Chapter 12

1 Mixtures


1.1 Solutions

Solutions are homogeneous mixtures composed of two or more substances, made up of a solute and a solvent. Solutes are substances
which dissolve in another substance, known as a solvent. Gases may dissolve in liquids, such as carbon dioxide or oxygen dissolving in water.
Also, liquids may dissolve in other liquids. Determining a solution can be easy if the component parts are distinguishable, such as small
stones and plant matter which make up soil. On the other hand, some solutions are difficult to distinguish, such as salt water. When dissolved
in the water, the salt particles aren't visible.

1.2 Components of Solutions

Solutions are made up of a solution and a solvent. Solvents are the dissolving medium in a solution, and solutes are the substances dissolved
in a solution. In general, the component with a greater quantity is considered the solute.

1.3 Types of Solutions

Solutions can exist as gases, liquids or solids. Examples of solute-solvent combinations are listed in the below table. Most alloys, for example
14 karat gold (made of gold, silver, and copper), are solid solutions with uniformly mixed atoms.
Solute state
Solvent state
Example
Gas
gas
oxygen in nitrogen
Gas
liquid
carbon dioxide in water
Liquid
liquid
alcohol in water
Liquid
solid
mercury in silver and tin
Solid
liquid
sugar in water
Solid
solid
copper in nickel

1.3.1 Suspensions

When the particles in a solvent are large enough that they settle out unless the mixture is constantly sirred or agitated, the mixture is a
suspension. An easy example of a suspension is muddy water. The dirt particles will settle to the bottom of the container, unless the contents
of the container are mixed or stirred. The particles in a suspension are over 1000 nanometers.

1.3.2 Colloids

Colloids are formed when the particles are inbetween the size of those in solutions and in suspensions. Particles between 1nm and 1000nm
form colloids. Particles in colloids are small enough to be suspended throughout the solvent by the movement of the molecules in the colloid.
Examples of colloids include milk, gelatin, and smoke.

Whether or not a mixture is colloidal can be tested with the Tyndall Effect. Because the individual particles of colloids are too small to be seen,
the mixture may appear to be homogeneous, but the particles are big enough to scatter light. When shining a light through a colloid, the beam
of light should be visible in the mixture.

1.4 Electrolytes

An electrolyte is a substance that dissolves in water to give a solution which is able to conduct electric current. A substance that does not yield
a conductive solution when dissolved in water is naturally a nonelectrolyte. When an ionic compound dissolves, positive and negative ions
separate from each other and are surrounded by water molecules. Free to move, it is possible for an electric current to pass through the solution.

2 Solution Process


2.1 Variables Affecting Dissolution

There are many variables affecting the rate at which a solute dissolves in a solvent, or the rate of dissolution. For example, to speed up the rate
at which a cube of sugar dissolves in tea, one may stir the tea, heat it up, or grind the sugar cube into smaller grains of sugar.

2.1.1 Increasing Surface Area

The dissolution of a solute occurs at the surface of the solute. Thus, the greater the surface area of the solute, the greater quantity of the solute
may be dissolved at a time. Using the example of the sugar cube in the iced tea, if the sugar cube is crushed, increasing the surface area of the
sugar, then more of the solute may be dissolved in a period of time, speeding up the rate of dissolution.

Solution1.jpg
This shows the time lapse of the dissolution of a solute with a large surface area
Solution2.jpg
This shows the time lapse of the dissolution of a solute with a small surface area.

2.1.2 Stirring a Solution

Since the dissolving of a solute occurs at the surface of the particle, there is a high concentration of dissolved solute surrounding the solute
particle. With this highly concentrated area around the particle, it is difficult for more of the particle to dissolve. If the solution is stirred, or shaken,
then the highly concentrated solution around the particle will be shifted elsewhere in the solution, and more of the solute particle may be dissolved.

2.1.3 Increasing the temperature of the Solvent

An increase in the temperature of a solvent means more kinetic energy in the particles. When the molecules are moving faster, collisions
between solute and solvent particles are more common. These collisions disperse the solute particles more evenly amongst the solvent
particles, speeding up the dissolution process.

2.2 Saturated, Unsaturated, and Supersaturated Solutions

For every combination of a solvent and a solid solute, there is a limit to the quantity of the solute that can be dissolved. When a solute dissolves
into a solvent, some of the solute particles may return to the state of the original solute, or recrystallizes. As the concentration of the dissolved
molecules increases, more and more molecules recrystallize, until a solution equilibrium is formed. A solution equilibrium is when the opposing
processes of dissolution and crystallization of a solute occur at equal rates.

