Readings and Problems in Chang

1
Readings and Problems in Chang

• Chapter 12
• Read Sections 12.1 12.2 (without energy
  connections) 12.3 (molarity only) 12.4 12.6
  (descriptive only except for osmotic pressure)
• Problems 3-5 9-12 30 39 46 66 68
• Chapter 4
• Read Sections 4.1- 4.3 4.4 (without
  classifications) 4.5 4.7
• Problems 2 3 6-10 12 17-22 46 50 60 62
  64 66 70 72 74 78 86 88

2
Solutions

• Terminology
• Solubility in practice
• Working with solutions
• Molarity
• Other units of concentrations
• Colligative Properties of solutions
• Vapor pressure lowering
• Freezing point depression and boiling point
  elevation
• Osmosis

3
Terminology

• Solution - A homogeneous mixture of two or more
  substances
• Not a suspension or colloid
• Solvent - The component of the solution that has
  the same physical state as the final solution
• The larger component of the solution
• Solute - Any component that is dissolved in the
  solvent
• The smaller component of the solution
• Saturated solution - A solution that contains the
  maximum amount of solute at the prevailing
  condition (T)
• Supersaturated solution - A solution that
  contains the more than the maximum amount of
  solute at the prevailing condition (T)
• This is a temporary unstable situation that lead
  to either precipitation or separation into layers

4
Types of solutions

• Solution of gases (Air) homogeneous mixture
  nitrogen, oxygen, argon and other things
• Solution of solids (Alloy-bronze) homogeneous
  mixture of copper and tin (etc)
• Solution of liquids (Gasoline) homogeneous
  mixture of various hydrocarbons (octane, etc)
• Aqueous solutions most common type of solution
• Gas in water (natural water containing O2
  carbonated beverage)
• Liquid in water (various alcohols, antifreezes)
• Solid in water (salt water, countless reagent
  solutions)

5
The Solution Process Dissolving

• Intermolecular forces between solvent molecules
  (solvent-solvent attractions) have to be
  disrupted
• Interparticle forces between solute particles
  (solute-solute attractions) have to be disrupted
• New interactions between the solute particles and
  the solvent molecules (solute-solvent
  attractions) have to occur
• The strengths of the new attractions have to be
  equivalent to or stronger than solvent-solvent
  and solute-solute attractions

6
Factors Influencing Solubility

• Matching solute and solvent attractions
• Strong solvent solvent interactions with strong
  solute solute interactions
• Water H-bonds
• Ionic substances H-bonding covalent substances
• NaCl, NH3, and CH3CH2OH dissolve in water
• weak solvent solvent interactions with weak
  solute solute interactions
• Heptane London forces
• Nonpolar substances
• candle wax and grease dissolve in heptane
• Mis-matching solute and solvent attractions
• Strong solvent solvent interactions with weak
  solute solute interactions
• candle wax is not soluble in water
• Weak solvent solvent interactions with strong
  solute solute interactions
• NaCl is not soluble in gasoline

7
Solubility in Water - Review

• Nonpolar substances and large network covalent
  substance have limited water solubility
• Polar, H-bonding covalent solutes
• Interactions between water molecules in pure
  water are strong.
• Molecular substances will only dissolve in water
  if they can form hydrogen bonds or dipole-dipole
  interactions with water
• Ionic substances
• Attractions between cations and anions are strong
  in ionic solids.
• Ion-water interactions are also strong.
• ionic compounds are often soluble in water IF
• the charges on the ions are 1 or -1 (and
  sometimes 2 or -2)
• the attractions between ions in a crystal are not
  compact and strong

8
Solubility of gases in water

• Solubility of gases in any solvent is limited
• Gases (by definition) have high kinetic energy
• Being solvated decreases kinetic energy without
  an equivalent gain in potential energy
• Entropy (freedom of motion) plays a big role
• Solubility of gases decreases as temperature
  increases
• Gas molecules will have even more kinetic energy!
  
