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Chapter 15 notes
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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