Saturated Solutions - A solution which contains the maximum quantity of dissolved solute possible is considered saturated.
Unsaturated Solutions - A solution which contains less than the maximum possible amount of dissolved solute.

A supersaturated solution is one that contains more dissolved solute than a saturated solution. When saturated solutions are cooled, excess
solute comes out of the solution to keep the solution saturated at the lower temperature. But with some solutions, if left undisturbed to cool,
the excess solute won't seperate, and that is when a supersaturated solution is created.

The 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. The temperature needs to be determined because the solubility of a substance will change at different temperatures.

2.3 Solute-Solvent Interactions

A rough but useful rule for predicting whether a substance will dissolve in another is "like dissolves like." What makes substances similar depends
on the type of bonding, polarity, and intermolecular forces between the solute and solvent.

2.3.1 Dissolving Ionic Compounds in Aqueous Solution

The polarity of water molecules is important in the formation of solutions of ionic compounds in water. The charged parts of water molecules attract the ions in the compounds and surround them and keep them seperated from the other ions in the solution.
ionic_compound.JPG
After reacting with sodium chloride, the water molecules seperated the Cl- from the Na+. The positively charged end of the H2O molecule attaches to the Cl- and the negatively charged end attaches to the Na+.

2.3.2 Liquid Solutes and Solvents

Nonpolar substances such as fats, oils, and grease are usually soluble in nonpolar liquids such as gasoline. Liquids that are not soluble in each other are immiscible. The only attractions in nonpolar liquids are london dispersion forces, which are very weak. Liquids that can dissolve in each other are considered miscible. Since the nonpolar molecules do not exert any strong forces, the molecules are free to mix amongst each other.

2.3.3 Effects of Pressure on Solubility, and Henry's Law

Changes in pressure do not greatly effect the solubility of liquids or solids in liquid solvents, but it does affect the solubility of gases in liquids. When gas is in contact with a liquid's surface, gas molecules can enter the liquid. As the amount of dissolved gas increases, gas molecules begin to reenter the gas phase, and eventually an equilibrium is reached in which the rates of gas molecules leaving and entering the liquid phase are the same. An increase in pressure causes the equilibrium to shift so that more molecules are in the liquid phase.
Henry's 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.

2.3.4 Effects of Temperature on Solubility

Since an increase in temperature means an increase in kinetic energy, then as the temperature rises the molecules in the solution get faster, providing for more collisions to occur. The more common these collisions, the less gas molecules are needed in the solution. Therefore increased temperatures decrease the solubility in gas. The effect of temperature on the solubility of solids in liquids is less predictable though, and the change in solubility can range from large increases to small fluctuations.

3 Concentrations

Concentration of a solution is a measure of the amount of solute in a given amount of solvent or solution.

3.1 Molarity

Molarity is the number of moles of solute in one liter of solution.
molarity.JPG

3.2 Molality

Molality is the concentration of a solution expressed in moles of solute per kilogram of solvent.
molality.JPG

Sample Problems


Molality

1) What is the molality of a solution made by dissolving 0.1356g MgSO4 in 200.0mL of water?

2) What is the molality of a solution of 6.44g Cd(NO3)2 in 375.00g of water?

3) How many grams of Sr(ClO4)2 are required to make a solution with 600g water and a molality of 0.30?

4) How many grams of BiCl3 are needed to make 500.0g of a solution with a molality of 0.10?

Molarity

5) What is the molarity of a solution made by dissolving 0.479g of Li2CO3 to make 250.0mL of solution?

6) What is the molarity of a solution made by adding water to 0.33 mol of CaBr2 until there is 500. mL of solution?

7) How many grams of AlBr3 are required to make 500.0mL of AlBr3 with a molarity of 0.2?

8) How many mg of silver nitrate are required to make 1.0 mL of a solution with a molarity of 0.042?





Answers

1) 5.633x10^-3m

2) 0.0726m

3) 52g

4) 15g

5) 0.0259M

6) 0.66M

7) 27g

8) 7.1mg





Works Cited

  • All information came from the text book Modern Chemistry and the online text book. Information also came from Mr. P, our chemistry teacher.
  • Pictures are form google images.