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Solutions and Mixtures

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Title: Solutions and Mixtures


1
Solutions and Mixtures
2
Solutions
  • Solutions are stable, homogeneous mixtures
  • Particles are evenly distributed through the
    mixture
  • Will not spontaneously separate
  • There are two parts solute and solvent

3
Parts of a Solution
  • Solute the substance being dissolved
  • Solvent the substance doing the dissolving
    (think water universal solvent)

4
Classes of Solutions
  • Classes are based on the final state of matter of
    the soln
  • Three classes
  • Gaseous solns mixtures of two gases
  • Liquid solns gas, liquid, or solid is
    dissolved in a liquid
  • Solid solns solid and a liquid or two solids
    are melted, mixed, or cooled
  • Alloys solid solutions containing metals

5
Solution of a gas (hydrogen) in a solid
(palladium)
6
Examples of solid solutions
7
Solution Terms
  • Most solns are combinations of liquids and
    solids
  • Miscible liquids or gases that will dissolve in
    each other
  • Immiscible liquids or gases that will not
    dissolve in each other
  • Soluble able to be dissolved in a particular
    solvent
  • Insoluble not able to be dissolved in a
    particular solvent

8
Concentrations
  • Concentrated solution a large amount of solute
    is dissolved in the solvent
  • Dilute solution a small amount of solute is
    dissolved in the solvent

9
Saturation
  • Saturated soln a soln that has dissolved all
    the solute it can hold at a given temp
  • NOT the same as concentrated
  • Soln can be saturated but not concentrated and
    vise versa

10
Saturation (cont)
  • Unsaturated soln any soln that can dissolve
    more solute at a given temp
  • Supersaturated soln soln that contains more
    solute than a saturated one has at that temp
  • It is unstable
  • Crystallization from Supersaturated Solutions of
    Sodium Acetate

11
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12
Determining Saturation
  • Add a solute crystal to a soln
  • If dissolves unsaturated
  • If does not dissolve saturated
  • If excess solute comes out supersaturated

13
The Formation of Solns
  • The process of dissolving takes place at the
    surface of the solid solute
  • Water molecules orient themselves on the surface
    of the crystal so that they can separate the
    pieces and pull them into soln
  • Once separated, the pieces are surrounded by
    water molecules

14
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15
Formation of Soln (cont)
  • Solvation process of surrounding solute
    particles with solvent particles
  • Hydration solvation involving water
  • Formation of a soln involves
  • Breaking the attractions among solute particles
  • Breaking the attractions among solvent particles
  • Formation of attractions between solute and
    solvent particles

16
Heat of Solution
  • Defined as overall energy change that occurs
    during solution formation
  • Energy is required for formation of solns
  • Breaking attractions endothermic
  • Forming attractions exothermic
  • Overall process (net change) depends on the
    balance between breaking and forming attractions
  • If breaking requires more energy endothermic
  • If forming releases more energy - exothermic

17
Solubility
  • Defined as the maximum amount of a solute that
    will dissolve in a given amount of solvent at a
    specific temp
  • Determined experimentally
  • Usually expressed in grams of solute per 100
    grams of solvent at a specific temp and pressure

18
Solubility
  • Based on the idea of attractions between
    particles
  • Overall polarity determines solubility
  • Like dissolves like
  • Polar dissolves polar
  • Nonpolar dissolves nonpolar

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20
Picture of a soap micelle.
21
Solvation
  • The extent to which one substance dissolves in
    another depends on
  • State of solute and solvent
  • Temp and pressure
  • Solubilities change with temp (the key to
    preparing a supersaturated soln)
  • Solubility of solids and liquids increases
  • Solubility of gases in liquids decreases

22
Solubilities of Substances in Water
23
Solubilities of Gases in Water
24
Henrys Law
  • Solubilities of solids and liquids are not
    appreciably affected by pressure, but solubility
    of a gas in a liquid is strongly influenced by
    pressure
  • Henrys Law the amount of gas dissolved in a
    solvent is proportional to the partial pressure
    of that gas over the solvent

25
The solubility of a gas depends upon its partial
pressure above the solution.
26
Factors that Affect Rate of Solvation
  • Must increase the collisions between solute and
    solvent particles to increase rate at which
    solute dissolves.
  • Three ways
  • Agitate mixture (stir, shake)
  • Increase surface area (breaking solute into small
    pieces)
  • Increase temp of solvent

27
  • Because dissolving occurs at the surface,
    stirring/shaking allows more solvent to come in
    contact with solute
  • Increasing surface area does the same thing
  • As temp increases, solvent particles move faster,
    which causes more particles to come in contact
    with the solute.
  • Particles of solvent have more energy to remove
    particles from the solid solute

28
Aqueous Solutions
  • Ionic compounds dissociate in water
  • Dissociate a process using energy to separate a
    compound into ions in water
  • Molecular compounds may also dissociate in water
  • Some aqueous solutions conduct electricity

