Solutions

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Solutions
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• Ask a chemist, they always have

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Definitions
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• Mixture several pure substances mixed together
  in an indefinite ratio
• Homogeneous
• Heterogeneous
• Solution a homogenous mixture that form when one
  or more substances dissolve into another.
• Suspensions cloudy mixtures that form when two
  or more substances mix but do not dissolve.
• Emulsions suspension of 2 liquids

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Solutes and Solvents
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• Solution a homogenous mixture
• Solute thing that dissolves
• Solvent thing that does the dissolving (found in
  the largest amounts)
• If the solvent is water, then it is called an
  aqueous solution

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Solubility
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• Why does sugar disappear in your iced tea?
• How do fish breathe underwater?
• Why does soda go flat faster when left out than
  when it is refrigerated?
• It is all based on solubility!

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Solubility
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• Example iced tea
• Solute
• sugar
• tea
• Solvent
• water

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States and Solutions
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• Solutions can be any state of matter
• Solid-solid alloys (gold jewelry, brass, etc.)
• Solid-liquid salt water, sugar water
• Liquid-liquid vinegar, peroxide, rubbing alcohol
• Liquid-gas soda, champagne, O2 in H2O
• Gas-gas air, air tanks (scuba)

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Solubility Ionic Compounds
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• Ions form, separate (dissociate) and move
  throughout the solution
• The forces that hold the ions together are
  overcome by the ions attractions to polar water.
• Ion- dipole interaction
• Because ions are present, ionic solutions can
  conduct a current
• Current is just movement of electrons

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Figure 15.1 Dissolving of solid sodium chloride.
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Solvation animation Animation with Audio
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Figure 15.2 Polar water molecules interacting
with positive and negative ions of a salt.
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Solubility Polar Compounds
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• Like dissolves like
• Typically, hydrogen bonding occurs between the
  substance being dissolved and the polar water
  molecules
• Example
• Sugar in water
• Ethanol in water

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Soluble and Miscible
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• Soluble refers to a solid or gas
• Miscible refers to a special case of solubility
• In liquids, when both liquids are completely
  soluble in each other
• Ex you can pour as much water into isopropyl
  alcohol as you can alcohol into water. At no
  point will it begin to be saturated

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Figure 15.3 The polar water molecule interacts
strongly with the polar OH bond in ethanol.
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Figure 15.4 Structure of common table sugar.
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Get interactions between water molecules the
polar regions on the sugar (the Os) , and some
hydrogen bonding at the -OH groups
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They go together like oil and water. (things
that dont dissolve or mix)
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• Anything nonpolar will not mix well with anything
  polar
• Examples
• Oil spill
• Salad dressing
• Can mix when shaken (LDF) and then may separate
  out (other forces)

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How Things Dissolve
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• Need to find/ create holes in water for the
  dissolving substance to move into
• Need to over come hydrogen bonding between water
  (or solvent) molecules
• Get interactions between water molecules and
  molecules of the solute
• Ion-dipole interactions
• Dipole-dipole (and H bonding)

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How much is too much?
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• There is a limit to the amount of a substance
  dissolved
• Saturated the solution holds as much solute as
  possible at that temperature.
• Unsaturated solution has not reached the limit

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• Can you have too much? YES!
• Supersaturated have as much solute dissolved as
  possible, then cooled and all the solute stays
  dissolved.
• In other wordsthe solution contains more
  dissolved solid than a saturated solution created
  at the same temperature.
• These can begin crystallization of the solute at
  the slightest change

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Cant this go any faster?
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• Q Why are sugar packets better than sugar cubes?
• Ever wonder why we stir things?
• Why does some of the sugar sink to the
  bottom of the glass when making iced tea-no
  matter how much you stir?
• A It has to do with the 3 factors that affect
  dissolution surface area, stirring and
  temperature.

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Factors Affecting Rate (Kinetics)
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• Surface Area
• Stirring
• Temperature

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Surface Area
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• Dissolving occurs at the surface of the solute
  (where they meet).
• The greater the surface area available, the
  faster it will dissolve.
• Ex rock salt vs. table salt
• Ex granulated sugar v. rock candy

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Stirring
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• When particles dissolve, they stay close to the
  surface of the other molecules still waiting.
• Stirring removes already dissolved particles away
  and exposed new ones.
• Just try putting sugar into tea without stirring,
  and see how long it takes to dissolve.

