Solutions

Chapter 15Chapter 15 Solutions

Characteristics of Solutions

• Solute – substance that dissolves

• Solvent – dissolving medium

• Soluble – substance dissolves in a solvent

Immiscible – two liquids can be mixed but separate shortly after they cease to be mixed.

Solvation in Aqueous Solutions

• Solvation – the process of surrounding solute particles with solvent particles to form a solution

• General Rule: “like dissolves like” – Examine the bonding, polarity and intermolecular

forces of the particles.

• Solvation occurs when the attraction between the dipoles and ions of the solute and solvent is greater than the attraction of the ions in the solute.

Rate of Solvation

• Three common ways to increase rate:1. Agitating the mixture.

2. Increasing the surface area of the solute.

3. Increasing the temp of the solvent.

• Heat of Solution – the overall energy change that occurs during the solution formation process

Solubility• Solubility – the maximum amount of solute that will

dissolve in a given amount of solvent at a specified temp and pressure.

• Saturated Solution – max amount of dissolved solute for a given amount of solvent (at specific temp and pressure)

• Unsaturated Solution – dissolved solute for a given temp and pressure is less than a saturated solution

• When solvation rate equals the crystallization rate, a dynamic equilibrium has been reached.

Factors That Affect Solubility• Generally, solubility increases with increased

temperature

• Allows formation of supersaturated solutions.

• Supersaturated Solutions – contains more dissolved solute than a saturated solution at the same temp.

• video

• Solubility of gas increases as its external pressure increases.

Henry’s Law

• S1 S2

P1 P2

• Most commonly solved for S2:

S1P2

P1

Do page 461 1-2

=

S2 =

Solution Concentration

• Concentration – a measure of how much solute is dissolved in a specific amount of solvent or solution.

• Percent by mass is usually used to describe solution of a solid dissolved in a liquid. mass of solute

mass of solutionPercent by mass = X 100

P463 8-10

Concentration

• Percent by volume is usually used to describe solution of a liquid dissolved in a liquid. volume of solute

volume of solution• Do page 464 11-13

• Molarity (M)– the number of moles of solute dissolved per liter of solution.

moles of solute

liters of solution

X 100Percent by volume =

Molarity (M) =

• Percent by volume is usually used to describe solution of a liquid dissolved in a liquid. volume of solute

volume of solution• Do page 464 11-13

• Molarity (M)– the number of moles of solute dissolved per liter of solution.

moles of solute

liters of solution

X 100Percent by volume =

Find M

• What is the molarity of a solution prepared by adding 6.3 grams of KCl to enough water to make 3.4 liters of solution?

Preparing Molar Solutions

• Example: How many grams of MgCl2 would be dissolved in 1.0 liter of a 0.50 molar solution of MgCl2?

1.0 L 0.50 moles 95.21 grams MgCl2 1 L 1 mole MgCl2

= 48 grams MgCl2Do page 466 17-20

Diluting Solutions• Dilution lowers the concentration because more

solvent particles are added as the number of solute particles remains the same.

• This relationship as an expression:

M1V1 = M2V2

• Example: What volume of a 3.50M CaI2 stock solution would you use to make a .600 L of a 1.75M CaI2 solution?

• Do page 468 21-23

Molality• Molality (m) – the ratio of the number of moles

of solute dissolved in 1 kg of solvent.

moles of solute moles of solute

kilogram of solvent 1000 g of solvent

• Example: What is the molality of 25.0 g of CaCO3 dissolved in 500.0 g of water?

• Do page 469 24+25

=Molality (m) =

Mole Fraction• Mole Fraction (X) – the ratio of the number of

moles of solute in solution to the total number of moles of solute and solvent.

nsolute

nsolute + nsolvent

• Example: What is the mole fraction of HNO3 in an aqueous solution that contains 36.87% HNO3 by mass?

• What is the mole fraction of water?• Do page 470 26+27

Xsolute=

Colligative Properties• Colligative Properties – a physical property of a

solution that depends on the number, but not the identity, of the dissolved solute particles.

• Depends on amount (how much/how many).

• Electrolytes– Strong Electrolytes: produce many ions in solution– Weak Electrolytes: produce only a few ions in solution– Nonelectrolytes: doesn’t produce ions or conduct

electricity.

Vapor Pressure Lowering• The greater the number of solute particles in

solvent, the lower the resulting vapor pressure. (Colligative Property)

• Relative effect can be predicted based on whether the solute is an electrolyte or a nonelectrolyte.

Incr

easi

ng P

ress

ure

Increasing Temp

Pure Solventsolution

SOLID

GAS

LIQUID

P

Tf Tb

Boiling Point Elevation• Boiling Point Elevation – the temp difference

between a solution’s boiling point and a pure solvent’s boiling point.

•

• For nonelectrolytes: ΔTb = Kbm

• For electrolytes: ΔTb = iKbm

• Kb for water is 0.512◦C/m (see table 15-4)

Freezing Point Depression

Freezing Point Depression – the difference in temp between its freezing point and the freezing point of its pure solvent

For nonelectrolytes: ΔTf = Kfm

For electrolytes: ΔTf = iKfm

Kf for water is 1.86 ◦C/m (see table 15-5)

Do page 475 33-36

• Osmosis – 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 allow

some but not all kinds of particles to

cross it.– Ex: membranes surrounding all living

cells

• Osmotic Pressure (¶) – the amount of additional pressure caused by the water molecules that moved into the solution.

Heterogeneous Mixtures

• Mixture – combination of two or more substances that keep their basic identity

• Contain substances that exist in distinct phases.

• Two types:– Suspensions– Colloids

Suspensions• Suspension – mixture containing particles

that settle out if left undisturbed.– Ex: cornstarch in water, muddy water

• Suspended particles (1000nm) are much larger than solvated particles (1nm) so gravity quickly pulls them out of solution.

• Thixotropic – substances that are in a solid-like state but quickly begin to flow like a liquid when stirred. – Ex: house paint that quickly thickens

to a solid

Colloids• Colloid – heterogeneous mixture of intermediate size

particles (between 1nm and 1000nm).

• Colloids are categorized by the phase of their dispersed particles and dispersing mediums.

Category Dispersed Particles

Dispersing Particles

Example

Solid Sol Solid Solid Colored gems

Sol Solid Liquid Blood, gelatin

Solid Emulsion

Liquid Solid Butter, cheese

Emulsion Liquid Liquid Milk, mayonnaise

Solid Foam Gas Solid Marshmallow, soaps that float

Foam Gas Liquid Whipped cream, beaten egg white

Aerosol Solid Gas Smoke, dust in air

Aerosol Liquid Gas Spray deodorant, clouds

Types of Colloids

Brownian Motion• Brownian Motion - the erratic movement of

colloid particles.

• Occurs when particles of the dispersion medium collide with the dispersed particles

• Electrostatic forces keep the particles in the colloid from settling to the bottom.

• Heat, addition of an electrolyte, or any other interference with the electrostatic layering will destroy the colloid.

The Tyndall Effect• Dispersed colloid particles are large enough to

scatter light.– known as the Tyndall Effect

• Solutions never create the Tyndall Effect.• Suspensions do create this effect.

• Examples: rays of sunlight passing through smoke-filled air, lights through fog at night