29
Aqueous Solns
  • Conductance the measurement of a solns
    ability to conduct electrical energy
  • Electrolyte any substance that will conduct an
    electric current (by means of movement of ions),
    when dissolved in soln
  • Nonelectrolyte any substance that will not
    conduct an electric current when dissolved in
    water

30
Ways to Describe Solutions
  • There are three ways to describe concentrations
  • Molarity
  • Molality
  • Mole fraction

31
Molarity
  • Molarity (M) of moles of solute dissolved in
    each liter of soln
  • M moles of solute
  • liters of soln
  • Four different labels molarity, molar, M, or
    mol/L
  • Dependent on temp
  • Volumetric flasks are the best containers for
    making a soln of a precise molarity

32
  • What is the molarity of a soln prepared by
    dissolving 16.0 g of barium chloride in enough
    water to give 450 mL of soln?

33
  • How many grams of potassium chloride are needed
    to prepare 0.750L of a 1.50M soln of potassium
    chloride?

34
Molality
  • Molality (m) of moles dissolved in each
    kilogram of solvent
  • m moles of solute
  • kg of solvent
  • Independent of temp

35
  • Calculate the molality of a soln prepared by
    dissolving 20.4 g of sodium chloride in 192g of
    water.

36
  • What is the molality of a solid soln containing
    0.125g of chromium and 81.3g of iron?

37
Mole Fraction
  • Mole fraction (X) of moles of one component
    divided by the total of moles in the soln
  • X moles of component
  • total moles of soln
  • Used to compare solute and solvent

38
  • What are the mole fractions of ethyl alcohol
    (C2H5OH) and water in a soln prepared by adding
    50.0g of ethyl alcohol to 50.0g of water?

39
  • A gas mixture contains the following gases with
    the mole fractions indicated CH4 (0.510), C2H6
    (0.431), C3H8 (0.011), and C4H10 (0.013). The
    mixture also contains the gas acetylene (C2H2).
    What is the mole fraction of acetylene?

40
Colligative Properties
  • A physical property that is dependent on the of
    particles present rather than on the size, mass,
    or characteristics of those particles
  • Four main properties
  • Vapor pressure reduction
  • Boiling point elevation
  • Freezing point depression
  • Osmotic pressure

41
Vapor Pressure Reduction
  • Vapor pressure of a solvent containing a
    nonvolatile solute is lower than the vapor
    pressure of the pure solvent
  • Volatile means tending to evaporate readily
  • Vapor pressure reduction is proportional to
    concentration

42
When a solute is added to the solvent, some of
the solute molecules occupy the space near the
surface of the liquid, as shown in the figure to
the left. When a solute is dissolved in a
solvent, the number of solvent molecules near the
surface decreases, and the vapor pressure of the
solvent decreases.
43
Boiling Point Elevation
  • Because vapor pressure is lower, a higher temp
    will be needed to make the soln boil
  • The amount by which the boiling point is raised
    is the boiling point elevation (?Tb)
  • It is directly proportional to the solns
    molarity

44
A close-up plot of vapor pressure versus
temperature for pure water (red curve) and for a
1.0 M NaCl solution (green curve). Pure water
boils at 100.0C, but the solution does not boil
until 101.0C.
45
Freezing Point Depression
  • The presence of solute molecules will lower the
    freezing point of a substance
  • The more solute particles in soln, the greater
    the lowering of the freezing pt
  • Freezing pt depression the ability of a
    dissolved solute to lower the freezing point of
    its soln
  • Why does salt melt ice?

46
Osmotic Pressure
  • When a semipermeable membrane is placed between 2
    solns so that only solvent particles can move
    through it, a net flow of solvent molecules will
    occur from the less concentrated soln to the
    more concentrated soln (osmosis)
  • Osmotic pressure the pressure required to
    prevent osmosis
  • If 2 different solns have identical osmotic
    pressure, no osmosis will occur (isotonic)

47
A solution inside the bulb is separated from pure
solvent in the container by a semipermeable
membrane. Net passage of solvent from the
container through the membrane occurs, and the
liquid in the tube rises until an equilibrium is
reached. At equilibrium, the osmotic pressure
exerted by the column of liquid in the tube is
sufficient to prevent further net passage of
solvent.
48
Heterogeneous Mixtures
  • Suspension mixture that appears to be uniform
    while being stirred, but separates into different
    phases when agitation ceases
  • Colloid heterogeneous mixture that contains
    intermediate size particles evenly distributed
    through a dispersion medium

49
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50
Heterogeneous Mixtures
  • Brownian Motion jerky, erratic movement of
    dispersed particles
  • Prevent colloids from settling out
  • What is Brownian Motion?
  • Tyndall effect scattering of light by dispersed
    particles

51
The light beam is not visible as it passes
through a true solution (right), but it is
readily visible as it passes through colloidal
silver in water.
52
Tyndall effect in clouds.
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