Chapter 15 Solutions
15-1 The Nature of Solutions
-
Mixture- blend of two or more pure substances that are chemically combined
o Very few things are
pure substances in nature
o Usually heterogeneous
(visibly different parts) or homogeneous mixtures (not visibly different parts)
-
Solution- a homogenous mixture of two ore more substances in a single physical state.
Properties of Solutions
-
Basic characteristics:
o Small particle size-
not visibly seen
o Particles evenly distributed-
different samples of the same solution will have the same concentration
o The particles in a solution
will not separate no matter how long it is left standing under constant conditions.
-
Solute- the substance dissolved or broken down (lesser quantity)
-
Solvent- the substance doing the dissolving (greater quantity)
-
Soluble- the ability to be dissolved in another substance
-
Insoluble- the inability to be dissolved in another substance
Types of Solutions
-
Solid Solutions- contain two or more solids
o Alloys- solid solutions
of two or more metals
§
Properties are often different than the individual elements
§
May improve melting points, durability, resistance to corrosion.
-
Gaseous solutions- mixture of two gases
o Gas particles are far
apart- mix readily
o If the gases do not
react with each other, they become a solution readily.
o Properties of gas solution
depend on the properties of the gasses added
-
Liquid solutions- a mixture that results in the liquid state of matter.
o The solvent and the
solution are liquids
o Solute can be either
solid, liquid or gas (soda, vinegar, sugar-water)
o Miscible- able to mix
in any amounts
o Immiscible- liquids
that cannot mix in any proportions
-
Aqueous Solutions- solutions in which water is the solvent
o Since water can dissolve
so many things- it is called the universal solvent
o Electrolyte- a substance
that when dissolved in water will conduct an electric current – usually contains some number of ionic bonds.
o Non-electrolyte- a substance
that when dissolved in water will not conduct an electric current- usually does not contain any ionic bonds
15-2 Concentration of Solutions
-
Concentration- the amount of solute in a given amount of solvent or solution
Molarity (M)
-
Molarity- the number of moles of a solute dissolved in each liter of solution
Molarity = moles
of solute/liters of solution
-
Example:
o 10.0 grams of NaOH in
enough solvent to make .100 liters of solution.
(10.0 g NaOH/.100L solution
)(1 mol NaOH/ 40.0 g NaOH)
= 2.50
mol NaOH/ 1 L solution
=
2.50 M NaOH
-
Volumetric Flasks are the best to use to make solutions
o Use balance to get proper
amount of solute
o Then add enough liquid
to flask to attain a liter.
Molality (m)
-
Molality- number of moles of a solute dissolved in each kilogram of solvent.
Molality = moles of
solute/kilograms of solvent
Mole Fraction (X)
-
Mole fraction- number of moles of one component divided by the total number of moles in the solution.
Mole fraction = moles of component/total moles of solution
-
Can figure out the mole fraction of the solute or the solvent
Xsolute
= moles of solute/total moles of solution
Xxolvent
= moles of solvent/total moles of solution
-
The total mole fraction must equal 1
Saturation
-
Is there a limit to how concentrated a solution can be?
-
Saturated-a solution in which no more solute can be dissolved- under the existing conditions of temperature and pressure.
o Saturated and concentrated
are not the same thing
§
May take only a very little solute to cause a solution to be saturated.
-
Unsaturated solutions- a solution that has less than the maximum amount of solute
-
Supersaturated- a solution that contains a greater amount of solute that that needed to form a saturated solutions
o Very unstable
o Will often form precipitate
15-3 The Formation of Solutions
How a Solution Forms
-
It deals with the ability of the solvent to break intermolecular and intramolecular forces.
-
Dissolving takes place at the surface of the solvent- in the case of NaCl and water
o Water molecules orient
themselves so that they can separate the ions and pull them into solution- called solvation (hydration when the solvent is
water)
o The solvent and solute
particles are intermingled
§
So intermolecular forces in the water must break, attractive forces in the salt must break- and new forces between
the salt and water must form
§
Any time attractions are broken- energy is required (endothermic)
§
The formation of new attractions releases energy (exothermic)
·
Whether or not heat is given off or taken in depends on which process requires more
Solubility
-
Why are some substances soluble and some not?
-
Why do different solutes dissolve to different rates in the same solvent?
-
Solubility- the amount of a solute that will dissolve in a specific solvent under given conditions- or the amount of
solute required to saturate a solution.
o Must be determined experimentally
o Usually given in grams
solute per 100 grams solution.
-
Factors that affect solubility:
o Nature of the solute
and solvent
o Temperature
o Pressure (for gasses)
Nature of the Solute and Solvent
-
Polar solvent- a liquid made up of polar molecules (large difference in electronegativity)
o Polar molecules tend
to dissolve in polar solvents
o Ionic substances tend
to dissolve more readily in polar solvents than they due in non-polar solvents due to the charged nature of the ions.
o Many ionic substances
are only slightly soluble in water
-
Non-polar solvent – a liquid made up of non-polar molecules (not a large difference in electronegativity)
o Non-polar molecules
tend to dissolve in non-polar solvents
Temperature
-
Soda may go flat when left out on a table due to the solubility difference of carbon dioxide in warm temperatures
o Carbon dioxide is more
soluble at colder temperatures
-
Solutions of gases dissolved in liquids are greatly affected by changes in temperatures
o As temperature increases,
the gas gains more kinetic energy and is more likely to escape from the surface of the liquid
o As temperature increases,
solubility of a gas decreases.
