pressure.
• (Fig. 15-22, p 520)
• This results in an increase in the boiling point
• of the solvent, and a decrease in its freezing
  point.
• Boiling point elevation
• Freezing point depression
• Applications?

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Freezing Point Depression

• This is the ability of a dissolved solute to
  lower the freezing point of a solution.
• Example Antifreeze is added to a cars coolant
  system to prevent freezing of the water in
  winter.
• Decrease of freezing point is directly
  proportional to the molality (m) of the solute.
• Calculated from ?Tf Kfm
• where ?Tf is the temperature depression, m is
  molality and Kf is the freezing point depression
  constant.

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Boiling Point Elevation

• This is the ability of a dissolved solute to
  raise the boiling point of a solution.
• Example The antifreeze added to a cars coolant
  system also prevents overheating in summer!
• Increase of boiling point is also directly
  proportional to the molality (m) of the solute.
• Calculated from ?Tb Kbm
• where ?Tb is the temperature depression, m is
  molality and Kb is the freezing point depression
  constant.

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Post Lab Questions

• 1. Which compound types (ionic or covalent)
  produce more particles when dissolved in water
  and why? Remember this is related to molality.
•  2. Which type of compound (ionic or covalent)
  will have a greater effect on the colligative
  properties of a solution? Explain.   

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Determining Molar Mass

• A solution containing 16.9g of a nonvolatile
  molecular compound in 250g of water has a
  freezing point of -0.744 C. What is the molar
  mass of the compound?
• Solution
• First find the molality from ?Tf Kfm.
• 0.744 C (1.86 C/m) x m,
• so the molality 0.400 molal
• But molality moles/kg, so
• 0.400 m ??? moles/0.250 kg,
• from which ??? moles 0.100 mole
• -Since 0.100 mole 16.9g of solute, 1.00 mole of
  solute 169 g, giving a molar mass of 169 g/mol.