Solutions

1
Solutions

• Chapter 13
• Pages 395 - 418

2
Types of Mixtures

• Solutions
• Suspensions
• Colloids

3
Solutions
Solution a homogeneous mixture of 2 or more
substances in a single phase
-atoms, molecules, or ions are thoroughly mixed,
resulting in a mixture that has the same
composition and properties throughout
Soluble capable of being dissolved
4
Particle Model for a Solution
5
Components of Solutions
Solvent the dissolving medium in a solution
Solute the substance dissolved in a solution
(usually the component that is lesser quantity)
Types of Solutions
-exist as gases, liquids, or solids
Alloy solutions of solids uniformly mixed
(brass, sterling silver)
6
Particle Models for Gold and Gold Alloy
7
Properties of Solutions

• Particle size 0.01 1 nm can be atoms, ions,
  molecules

• Do not separate on standing

• Cannot be separated by filtration

• Do not scatter light

• Homogeneous

8
Suspensions
Suspension a mixture in which the particles of
a solvent are so large that they settle out
unless the mixture is constantly stirred or
agitated
Examples muddy water, Italian salad dressing
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Particle Model for a Suspension
10
Properties of Suspensions

• Particle size over 1000 nm, suspended can be
  large particles or aggregates

• Particles settle out

• Can be separated by filtration

• May scatter light, but are not transparent

• heterogeneous

11
Colloids
Colloids particles that are intermediate in
size between those in solutions and suspensions
form mixtures known as colloidal dispersions
-particles are small and suspended by constant
movement
-dispersed phase colloidal particles
-dispersing medium water
Examples emulsions (mayonnaise), foam (shaving
cream), gelatin, aerosols
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Tyndall Effect
-occurs when light is scattered by colloidal
particles dispersed in a transparent medium.
This property can be used to distinguish between
a solution and a colloid.
13
Properties of Colloids

• Particle size 1-1000 n, dispersed can be
  aggregates or large molecules

• Do not separate on standing

• Cannot be separated by filtration

• Scatter light (Tyndall effect)

• heterogeneous

14
Solutes Electrolytes vs. Nonelectrolytes

• Substances that dissolve in water are classified
  according to whether they yield molecules or ions
  in solution.

• Ionic compounds dissolve to yield and ions
  surrounded by water molecules.

• Ions are free to move and therefore conduct
  electricity.

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Particle Models for Electrolytes/Nonelectrolytes
in Solution
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Definitions
Electrolyte a substance that dissolves in water
to give a solution that conducts electric current
Nonelectrolyte a substance that dissolves in
water to give a solution that does not conduct an
electric current
Examples NaCl, HCl (highly polar)
Example sugar solution
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Factors Affecting the Rate of Dissolution

• Increasing the surface area of the solute

• Agitating the solution

• Heating the solvent

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Increasing the Surface Area
To dissolve

• Molecules or ions of the solute are attracted by
  the solvent

• Dissolution occurs at the surface of the solute

• Increasing surface area of solute speeds up the
  process

• The more finely divided a substance is, the
  greater the surface area per unit mass and the
  more quickly it dissolves

19
Agitating a Solution

• Concentration of dissolved solute is high near
  surface of a solute

• Stirring or shaking disperses solute particles
  and brings fresh solvent in contact with the
  solute surface

• Stirring is similar to crushing a solid contact
  between solvent and solute surface is increased

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Heating a Solvent

• With increase in solvent temperature, solvent
  molecules move faster and KE increases

• At higher temperatures, collisions between
  solvent and solute molecules are more frequent
  and of higher energy

• This separates solute molecules from one another
  and disperses them among the solvent molecules

