Chapter 15Solutions

ByDavid B. Oestreicher

Desired Learning Objectives

1. You will be able to describe and categorize solutions

2. You will be able to calculate concentrations of solutions

3. You will be able to analyze the colligative properties of solutions

4. You will be able to compare and contrast heterogeneous mixtures

Sample Solutions

• Air• Lakes• Steel• Cell solutions• Ocean water• Pool water

Solutions

Solute – is the substance that dissolves in the solvent (e.g. sugar, salt, tea)

Solvent – is the dissolving medium (e.g. water)

A solution can exist as a gas, liquid, or solid depending on the state of its solvent.

Soluble, Insoluble, Immiscible, and Miscible

A substance that dissolves in a solvent is said to be soluble in that solvent (e.g. sugar in water).

A substance that does not dissolve in a solvent is said to be insoluble in that solvent (e.g. sand in water).

Oil and vinegar are said to be immiscible. They do not mix with one another.

Liquids that mix with one another are miscible with one another.

Demonstrations

• Water and Vinegar• Water and Oil• Water and red/blue dye

1. Which are solutions?2. Which are miscible?3. Which are immiscible?

Solvation• Attractive forces exist between the pure solvent

particles, and between the solute and solvent particles.

• When a solid solute is placed in a solvent, the solvent particles completely surround the surface of the solute.

• If the attractive forces between the solute/solvent particles are greater than between those of the solute/solute, the solvent pulls the solute particles apart and surround them (e.g. girl/boy)

Solvation

“Like dissolves like”

To determine whether solvation will occur in a specific solvent, one must determine whether a solvent and solute are alike:

1. What is the bonding of each?2. What is the polarity of each?3. What are the intermolecular forces between the

particles?

Water and NaCl

Water and NaCl

• The attractive forces between the H2O molecules are stronger than those between the Na-Cl molecules.

• The H2O molecules are polar (O- and H+) as are the NaCl molecules that have ionic bonding (ions of Na+ and Cl-)

• Hence, NaCl dissolves pretty readily and completely in water

Water and GypsumSolution or Mixture?

1. Gypsum is composed of calcium and sulfate ions2. Mixed with water is makes plaster3. It is a mixture, not solution4. Gypsum is insoluble in water5. Attractive forces between calcium and sulfate

molecules are much stronger than those exerted by water.

6. Solvation will not occur.

Aqueous solutions of molecular compounds

• Sugar –> O-H bonds• Detergents -> hydrophilic and lipophilic ends

Factors that affect rate of solvation

1.Agitation2.Increasing the surface area of

the solute3.Increase the temperature of

the solvent

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Solubility Question

Heat of solution1. Energy is required for the solvent molecules

to separate from one another2. Energy is required for the solute molecules to

separate from one another3. Both reactions are endothermic4. When solute and solvent molecules then

form bonds, they give off energy, exothermic5. The overall energy change is call the “heat of

solution”

Solubility

• Only a limited amount of solute can dissolve in a given amount of solvent at a given set of conditions (e.g. temperature, etc.)

• Each solute has a characteristic solubility.• Solubility is the maximum amount of solute

that will dissolve in a given amount of solvent at a specified temperature and pressure (usually expressed in grams of solute per 100 g of solvent).

Solubility graph(solute and temperature)

Saturated vs. Unsaturated Solutions

• In a saturated solution, the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure has been reached at equilibrium

• In an unsaturated solution, the solution contains less than the maximum amount of solute for a given temperature and pressure. In other words, more solute could be dissolved in an unsaturated solution.

Factors that affect solubility• Temperature and solubility– Some substances are more soluble at high

temperatures than at low temperatures

Supersaturated solutions

• A supersaturated solution contains more dissolved solute than a saturated solution at the same temperature. To make a supersaturated solution, a saturated solution, a saturated solution is formed at a high temperature and then cooled slowly. The slow cooling allows the excess solute to remain dissolved in solution at the lower temperature. Unstable.

Sugar candy demonstration

• Crystallization– Scratching the container– Seed crystals– Seed particle– Physical shock– Tapping– Stirring– Cloud seeding (Silver Iodide, AgI) to make rain

Pressure and solubility

• Carbonation in sodas• Demonstration

Henry’s Law

• The decreased solubility of the carbon dioxide in a carbonated beverage can be described by 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.

Henry’s Law

S1 = S2

P1 P2

S1P2 = P1S2

Solution Concentrations, pg. 462

• Percent by Mass• Percent by Volume•Molarity•Molality•Mole fraction

Percent by Mass

mass of solute x 100mass of solution

Percent by Volume

volume of solute x 100volume of solution

Molarity

Moles of soluteLiter of solution

Molality

Moles of soluteKilogram of solvent

Mole Fraction, pg. 462

Moles of soluteMoles of solute + moles of solvent

Preparing a Molar Solution

• Demonstrate with sugar or Copper Sulfate• Put in molar quantity• Put in some solvent• Mix• Fill up to the line on a volumetric flask

Diluting Solutions

M1V1 = M2V2

WhereM1 = Initial concentration

M2 = Final concentration (diluted condition)

V1 = Initial volume

V2 = Final volume (diluted condition)

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Dilution Problem

Electrolytes and Colligative Properties

• Colligative properties are those properties that are changed by the number of dissolved particles in a solution, but not necessarily because of the specific solute (colligative means “depending on the collection.”)

