Pre AP Chemistry

Chapter 13

“Solutions”

Suppose you drop a lump of sugar into a glass of water.

What will eventually occur? The sugar will eventually dissolve in the water.

This will form a mixture of sugar and water. If this process is continued, a point will be

reached in which no additional sugar will dissolve in the mixture.

If the mixture is homogeneous, it is referred to as a solution.

Solution Terminology

solution - a homogeneous mixture of two or more substances, the composition of which may vary within definite limits.

solute - the substance dissolved in a solution. solvent - the dissolving medium in a solution. A solution is a solute dissolved in a solvent. In the example of the sugar-water mixture,

sugar-water is the solution, sugar is the solute, and water is the solvent.

Suspension Terminology

If clay is mixed with water, it will not dissolve. Clay particles will be suspended in the water. This is called a suspension.

suspension - a heterogeneous mixture Colloids are large suspended particles. They

may be homogeneous, but they are not true solutions because the large particles are not dissolved.

Examples : mayonnaise, catsup, mustard

Electrolytes

electrolyte - a substance whose water solution conducts electricity

Examples : acids, bases, salts nonelectrolytes - a substance whose

water solution does not conduct electricity.

Examples : most of what is discussed in the remainder of this chapter

Solutions and Phases of Matter

It is useful to break solutions into three main types.

1. Gaseous solutions - all mixtures of gases are solutions because the particles are so far apart.

2. Liquid solutions - examples include cokes, radiator coolant, and soap.

3. Solid solutions - alloy - solid solutions in which atoms of two or more metals are uniformly mixed - brass

Types of Solutions

air coke cake rain alcohol and water wet sponge smoke saltwater brass

gas in gas

gas in liquid

gas in solid

liquid in gas

liquid in liquid

liquid in solid

solid in gas

solid in liquid

solid in solid

Solution Equilibrium

Dissolving is a reversible process. What is the reverse of dissolving? __________________

A solution in equilibrium exists when the opposing processes of dissolving and crystallization of a solute occur at equal rates.

A saturated solution is one in which the dissolved and undissolved solute are in equilibrium.

Solubility is the ability of a substance to dissolve and form a solution.

Scenario 1. You drop a cube of sugar into a glass of

water. What occurs? ______________. 2. As you add more sugar cubes , the solution

will reach an ________________. 3. If you continue to add more cubes, the

solution will become _________________. 4. If the process is continued, the reverse of

dissolving will occur and _____________ will develop in the solution.

Assignment Page 400, 1 - 5

The Solution Process Consider the following combinations of solutes

and solvents in solution. Which would be most affected by prevailing pressure on the solution?

1. gas in a liquid 2. liquid in a liquid 3. solid in a liquid Prevailing atmospheric pressure would

most likely affect a gas in a liquid.

Effervescence

Carbonated beverages exhibit effervescence when uncapped. They are bottled under pressure to increase the amount of gas, CO2, in the liquid.

effervescence - the rapid escape of a gas from a liquid

What would you do to increase the amount of carbonation in a beverage, increase the pressure or decrease the pressure?

Henry's Law

The solubility of gas in a liquid is directly proportional to the pressure of the gas above the liquid.

If the pressure on a coke is increased, the amount of gas, carbonation, in the coke ________________.

When the top on a coke is removed, the solutility of the gas in the coke _______.

Temperature and Solubility

1. gases in liquids - As the temperature of a solution consisting of a gas in a liquid is increased, the solubility_______________ . Hint : What happens to a coke if it is heated?

2. liquids in liquids - No great effect. 3. solids in liquids - Increasing the

temperature usually increases the solubility of solids in liquids.

Example : If tea is sweetened while it is hot, less sugar is required.

Increasing the Rate of Dissolving Since most solutions consist of solids in liquids, we

will consider these types of solutions only. There are three ways of increasing the rate of dissolving a solid in a liquid.

1. stirring - This increases the contact between solute and solvent molecules.

2. heating - This increases molecular activity of solute and solvent molecules.

3. powdering the solute - This increases the contact between solute and solvent molecules.

Assignment Page 410 , 1 - 5

Concentration of Solutions

The concentration of a solution depends on the relative amounts of solute and solvent present in a solution.

The terms concentrate and dilute are qualitative. This means they only give "what" is present in a solution and not "how much" is present.

Percent by Mass Percent by mass of a solute in solution is the

number of grams of solute dissolved in 100g of solution.

mass of solute % by mass = --------------------------------- x100% mass solute + solvent Find the percent by mass of a solution made by

dissolving 10g of sodium hydroxide in 90g of water.

Molarity (M) Molarity is the number of moles of solute in one

liter of solution. moles of solute Molarity = ------------------------ liters of solution Find the molarity of a solution made by

dissolving 20g of sodium hydroxide in 500mL of solution.

How many moles of HCl are present in 0.8 L of 0.5 M HCl solution?

Sample Problem

Find the mass of sodium chloride and volume of water needed to make 350mL of a 4-M solution.

Assignment: Page 421; 15 - 18

Molality molality (m) - an expression of moles of solute in

a kilogram of solvent moles of solute molality = ------------------------------- mass of solvent (kg) A solution consists of 17.1g of sucrose

(C12H22O11) dissolved in 125g of water. Find the molal concentration of this solution.