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Solubility
Saturated
Unsaturated
Supersaturated
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Solubility
-for every combination of solvent and solid
solute at a given temperature, there is a limit
to the amount of solute that can be dissolved
-the point at which this limit is reached depends
on the nature of the solute, the nature of the
solvent, and the temperature
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Model
-sugar dropped in water
-as more solid dissolves, collisions increase
-sugar molecules leave solid and move at random
in solvent
-eventually, molecules are returning to the
crystal at the same rate at which they are going
into solution
-some may collide with solid and remain there
Solution equilibrium the physical state in
which the opposing processes of dissolution and
crystallization of a solute occur at equal rates
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Saturated
Saturated solution a solution that contains the
maximum amount of dissolved solute
How to tell a solution is saturated when more
solute is added it falls to the bottom and does
not dissolve
25
Unsaturated
Unsaturated solution a solution that contains
less solute than a saturated solution under the
existing conditions
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Supersaturated
-if solubility increases w/temperature, the
excess solute usually comes out of solution,
leaving the solution saturated at the lower temp
-sometimes, if the solution cools undisturbed,
the excess solute does not separate
Supersaturated solution a solution that
contains more dissolved solute than a saturated
solution contains under the same conditions
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-dropping a small crystal of the solute into a
supersaturated solution (seeding) or disturbing
the solution causes a rapid formation of crystals
by the excess solute
-once crystals begin to form, the process
continues until equilibrium is established at the
lower temp
Solubility of a substance is the amount of that
substance required to form a saturated solution
with a specific amount of solvent at a specified
temperature
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-temperature must be specified because solubility
varies with temperature (for gases, pressure must
also be specified)
-the maximum amount of solute that dissolves and
reaches equilibrium is always the same under the
same conditions
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Solute-Solvent Interactions
Rule of Thumb like dissolves like
-substances are alike when similar in type of
bonding, the polarity or nonpolarity of
molecules, and the intermolecular forces between
solute and solvent
30
Dissolving Ionic Compounds in Aqueous Solution
-polarity of water plays important role
-charged ends attract ions in ionic compounds and
surround them to keep them separated from other
ions in the solution
-attraction is strong enough to draw ions away
from crystal surface and into solution
Hydrated the solution process with water as the
solvent
-ions are said to be hydrated
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-entire crystal dissolves and hydrated ions are
uniformly distributed
-as crystals reform they retain specific ratios
of water molecules and are known as hydrates
(CuSO4 . 5H2O)
-a hydrates behavior is just like that of the
anhydrous form dissolving results in hydrated
ions and water
-ions and polar molecules are linked to water
with dipole-dipole attractions
32
Nonpolar Solvents
-generally, ionic compounds are not soluble in
nonpolar solvents
Examples CCl4 C6H5CH3 (toluene) gasoline
-nonpolar solvent molecules do not attract ions
strongly enough to overcome the forces holding
the crystal together
-nonpolar molecules are linked to nonpolar
solvents with London dispersion forces
33
Liquid Solutes and Solvents
Immiscible liquid solutes and solvents that are
not soluble in each other
Miscible liquids that dissolve freely in one
another in any proportion
-the nonpolar molecules of these substances exert
no strong forces of attraction or repulsion, and
the molecules mix freely
Examples nonpolar with polar, toluene with
water, gasoline with water, oil and water
Examples nonpolar w/nonpolar, gas with fats,
oils, and greases
34
Effects of Pressure on Solubility
-pressure changes have very little effect on the
solubilities of liquids
-increases in pressure increase gas solubilities
in liquids
-equilibrium eventually reached between rates at
which gas molecules enter and leave the gas
phase gas solvent solution
35
-increasing the pressure of the solute gas above
the solution puts stress on the equilibrium
-an increase in gas pressure causes the
equilibrium to shift so that fewer molecules are
in the gas phase
Example gas escapes immediately when pressure
is reduced by opening a soda bottle. The soda
effervesces when the bottle is opened and the
pressure is reduced.
36
Henrys Law
Henrys Law the solubility of a gas in a liquid
is directly proportional to the partial pressure
of that gas on the surface of the liquid
-named after English chemist, William Henry
-in a mixture of gases, each gas exerts a
pressure. Assuming they dont react, each gas
will dissolve to the same extent it would if no
other gases were present.
37
Application of Henrys Law
In sodas, the solubility of CO2 is increased by
increasing the pressure. Its raised to 5-10
atm. When the cap is removed, the pressure is
reduced to 1 atm and some of the CO2 escapes.
Effervescence the rapid escape of a gas from a
liquid in which it is dissolved
38
Effects of Temperature on Solubility
On Gases
On Solids
-increasing temperature usually decreases gas
solubility
-more difficult to predict
-often, increase in temp increases solubility of
solids
-increase in temperature increase in KE
-sometimes the increase in solubility is large
and sometimes only slight (see solubility table )
-therefore, more molecules overcome attraction
and return to gas phase
-at higher temps, equilibrium is reached with
fewer gas molecules in solution
-in some cases, solubility decreases with an
increase in temp
39
Temperature Solubility
40
Heats of Solution
-formation of a solution means an energy change
-dissolving some solids results in heat formation
exothermic (hot)
-dissolving others results in the absorption of
heat endothermic (cold)
-both solvent and solute must undergo changes in
the forces holding them together
-energy is required to separate solute molecules
and solvent molecules
41
-solvated a solute particle that is surrounded
by solvent molecules
Heat of Solution net amount of heat energy
absorbed or released when a specific amount of
solute dissolves in a solvent
-since heating decreases solubility of a gas,
dissolution of gases is exothermic (negative
values)
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Energy Changes in the Solution Process