• Colligative properties include:– Vapor pressure lowering– Boiling point elevation– Freezing point depression, and– Osmotic pressure

Nonelectrolytes and electrolytes in aqueous solution

• Sucrose in solution (nonelectrolyte)–Does not conduct electricity

• NaCl in solution (electrolyte)–Does conduct electricity

Vapor Pressure Lowering (pg. 472)

• The greater the number of solute particles in a solvent, the lower the resulting vapor pressure

• Thus, vapor pressure lowering is due to the number of solute particles in a solution and is a colligative property of solutions

• Nonelectrolytes have the same relative effect on VPL

• However, electrolytes have a greater effect on VPL because of the increased number of ions

Boiling Point Elevation

1. Because a solute lowers a solvent’s vapor pressure, it also affects the boiling point,

2. Because a liquid boils only when it’s vapor pressure equals the atmospheric pressure,

3. A solution must be heated to a higher temperature to supply the additional kinetic energy needed to raise the vapor pressure to atmospheric pressure

4. The difference between a solution's boiling point and a pure solvent’s boiling point is the BOILING POINT ELEVATION, where

ΔTb = Kbm

ΔTb = Kbm

• ΔTb is the change in temperature caused by the solute and is directly proportional to the solute’s MOLALITY

• Kb is the molal boiling point elevation constant for a 1m nonvolatile, nonvolatile, nonelectrolyte solution and a pure solvent, and

• m is the solution’s molality

Freezing Point Depression

ΔTf = Kfm1. The freezing point of a solution is always

lower that that of a pure solvent2. Because the particle's of the solute interfere

with the pure solvent’s particles in entering the solid-state, its normal freezing point is lowered

ΔTf = Kfm

• ΔTf is the change in freezing point

• Kf is the freezing point constant, and• m is the molality of the solutions

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Boiling Point – Freezing Point Problem

Osmosis and Osmotic Pressure

• Osmosis is the diffusion of solvent particles across a semipermeable membrane from an area of higher solvent concentration to an area of lower solvent concentration

• Semipermeable membranes are those with tiny pores that allow some, but not all kinds of particles to cross

Osmosis

• The membranes surrounding all living cells are semipermeable• Osmosis plays an important role in

many biological systems such as kidney dialysis and the uptake of nutrients by plants

Osmotic Pressure

Diffusion vs. Osmosis

• Diffusion – higher concentration of solute going to lower concentration (e.g. gases, smells, liquids, etc.)

• Osmosis – higher concentration being lowered by the uptake of water (it seems to be the opposite of what we’d expect)

Osmotic Pressure

• The amount of additional pressure caused by the water molecules that moved into the solution is called the osmotic pressure

• Osmotic pressure depends on the number of solute particles in a given volume of solution

• Osmotic pressure is a colligative property

Osmotic pressures on cells

Suspensions

• Cornstarch demonstration - SUSPENSION– It looks like milk, but will settle out over time with

two distinct layers. A thick pastelike substance on the bottom and the water on top.

– If filtered, the cornstarch can be removed– Other examples: fine sand in water, mud in water,

etc.

Suspended particles (pg. 476)

• Suspended particles are large compared to the solvated particles

• Gravity acts on the suspended particles quickly causing them to settle out

• The cornstarch that settles form a solid that can easily be stirred to become liquid again

• Substances that behave like this are called “THIXOTROPHIC” (Paints are thixotrophic)

Colloids (pg. 477)

• Particles in a suspension are large compared to the solvent’s particles

• Particles in a COLLOID are intermediate in size (1 – 1000 nm in diameter)

• Milk is a colloid• Although milk looks like the cornstarch

solution, milk cannot be filtered the same way

Colloids

• Colloids are categorized by their dispersed particles and dispersing mediums.– Milk is a colloidal emulsion because liquid

particles are dispersed in a liquid medium– Other colloids: blood, gelatin, butter, cheese, milk,

mayonnaise, marshmallows, whipped cream, beaten egg whites, smoke, dust in the air, spray deodorant, clouds

Brownian Movement

• The erratic movement of colloid particles is called Brownian Movement, named after the botanist, Robert Brown, who noticed the random movements of pollen grains dispersed in water.

• Brownian movement results from collisions of particles of the dispersion medium with the dispersed particles. These collisions prevent the particles from settling out.

How to destroy the Brownian Motion?

• Destroy the electrostatic layers– Stir in an electrolyte. The dispersed particles

increase in size and destroy the colloid– Heat the colloid. This gives the colliding particles

enough kinetic energy to overcome the electrostatic forces and settle out.

The Tyndall Effect

• Dispersed colloid particles are large enough to scatter light, the Tyndall Effect– A beam of light through a solution– A beam of light from a street lamp hitting dust or

water particles– A beam of sunlight coming through a dusty, air-

filled room (smoke too)