• Solubility of gases in water
• High solubility ONLY if gas reacts with water
• NH3, HCl, HF, SO3
• Gases can be forced into solution with pressure
• Carbon dioxide (carbonated beverages
• O2 solubility
• Critical for aquatic life
• Decreases with increasing temperature ? thermal
  pollution
• Nonpolar gases have some solubility ? Cl2 (aq)

9
Solubility (miscibility) of liquids in water

• Miscibility determined by balance of polar and
  nonpolar components of solvent
• Alcohol solubility
• CH3OH, CH3CH2OH, CH3CH2CH2OH miscible in all
  parts with water
• CH3CH2CH2CH2OH partially miscible
• CH3CH2CH2CH2CH2OH and larger alcohols immiscible
• Immiscible liquids
• One polar (water) other nonpolar (usually
  hydrocarbon)
• Partially soluble in each other but layers formed
• Less dense layer on top
• Heptane top and H2O bottom but CCl4 bottom and
  H2O top
• Extraction
• Ability to move a substance from one solvent to
  another
• Based on relative solubility of substance in two
  solvents
• Halogens move to heptane from water

10
Temperature and solubility of solids

• Solubility of most solids increases with
  temperature
• Exceptions
• Differential solubility allows for fractional
  crystallization
• Temperature solubility effect related to entropy

11
Solubility Qualitative to Quantitative

• Solubility is a relative concept
• Enough solvent everything is soluble
• Excess solute nothing is soluble
• General solubility expressed per 100 mL
• gt 10 g/100 mL very soluble
• 1-10g/100 mL moderately soluble
• lt 1g/100 mL insoluble
• Statements of solubility of various substances
  are based on these parameters

12
Expressing Concentration

• Concentration amount of solute in relationship
  to solvent
• Molarity (M) - The number of moles of solute in 1
  liter of solution
• the most important concentration unit
• Percent by mass ( m/m) - The number of grams of
  solute in 100 grams of solution
• Percent by volume ( v/v) - The number of mL of
  solute in 100 mL of solution
• Mass-volume Percent ( m/v) - The number of grams
  of solute in 100 mL of solution

13
Switching Yard - review
PV nRT
14
Colligative Properties

• Colligative Properties Properties of a solution
  that depend only on the concentration of solute
  particles
• Vapor pressure lowering
• The vapor pressure of a solution decreases as the
  particles of solute increases
• Freezing Point Depression
• A solution freezes at a lower temperature than
  the pure solvent.
• Boiling Point Elevation
• A solution boils at a higher temperature than the
  pure solvent.
• Osmosis
• Solvent molecules pass through a semipermeable
  membrane dividing two solutions in such a way as
  equalize the concentration of the solute

15
Osmosis on the molecular level

• There are holes only big enough to allow the
  passage of water molecules
• Solute molecules can sometimes block the holes
• Water molecules in the dilute solution have less
  solute molecules blocking holes than do water
  molecules in the concentrated solution
• Water will move to the more concentrated solution

16
Osmotic Pressure

• The natural flow of water to the more
  concentrated solution can reversed by applying
  pressure
• The amount of pressure applied to counteract the
  natural flow osmotic pressure
• P MRT
• M molarity of solution R O.082 L-atm/mol-K
  T in K
• A way to determine molecular weight

17
Determining molecular weight
What is the molecular weight of a molecule if
1.12 g dissolved in 100.0 mL of solution yields
an osmotic pressure of 567 torr at 25 oC?

• MRT
• 567 / 760 atm M (0.0821 L.atm/mol.K)(298 K)
• 0.746 / (0.0821 x 298) mol/L M
• 0.0305 mol/L M
• 0.0305 M x 0.1000 L 0.00305 mol
• 1.12 g / 0.003049 mol 367 g/mol

18
Osmosis in action

• Only water can cross membrane
• Movement in both directions
• Movement toward more solute favored
• Equilibrium when concentration of solute on both
  sides equal (if possible)

19
Osmosis in blood

• Intravenous solutions have to have the same
  concentration of particles as in blood
• Isotonic solution
• Hypertonic too much solute
• Hypotonic too little solute

20
Additional Applications of Colligative Properties

• Determination of Molecular weight
• Freezing point depression, boiling point
  elevation
• DT proportional to moles in solution
• Antifreezes and Coolants
• Mixtures of solvents (glycols)
• Road ice melting
• NaCl, CaCl2
• Distinguishing between electrolytes and
  non-electrolytes
• Ice cream making

