Ch 15 16 Aqueous Systems

1
Ch 15 - 16 Aqueous Systems

• Vocab
• Solution Homogeneous mixture solid liquid, or
  gas
• Soluble Capable of being dissolved
• Solute Substance that is dissolved, present in
  lesser amounts
• Solvent Dissolving medium present in greater
  amounts

2
Types of Mixtures

• Solutions particles lt 1nm cannot be seen no
  scattering of light (salt water)
• Suspensions particles gt 1000 nm settle to the
  bottom (sand water)
• Colloids particles 1-1000 nm suspended
  throughout the medium exhibit Tyndall effect
• Tyndall Effect Scattering of light so that the
  beam of light can be seen like headlights in fog
  seen in colloids

3
Hydration

• When crystallized from solution, some ionic
  compounds form crystals that incorporate water
  molecules into their crystalline structure.
  These are known as hydrates and have specific
  ratios of water to compound. Heating can drive
  off the water and leave the anhydrous salt
• CuSO4 5H2O
• (copper (II) sulfate pentahydrate

4
Types of Solutes

• Electrolytes Conduct an electric current when
  dissolved ionic compounds
• Nonelectrolytes Do not conduct an electric
  current when dissolved covalent compounds

5
Dissociation

• Sodium Chloride
• NaCl(s) ? Na1(aq) Cl-1(aq)
• Magnesium nitrate
• Mg(NO3)2(s) ? Mg2(aq) 2NO3-1(aq)
• Dissociation is the breaking apart of ions as
  an ionic compound dissolves in water.

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Factors Affecting Rate of Dissolution

• Surface Area Greater surface area (smaller
  particle size) faster dissolution
• Agitation stirring faster dissolution
• Heating generally means faster dissolution
• Pressure increasing the pressure results in
  faster dissolving time

7
Like Dissolves Like

• The polarity of water molecules plays an
  important role in the formation of solutions of
  ionic compounds and other polar compounds in
  water.
• The positive ends of the water molecules attract
  negative ions (dipoles) and the negative ends of
  water molecules attract positive ions (dipoles),
  pulling them from the surface of the crystal.

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Miscible/Immiscible

• Nonpolar molecules do not attract ions of
  crystalline solids strongly enough to overcome
  forces holding them together and therefore cannot
  dissolve polar or ionic compounds.
• Miscible Two liquids mix soluble in one
  another (Ex Ethanol Water)
• Immiscible Do not mix insoluble in one another
  (Ex Oil Water)

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Relative Concentration

• Concentrated Large amount of solute
• Dilute Small amount of solute
• Unsaturated Less than the theoretical amount of
  solute is dissolved there is room to dissolve
  more
• Saturated The maximum amount of solute that can
  be dissolved at a certain temperature has been
  reached
• Supersaturated More than the theoretical amount
  of solute is dissolved in solvent at a given
  temperature

10
Solubility/Recrystallization

• For every combination of solvent with a solid
  solute at a given temperature, there is a limit
  to the amount of solute that can be dissolved.
  (The solution becomes saturated.)
• At the same time substances are breaking apart,
  there are also particles attracted to each other
  and remaining together RECRYSTALLIZATION
• Solution Equilibrium Rate of dissolution Rate
  of recrystallization

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Gas Solubility

• Henrys Law S1 /P1 S2 / P2
• The solubility of a gas in a liquid is directly
  proportional to the partial pressure of that gas
  on the surface of the liquid.
• Open a can of pop!! Carbon dioxide is forced
  into solution of flavored water at 5-10 atm.
• Effervescence The FIZZING that happens when you
  open the pop the release of gas from a solution
• How does the carbonation in a cold can vs. a warm
  can compare?

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Heats of Solution

• The formation of solutions recrystallization
  are accompanied by energy changes
• (absorb) Energy to overcome solute-solute
  interactions (Endothermic step)
• (absorb) Energy to overcome solvent-solvent
  interactions (Endothermic step)
• -(give off) Energy when solvent particles are
  attracted to and solvating solute particles
  (Exothermic step)

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Heats of Solution

• ?Hsolution can be endothermic ( value) and
  written as a reactant or exothermic (- value)
  and written as a product
• AgNO3 (s) 22.8 kJ/mol ? Ag(aq) NO3-1(aq)

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Concentrations of Solutions

• Concentration Amount of solute dissolved in a
  given amount of solvent
• Molarity M mol solute
• L solution
• Molality m mol solute
• kg solvent
• Percent by mass m/v mass solute x 100
• 
  mL solution
• Percent by volume v/v vol solute x 100
• 
  vol solution

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Dilutions

• You have the following stock solutions available
  2.0 M NaCl 4.0 M KNO3 0.5M MgSO4
• Calculate the stock volumes you must dilute to
  make the following solutions using
• M1V1 M2V2
• 500 mL of a 0.50 M sodium chloride solution
• 2.0 L of a .20 M magnesium sulfate solution

16
Examples/Molarity

• What is the molarity of a solution that contains
  212.5 g of sodium nitrate (NaNO3) in 3.0 liters
  of solution?
• What mass of sucrose, C12H22O11 is needed to make
  300 mL of a 0.50 M solution?

17
Examples/Molality

• Calculate the molality of a solution made by
  dissolving 45.0 g of dextrose, C6H12O6 in 500.0 g
  of water.
• What is the mass of water required to prepare a
  1.00 molal solution containing 10.0 g of NaOH?
• Calculate the molality of a solution prepared by
  dissolving 6.3 moles of KCN in 633 g of water

18
Percent Problems

• m/v Calculate the number of grams of solute
  required to make the following solutions
• 2.5 L of saline solution 90 NaCl
• 50 mL of 4 magnesium chloride
• v/v 10 mL acetic acid is diluted with water to
  a total solution volume of 200 mL. What is the
  v/v of acetic acid?
• 25mL of ethanol and 75 mL of water are mixed.
  What is the v/v of the solution?