Solutions and Mixtures

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Solutions and Mixtures
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Solutions

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

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Parts of a Solution

• Solute the substance being dissolved
• Solvent the substance doing the dissolving
  (think water universal solvent)

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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

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Solution of a gas (hydrogen) in a solid
(palladium)
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Examples of solid solutions
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Ways to Describe Solutions

• There are three ways to describe concentrations
• Molarity
• Molality
• Mole fraction

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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

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• 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?

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

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Molality

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

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

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

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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

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• 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?

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• 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?

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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

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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

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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

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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

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Determining Saturation

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

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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

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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

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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

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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

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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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Picture of a soap micelle.
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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

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Solubilities of Substances in Water
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Solubilities of Gases in Water
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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

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The solubility of a gas depends upon its partial
pressure above the solution.
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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

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• 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

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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

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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

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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

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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

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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.
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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

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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.
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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?

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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)

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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.
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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

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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

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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.
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Tyndall effect in clouds.