21
Reactions in Aqueous Solution

• The Solvent Water
• Possible roles
• Reaction with added components
• Solvation of added components
• Media for transport of added components
• Dissolved components solutes
• In solution before or after reaction
• Expressed as actual species present in water
• Non dissolved components
• Solids, liquids or gases that enter as reactants
• Solids, liquids or gases that leave as products

22
Whats reacting and whats not?

• Whats not
• substance dissolving in water without changing
  the chemical characteristics of the species
  comprising the substance
• Ionic substances forming cations and anions in
  solution
• Covalent compounds dissolving in solutions
  without reacting with water
• Species that have the same status before and
  after other things have changed
• What is
• Any species that reacts with water
• Any species that changes location as a
  consequence of an interaction in water
  (precipitation)
• Any species that changes chemical properties by
  being placed in water
• Any species that reacts with another species in
  water

23
General reactions in water - 1

• Precipitations
• General form XY (aq)AB (aq) ? XB (aq)AY (s)
• Tip off XY and AB are both salts
• Metal followed by nonmetal ion or polyatomic ion
• Acid-base
• General forms
• HY(aq) XOH(aq) ? H2O (l) XY (aq)
• any acid with strong hydroxide base
• HY(aq) RNH2 (aq) ? RNH3Y (aq)
• any acid with weak amine base R can be H NH3
  (ammonia)
• Tip off HY and also a base

24
General reactions in water - 2

• Oxidation-reduction (redox)
• Most difficult to spot
• General forms
• Xo AY ? AX Yo
• Bo AY ? BY Ao
• Ao Yo ? AY
• Tip off species in elemental state metals,
  dissolved nonmetals gases, liquids
• No reaction!
• No interaction with water or other species
• Cations anions that can coexist dissolved
• Reactions that actually go in opposite direction
• You can write all kinds of reactions but that
  does not mean that they occur as written

25
Obtaining balanced complete equations

• Determine type of reactant equation
• Know tip offs
• Pair matching parts
• Oppositely charged ions for precipitation
  reactions
• H and OH to make water from acid plus hydroxide
  cation and anion to make salt
• Add H to amine and follow with anion
• Determine electron flow (metals ? cations
  nonmetals ? anions) and do transfer of electrons
• Balance equation
• Balance charges of oppositely charged ions
• Balance number of H and OH
• Balance electrons lost vs electrons gained

26
Obtaining net ionic equations

• What stays together?
• Precipitated salts
• Water
• Weak acids and bases
• Covalent compounds that do not ionize (for now
  that is everything that is not an acid or an
  amine)
• Metals
• What is separated into pieces?
• Soluble ionic salts (solubility rules!)
• Strong acids HCl HBr HI HClO4 HBrO4 HIO4 HNO3
  H2SO4
• What is cancelled out?
• Only those species that are exactly the same in
  species designation (s, l, g, aq) in products and
  reactants

27
Completing Equations example
NaCl (aq) AgNO3 (aq) ? ????????
Mixing, Matching Decision Box
NaCl (aq) AgNO3 (aq) ? NaNO3 (aq) AgCl (s)
28
Getting Net Ionic Equations
NaCl (aq) AgNO3 (aq) ? NaNO3 (aq) AgCl (s)

• Identify the stuff that is new
• NEW a different label than it or components had
  on the other side
• What did it come from (look at work box)

Ag (aq) Cl- (aq) ? AgCl (s)
29
Some examples

• Li3PO4 (aq) Fe(NO3)2 (aq) ?
• BaCl2 (aq) Mg(NO3)2(aq) ?
• H2SO4 (aq) NaOH (aq) ?
• H2SO3 (aq) NaOH (aq) ?
• NH3 (aq) HBr (aq) ?
• C3H7NH2 (aq) HCl (aq) ?
• Ca (s) CuSO4 (aq) ?
• MgBr2 (aq) Cl2 (aq) ?
• H2S (aq) NH3 (aq) ?
• Cr (s) O2 (g) ?
• Mg (s) H2SO4 (aq) ?