-
Solutions with solids dissolved in liquids is quite different
o As temperature increases,
solubility increases.
-
The relationship between solubility and temperature depends on the energy change during solution formation
o If the temperature drops
when the solute and solvent are mixed- raising the temperature will raise the solubility
o If the temperature stays
the same- changing the temperature will have little effect
o If the temperature increases
when the solute and solvent are mixed- raising the temperature will lower the solubility.
-
Supersaturated solutions are made by increasing the temperature so that more solute can be added to solution and then
allowing it to cool back down to room temperature.
Pressure
-
Solubility of a solid in a solvent is not significantly affected by pressure
-
Solubility of a gas in a liquid is significantly affected.
o The solubility of any
gas in a solution is increased when pressure above the solution is increased
§
The rate at which gas molecules hit the surface of the liquid increase
§
William Henry- solubility of a gas was proportional to the partial pressure of the gas above the liquid
Factors Affecting the Rate of Dissolving
-
Whether a solute dissolves quickly or slowly does not alter or depend on its solubility
Surface Area
-
Dissolving takes place at the surface of the solid
o To speed dissolving-
the surface area from which dissolving occurs needs to be increased
-
Grinding the solid tends to increase surface area- and thus dissolving
Stirring
-
Since dissolving occurs at the surface of the solid, dissolved solids tends to build up near the surface of the solid
o Stirring moves those
dissolved solids away from the surface to allow for quicker attack on the underlying solid.
o Increases contact between
solvent and solute
Temperature
-
As temperature increases solvent particles move faster
o When particles move
faster- more come in contact with the solute
o Also the solute particles
are moving faster and require less energy to be removed.
15-4 Colligative Properties
-
Some properties of liquid solutions differ from those of pure solvent
-
Colligative property- a property that depends on concentration of solute particles but is independent of their nature
o Depend on the collective
effect, not their chemical identity
Vapor Pressure Reduction
-
Vapor pressure- arises since some molecules of a pure liquid leave the liquid surface and enter the gaseous state (vaporization)
o Some return (condensation)
o At some point, in a
closed container- these two will be in equilibrium
-
When a non-volatile (will not vaporize) solute is added to a liquid, the vapor pressure of the solution becomes lower.
o The solute takes up
some of the space at the liquid’s surface- preventing some solvent molecules from vaporizing
o Since more molecules
leave the gas than enter it, the pressure of the gas is reduced
o Does not depend on the
chemical nature of the solute added.
o Raoult’s law-
the magnitude of vapor pressure reduction is proportional to solute concentration
Boiling Point Elevation
-
Boiling point is the point at which the vapor pressure of a liquid is equal to the external pressure on its surface-
generally atmospheric pressure
-
Since addition of solute lowers the vapor pressure, a higher temperature is required to get the vapor pressure of the
solution up to atmospheric pressure so the solution boils.
-
Boiling point elevation- the amount the boiling temperature rises.
o It is the difference
between the boiling point of the pure solvent and the solution
o Directly propotional
to the number of solute particles per mole of solvent particles- also known as molality
o DTb = Kbm
§
Kb is called the molal boiling elevation constant, and depends on the solvent. Each solvent has its own value.
Freezing Point Depression
-
Freezing point depression- the ability of a solute to lower the freezing point of its solution
-
The freezing point is the temperature at which the vapor pressures of the solid and liquid phases are the same.
o If the solute is non-volatile-
the vapor pressure of the solution is lowered in proportion to the mole fraction of the solute.
o DTf = Kfm
§
Kf is the specific effect of a solute on a given solvent. Each
solvent has its own value.
Osmotic Pressure
-
Osmosis- the movement of solvent particles from less concentrated solutions to more concentrated solutions through
a semipermeable membrane.
-
Osmotic pressure (P) is the pressure required to prevent osmosis
o When solvent flows to
opposite sides of a membrane- the levels become unequal
o When the levels are
so unequal that the pressure difference prevents more net movement, osmotic pressure is reached.
-
Isotonic- when two solutions have the same osmotic potential- results in no net movement of water and no osmosis
-
Hypotonic solutions- water enters cells and may rupture cell
-
Hypertonic solution- water exits cells and cells crenate
Determining Molar Mass
-
Colligative properties of solutions provide a useful means of experimentally determining the molar mass of an unknown
substance.
-
Any of the colligative properties can be use to determine molar mass by manipulating the basic equations.
Example: suppose
a 10.0 grams sample of an unknown compound is dissolved in a .100 kilogram of water.
The boiling point of the solution is elevated to .433 C above the normal boiling point of pure water. What is the molar mass of the unknown sample?
o DTb = Kbm
m = DTb
/Kb
-
The molality can also be used to determine the number of moles of solute.
m= mol
solute/ kg solvent
mol solute = m x
kg solvent
-
Once you have determined the moles of solute you can solve for molar mass
Mol solute = mass solute/
molar mass of solute
Molar mass = mass solute/
mol solute