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Temperature
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• Higher temperatures cause the solute (the
  substance doing the dissolving) to move more,
  increasing the speed substance dissolve.
• Ex easier to dissolve sugar in hot tea than it
  is in iced (cold) tea

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Temperature and Solubility of Gases in Liquid
Solutions
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• This is NOT true for dissolving gases in a liquid
• Bubbles evolve faster if the solution is hotter
  (more KE more gases reaching surface over the
  same period of time, meaning they leave quicker
• The colder the liquid, the greater the amount of
  dissolved gases
• The warmer the liquid, the lower the amount of
  dissolved gases
• Think leaving a soda out on the counter rather
  than leaving it in the fridge

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Energy changes and Solvation
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• Any chemical change (including solvation)
  requires a change in energy
• Energy removed from or added to the reactants
  from the surroundings
• NaOH(s)? Na (aq) OH- (aq) ?H -44.5 kJ/mol
• (thats 44.5kJ released, so exothermic, per mole
  of NaOH)
• Because you are breaking the ionic bond, energy
  must be either released when breaking the bond,
  or consumed when making the new ions
• ALL changes in formula indicate a change in
  energy.
• However, sometimes the energy change is so small,
  you cant tell that a change has occurred

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Why some coffees Put hair on your chest.
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• Strong coffee has more coffee dissolved in a
  given amount (say 1 pot) than weak coffee.
• Strong coffee concentrated
• Weak coffee dilute
• Concentration the amount of solute in a given
  amount of solvent (or solution).

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Molarity (M)
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• Most common way to express concentration
• Molarity is the number of moles of solute
  dissolved in each liter of solution
• Formula
• M moles of solute
• liters of solution
• Dependent on temperature
• The higher the molarity the stronger the
  concentration

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Practice Problems

• 1. What is the molarity when 6.0 moles of glucose
  is dissolved in water to make 3.0 L of solution.

2. How many moles of sodium chloride are there in
500 mL of 4.0 M solution?
3. What is the volume of 3.0 M solution that
contains 15 moles of glucose?
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Molality (M )
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• Another way to calculate concentration
• Formula
• M moles solute .
• kilograms of solvent
• Not dependent on temperature
• The higher the molality the stronger the
  concentration

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Normality (N)
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• 3rd way to calculate concentration
• Typically used with acids and bases
• Indicates amount of H and OH- available for
  acid/base reactions
• Formula
• N equivalents .
• 1 liter of solution
• Equivalent weight the mass in grams of acid/base
  that gives 1 mole of H/OH-
• The higher the normality the stronger the
  concentration

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How does something so strong become so weak?
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• The answer is dilution.
• The more dilute something is, the lower the
  concentration (its weaker).
• To accomplish this, add more solvent
• How do we know how much to add?
• M 1V1 M 2V2
• Typically start with a highly concentrated
  solution and dilute down to what you need

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Figure 15.8 Process of making 500 mL of a 1.00
M acetic acid solution.
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Colligative properties
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• In the winter, why do we throw salt when it
  snows?
• Why does Emeril add salt to boiling water when
  cooking pasta?

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Freezing point depression
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• By adding salt (or other solutes) to water, the
  temperature of freezing drops? it freezes at a
  lower temperature
• Because H bonding is disturbed
• Dependent on how much solute is added

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Freezing Point Depression Calcs
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• Antifreeze protects cars from freezing and
  overheating. Calculate the freezing point
  depression of a solution of 100. g of ethylene
  glycol (C2H6O2) antifreeze in 0.500 kg of water.
  Kf water 1.86 oC/m
• Formula
• Tf Kfm i
• Kf Molal Freezing Point depression constant
  (oC/m)
• i Pieces that the material dissociates into
  (for ionic compounds only)
• (Keep I at 1 (one) for covalent compounds)

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Freezing Point Depression and Boiling Point
Elevation

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Boiling point elevation
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• By adding salt (or other compounds) to water, the
  temperature of boiling goes up? it boils at a
  higher temperature
• Interrupts H bonding
• Need more vapor molecules and greater pressure
  to get bubbles to form
• Takes more time to get vapors to add to bubbles
• The molecules that do get into the bubbles need
  more energy
• Dependent on how much solute is added

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Boiling Point Elevation Calculations
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• Water with salt added boils at a higher
  temperature than pure water. By how much will
  the boiling point change if 100.g of salt is
  added to 500. g of water? Kbwater 0.52 oC/m
• Formula
• Tb Kbm i
• Kb Molal Boiling Point elevation constant
  (oC/m)
• i Pieces that the material dissociates into
  (for ionic compounds only)
• (Keep I at 1 (one) for covalent compounds)

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Freezing Point Depression and Boiling Point
Elevation
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Figure 15.10 Pure water.
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Figure 15.9 A bubble in the interior of liquid
water surrounded by solute particles and water
molecules.
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Figure 15.10 Solution (contains solute).
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Vapor Pressure Reduction

• Vapor pressure changes as IMFs change
• For the same reasons boiling point is disturbed
• What would evaporate faster
• Salt water
• Distilled water
• WHY?

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• Colligative properties interactive