Water and Aqueous Systems

Chapter 17

Objectives

1. Describe the hydrogen bonding that occurs in water

2. Explain the high surface tension and low vapor pressure of water in terms of hydrogen bonding

3. Account for the high heat of vaporization and the high boiling point of water in terms of hydrogen bonding

4. Explain why ice floats on water

The Water Molecule

• O-H bonds are highly polar covalent bonds.

• the O atom has a slightly negative charge. H has a slightly positive charge.

• Hydrogen bonding- occurs when hydrogen is attracted to a very electronegative element (N, O, and F).

Hydrogen bonding in water

The Unique Properties of Water

1. High surface tension

2. Low vapor pressure

3. High specific heat capacity

4. High heat of vaporization

5. High boiling point

6. Low density of ice

Surface properties

• Surface tension: the inward pull that tends to minimize the surface area of a liquid.

• Surfactant: surface active agent- surface tension is decreased by detergents (interferes with hydrogen bonding)

Why are water droplets spherical as they fall?

Surface tension

Surface tension

Water on your car….

Vaporization and Vapor Pressure

• Hydrogen bonding allows few water particles to vaporize, resulting in low vapor pressure above the surface of water

Water’s low vapor pressure

Specific Heat Capacity

• Water’s high specific heat capacity helps to moderate daily air temperatures around large bodies of water.

• (Water’s specific heat

is more than 4 times

that of most metals.)

Water’s high heat of vaporization

• Because of hydrogen bonding,

water absorbs large amounts

of heat as it vaporizes.

• Water releases large

amounts of heat as it

condenses.

Water’s high boiling point

• Hydrogen bonding causes water’s high boiling point

Ice

• Ice has an open framework structure, the density of ice is lower than that of water.

Objectives

1. Explain the significance of the statement “like dissolves like”

2. Distinguish between strong electrolytes, weak electrolytes, and nonelectrolytes, giving examples of each

What is a solution?

• Solution – homogeneous mixture – Solvent – substance present in largest amount

(the dissolving medium)– Solutes – other substances in the solution – Aqueous solution – water is the solvent

Solvation…

• the process that occurs as a solute dissolves.

• There are two types of solvation:– Ionic compounds dissolve by dissociation.– Covalent compounds dissolve by molecular

solvation.

Solubility of Ionic Substances: Dissociation

• The positive and negative ions of a salt come apart (dissociate) as a salt dissolves.

Dissociation vs. Molecular Solvation

• Dissociation:

NaCl(s) Na +(aq) + Cl -

(aq)

Note: the ions come apart from each other (electrolytes)

• Molecular solvation:

C6H12O6(s) C6H12O6(aq)

Note: no dissociation occurs (nonelectrolytes)

“Like dissolves like”

• Polar solutes dissolve in polar solvents (water is polar, so it dissolves polar substances, either ionic or molecular).

• Nonpolar solutes dissolve in nonpolar solvents (oil dissolves in kerosene).

Solubility of Polar Substances

• Ethanol is soluble in water because of the polar OH bond.

• Why is solid sucrose (C12H22O11), table sugar, soluble in water?

Substances Insoluble in Water

• Nonpolar oil does not interact with polar water.

Electrolytes

• Electrolytes- conduct an electric current in solution or in molten state.

(ionic compounds)• Nonelectrolytes- do not conduct an

electric current in solution or in molten state.

(molecular compounds)

Electrolytes

• Strong electrolytes: solute completely breaks apart in solution

[includes soluble salts (such as KCl), inorganic acids (such as HNO3), inorganic bases (such as NaOH)]

Electrolytes

• Weak electrolytes: only a fraction of the solute breaks apart into solution

[includes poorly soluble salts (such as PbCl2), organic acids (such as HC2H3O2), and organic bases (such as NH3)]

Electrolytes

• Nonelectrolyte – does not conduct when in solution- does not break apart into ions.

[includes most organic compounds (such as glucose)]

Water of hydration

• The water in a crystal is called water of hydration.

• A hydrate is a compound that includes water of hydration.

Water of hydration

• A hygroscopic substance removes water from the air. These substances are called dessicants.

Heterogeneous Aqueous Systems

• Suspensions are mixtures from which particles settle out upon standing. (The particles are much larger than those in a solution.)

• Colloids are heterogeneous mixtures that contain particles that are intermediate in size between suspensions and true solutions

Colloidal Systems

• Colloids exhibit the Tyndall effect, which is the scattering of light in all directions.

• Colloids include milk, mayonnaise, marshmallows, egg white, blood, and paint.

Solutions and their Behavior

Chapter 18

1. Identify factors that determine the rate at which a solute dissolves

2. Identify factors that affect the solubility of a solute in solution

3. Calculate the solubility of a gas in a liquid under various pressure conditions

Objectives

Factors affecting the rate of dissolving

• How could you speed up the dissolving of sugar in a glass of iced tea?

• Which dissolves faster, table salt or rock salt?

Factors affecting the rate of dissolving

• Temperature – increasing the temperature speeds up the rate of dissolving

• Agitation – stirring speeds up the rate of dissolving

• Particle size – smaller particles dissolve faster than large particles (surface area)

Solubility

• The solubility- the amount that dissolves in a given quantity of solvent at a given temperature.

Solubility

• A saturated solution contains the maximum amount of solute at a constant temperature.

Solubility

• An unsaturated solution contains less solute than a saturated solution.

• A supersaturated solution contains more solute than a saturated solution. (This occurs when a solution is saturated and then allowed to cool but all of the solid remains dissolved. It is an unstable solution, adding a crystal causes precipitation.)

What would happen…

• …if you added more sugar to a saturated sugar solution and stirred?

• …if you added more sugar to an unsaturated sugar solution and stirred?

Factors affecting the solubility of a substance

• How could you increase the amount of sugar that would eventually dissolve in a glass of tea?

Factors affecting the solubility of a substance

• Only two factors affect the amount of solute that can dissolve.

1. Temperature affects solubility of both solids and gases in liquid solvents.

2. Pressure affects solubility of gases in liquid solvents.

Factors affecting the solubility of a substance

• The solubility of most solid substances increases as the temperature of the solvent increases.

• For a few substances, the reverse occurs.

The Effect of Temperature on the Solubility of Solids

The Effect of Temperature on Gas Solubility

• Increasing the temperature of a dissolved gas solution decreases the concentration of the gas. Have you ever tried a hot Dr. Pepper? Heat it in a pan on the stove, pour a cup, and it has no bubbles!

Thermal pollution occurs when hot water is added to a lake, the dissolved oxygen levels fall in the water and it kills the fish.

The Effect of Temperature on Gas Solubility

The Effect of Pressure on Gas Solubility

• Increasing the pressure of a gas over the surface of a solvent increases the solubility of the gas in the solvent.

• In a bottled soda, the pressure of CO2 over the liquid is high and when the cap is opened, the pressure is reduced and bubbles begin to come out of the solution.

The Effect of Pressure on Gas Solubility

• Henry’s Law: at a given temperature the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid.

S1/P1 = S2/P2

Question:

• If the solubility of a gas in water is 0.77 g/L at 3.5 atm pressure, what is the solubility (in g/L) at 1.0 atm?

(The temperature is held constant at 25oC.)

• Answer: S1/P1 = S2/P2

0.77 g/L / 3.5 atm = S2 / 1.0 atm

S2 = 0.22 g/L