Solution Chemistry

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Chapter 13

• Solution Chemistry

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Tragedy in Cameroon

• Lake Nyos
• lake in Cameroon, West Africa
• on August 22, 1986, 1700 people 3000 cattle
  died
  
• Burped Carbon Dioxide Cloud
• CO2 seeps in from underground and dissolves in
  lake water to levels above normal saturation
  
• though not toxic, CO2 is heavier than air the
  people died from asphyxiation
  

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Tragedy in Cameroona Possible Solution

• scientist have studied Lake Nyos and similar
  lakes in the region to try and keep such a
  tragedy from reoccurring
  
• currently, they are trying to keep the CO2 levels
  in the lake water from reaching the very high
  supersaturation levels by pumping air into the
  water to agitate it

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Solution

• A homogeneous mixture of 2 or more substances
• Solvent the majority component
• Solute the minority component

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Solubility in Water
Dissolved molecules (sugar)
Dissolved ions (NaCl)
Electrolyte solution
Nonelectrolyte solution

• When ionic compounds dissolve in water they
  dissociate into ions
  

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Water the universal solvent

• Solutions in which water is the solvent are
  called aqueous solutions
  
• Polar molecule
• Oxygen atom has a partial negative charge
• Hydrogen atoms have a partial positive charge

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Solution Process - Ionic Compounds

• Attractive forces between anions cations in
  solid
  
• versus
• Attractive forces between water molecules and
  cations water and anions

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The Solution Process - Ionic Compounds

• Ions become surrounded by water molecules -
  hydrated
  
• When solute particles are surrounded by solvent
  molecules we say they are solvated

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The Solution ProcessMolecular Compounds

• Covalent molecules that are small and have
  polar groups tend to be soluble in water

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Likes dissolve likes

• Molecules that are similar in structure tend to
  form solutions
  
• Which of these molecules will dissolve in water?

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Solubility Definitions

• When one substance (solute) dissolves in another
  (solvent) it is said to be soluble
  
• Salt is soluble in water,
• Grease is soluble in turpentine
• When two liquids are soluble, they are miscible.
• When one substance does not dissolve in another
  they are said to be insoluble.
  
• Oil is insoluble in water
• When two liquids are insoluble, they are
  immiscible.
  

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In which phase will this molecule dissolve?
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Which of these compounds would you expect to be
the least soluble in water?

• CCl4
• CH3Cl
• H2S
• KF

• CH3OH
• C6H12
• CH3COCH3
• CH3CH20CH2CH3

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More definitions

• Solubility A property of the solute in the
  solvent.
  
• Each chemical will be listed with how much
  dissolves in the solvent. This information can
  be found in the Handbook of Chemistry and
  Physics.
• A concentrated solution has a high proportion of
  solute to solution
  
• A dilute solution has a low proportion of solute
  to solution

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Describing Solutions - Qualitatively

• A saturated solution has the maximum amount of
  solute that will dissolve in the solvent
  
• There is usually a limit to the solubility of one
  substance in another
  
• Gases are always soluble in each other
• Some liquids are always mutually soluble
• Depends on temperature
• An unsaturated solution has less than the
  saturation limit
  
• A supersaturated solution has more than the
  saturation limit
  
• Unstable

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Supersaturated Solutions

• Under some conditions, more solute will dissolve
  than saturated. Heat most salts in water, the
  solubility increases. Slowly cool down the
  solution. Extra solid doesnt come out easily.
  Something happens to cause crystals to form.
  Then, zap, they form.
• Examples fudge, honey

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Solubility and Temperature

• Each ionic compound is different but solubility
  of most increases with temperature
  

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Which picture has a higher gas pressure?
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Gas Solubility

• The solubility of gases in liquids
• Higher pressure Larger solubility
• Lower Temp Larger solubility

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Solution Concentration
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A laboratory procedure calls for 2.0 molal
aqueous solution. A student accidentally makes a
2.0 molar solution. Will the solution made by
the student bea) too concentratedb) too
dilutec) just rightd) it would depend on the
molecular weight of the solute
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Diluting Solutions

• Add more solvent to concentrated solution
• Volume x Molarity Moles
• V1M1 V2M2

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Solution Stoichiometry
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PROBLEM What volume (in mL) of 0.25 M Na2SO4
solution is needed to precipitate all of the
barium, as BaSO4(s), from 12.5 mL of 0.15 M
Ba(NO3)2 solution?

• Given 12.5 mL of 0.15 M Ba(NO3)2
• Want volume of 0.25 M Na2SO4
• Balance the reaction
• Na2SO4(aq) Ba(NO3)2(aq) ? NaNO3(aq)
  BaSO4(s)
  
• Convert to moles
• Solve for moles of wanted (or what is to be
  found)
  
• Convert from moles to volume.

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PROBLEM How many grams of lead(II) iodine will
precipitate when excess potassium iodide solution
is added to 50.0 mL of 0.811 M lead nitrate?

• Given 50.0 mL Pb(NO3)2
• Want mass of PbI2
• Balance the reaction KI(aq) Pb(NO3)2(aq) ?
  PbI2(s) KNO3
  
• Convert to moles
• Solve for moles of wanted (or what is to be
  found)
  
• Convert from moles to grams.

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Colligative Properties of Solutions

• Properties of solution differ from those of a
  pure solvent and depend on the amount of solute
  present
  
• Freezing Point Depression (?Tf)
• Antifreeze, wood frogs
• Boiling Point Elevation (?Tb)
• Kf and Kb are properties of the solvent

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The wood frog can survive at body temperatures as
low as -8C. Calculate the molality of a glucose
solution (C6H12O6) required to lower the freezing
point of water to -8C.

• Water
• Kf 1.86 C kg solvent/mol solute
• Kb 0.512 C kg solvent/mol solute

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Osmosis

• Water flows from an area of low solute
  concentration to an area of high solute
  concentration

Isotonic solution
Hypotonic solution
Hypertonic solution
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Osmotic Pressure

• The water flow changes a change in volume which
  leads to a build-up in pressure
  
• Osmotic pressure is the pressure difference
  between the two solutions