Determine the mass of copper(II) sulfate needed to prepare a 0.25-m solution using 250mL of water.

Assignment: Find m for 20g NaOH in 250ml solv

Colligative Properties of Solutions

As solute is added to a solvent , the freezing point of the solvent is generally lowered and the boiling point of the solution is generally raised.

The rate of freezing point depression and boiling point elevation is dependent on the molality of the solution and the type of solvent involved.

Freezing Point Depression The change in the temperature of the freezing

point is equal to the product of the molality and the molal freezing point constant (Kf) for the solvent. The molal freezing point constant for water is 1.86 0C/m.

Tf = Kf m

Tf = change in temperature of freezing point

Kf = molal freezing point constant m = molality

Sample Problem

A solution of calcium chloride and water consists of 55g of CaCl2 in 400mL of water. The Kf for water is 1.860C/m. Find the freezing point of the solution.

Boiling Point Elevation Just as the freezing point of a solution is

depressed, the boiling point of the solution is elevated as solute is added to a solvent. The molal boiling point constant (Kb) of water is 0.510C/m.

Tb = Kb m

Tb = change in temperature of b.p.

Kb = molal boiling point constant m = molality

Sample Problem

Find the boiling point of the solution in the previous sample problem.

Assignment:– Find the boiling and freezing point of

each of the following 2-m soln of salt and water 8g of NaOH in 200mL of water 100g of CaO in 100g of water

Chemical Equivalents

Chemical equivalents are the quantities of substances that have the same combining capacity in chemical reactions.

Chemical equivalents are calculated by dividing the molar mass by the total positive charge on the first element in the formula of the compound.

Sample Problem

molar mass 1 equiv = ------------------------------ total positive charge Determine the chemical equivalents

of each of the following. a. HCl ____________________ b. NaOH __________________ c. CaSO4 __________________

Normal Solutions Normality, (N), is an expression of gram

equivalents of solute per liter of solution. Before calculating normality, the chemical

equivalent must be calculated. mass solute 1 equiv 1000 mL N= ----------------- X ------------- X ----------- vol. soln. mass eq L

Sample Problem

Find the normality of 300mL of a solution containing 55 grams of calcium chloride.

In-Class Assignment

Find the normality of each of the following.– 20g of sodium hydroxide in 250mL of water– 200g of calcium carbonate in 500mL of water

Find the mass of solute needed to make;– 250mL of a 2-N solution of hydrochloric acid– 500mL of a 0.5-N of barium chloride

mass eq L Mass solute = N X ------------ X ------------- X vol soln 1 eq 1000mL

Ion Concentration in Water

Water undergoes autoprotolysis which is self ionization. The reaction equation for this process is;

2H2O H3O+ + OH-

The concentration of , [ ] , hydronium ions in one liter of water is 10-7 moles. What would be the concentration of hydroxide ions, [ OH ] , in one liter of water? _______

pH The pH of a solution is a numerical representation of

the degree of hydronium ion concentration. The term (from French pouvoir hydrogène, "hydrogen

power") is defined as the negative logarithm of the concentration of H+ ions (protons).

pH - the common logarithm of the reciprocal of the hydronium ion concentration

1 pH = log10 ------------

[H3O+]

Determining the pH of Acids

The concentrations of acids is usually given in molarity, M. Since molarity is an expression of Moles / L, the hydronium ion concentration can readily be determined.

Find the pH of a 0.001-M HCl solution.

Assignment

Find the pH of the following. 1. 0.01- M H2SO4

2. 0.0001- M H2S 3. 0.1 - M HCl Page 505 - 16,19

Determining pH of Bases

From previous data, the autoprotolysis of water produces hydronium at a concentration of 10-7 moles/liter. Likewise, the concentration of hydroxide is 10-7 moles/liter. This data can be used to determine the hydronium ion concentration of hydroxide solutions.

Calculation of pH of a Base

10-14

[H3O+] = ------------ [OH-] Find the pH of a 0.01 - M NaOH

solution. Find the pH of a 0.001 - M Ca(OH)2

solution

pH Scale

The pH scale is a numerical scale which indicates the acidity or basicity of a substance. A neutral substance, one which has an equal concentration of hydronium and hydroxide ions has a pH of 7 which is half way between 0 and 14.

acid base 0---------------------- 7 --------------------- 14

Approximate pH of Some Common

Materials MATERIAL pH gastric juice 2.0 lemons 2.3 vinegar 2.8 soft drinks 3.0 apples 3.1 tomatoes 4.2 bananas 4.6 bread 5.5 rainwater 6.2 milk 6.5 pure water 7.0 eggs 7.8 seawater 8.5 milk of magnesia 10.5

Assignment Page 491, 1-6

Acid-Base Titration

Titration is the progressive addition of an acid to a base to achieve neutralization so the concentration of an unknown solution may be found. The point at which neutralization occurs is known as the end point. Indicators are used for determination of this point. The previously discussed indicators will be used in our titrations.

Titration With Normal Solutions

VANA = VBNB

VA = volume of acid used in mL

NA = normality of acid used in N

VB = volume of base used in mL

NB = normality of base used in N

Sample Problem

A student performed a titration and determined 20mL of a 1.5-N HCl solution was needed to neutralize 35mL of a NaOH solution of unknown concentration. Determine the normality of the unknown solution.