Solutions

Solutions• Solutions are homogeneous mixtures of two or

more pure substances.• In a solution, the solute is dispersed uniformly

throughout the solvent.• Solutions where water is the solvent are called

aqueous solutions.

How and why do solutes dissolve in water?

Solution Formation is a 3 step process

c. Do all ionic solids dissolve in water?

NO, water is not strong enough to pull the ions aparthttp://www.youtube.com/watch?v=EBfGcTAJF4o

Solubility

• Solubility refers to the ability for a given substance, the solute, to dissolve in a solvent.

• It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution.

Solubility• Certain substances are soluble in all proportions with a given

solvent. An example for this is ethanol in water. This property is more correctly described as miscible.

• Under various conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is not very stable. The solvent is often a liquid, which can be a pure substance or a mixture.

• The species that dissolves, the solute, can be a gas, another liquid, or a solid. Solubilities range widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water.

• The term insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved.

• Staurated- solution containing all the dissolved solute possible at given conditions of temperature and pressure

• Unsatureted- solution containing less dissoleved solute than the maximum amount that can be dissolved at given conditions of temperature and pressure

• Supersaturated- unusual solution comtaining more dissolved solute than is normally possible at given conditions of temperature and pressure

• Dilute Solution- The amount of solute dissolved is small in relation to the amount of solvent present

• Concentrated Solution- The amount of solute dissolved is large in relation to the amount of solvent

A supersaturated solution

SOLUBILITY CURVES

Factors affecting solubility

• The solubility of one substance dissolving

in another is determined by • the balance of intermolecular forces

between the solvent and solute and • the entropy change that accompanies the

solvation. • Factors such as temperature and pressure

will alter this balance, thus changing the solubility.

The properties of solutions are dominated by the intermolecular

forces between solute and solvent.

Pressure

• Henry's law states that the solubility of a gas is directly proportional to the pressure of that gas, which may be written as:

• where k is a constant (for example, 769.2 L•atm/mol for (O2) at 298 K),

• p is the pressure of the gas

• c is the concentration of the gas

Henry's law• Henry's law

is used to quantify the solubility of gases in liquids as a function of the gas's partial pressure.

S1 = S2

P1 P2

Henry's law

Try:

If .024 g of a gas dissolves in 1.0 L of H2O at 1.5 atm of pressure, how much O2 gas will dissolve if the pressure is raised to 6.0 atm?

Cameroon: location of Lake Nyos

• Lake Nyos in Cameroon was the site of a natural disaster, in which a concentrated solution of carbon dioxide in water suddenly released enough carbon dioxide gas to suffocate 1700 people.

• The carbon dioxide concentration was high due to the high pressure deep in the lake: High pressure increases the solubility of carbon dioxide in water.

Pressure and solubility

• The higher the pressure above a liquid, the more soluble the gas is in the liquid.

• This explains why soda fizzes after the cap is removed.

Two soda cans, one with top popped

• A can of soda pop is pressurized with carbon dioxide.

• When the can is opened the pressure is released, lowering the solubility of carbon dioxide in the solution and causing it to come out of solution as bubbles.

• The higher the pressure above a liquid, the more soluble the gas is in the liquid.

• This explains why soda fizzes after the can is opened. The pressure of the open atmosphere is lower than the pressure inside the sealed can.

Polarity• A popular saying used for predicting solubility is "Like

dissolves like“ This indicates that a solute will dissolve best in a solvent that has a similar polarity to itself.

• This is a rather simplistic view, since it ignores many solvent-solute interactions, but it is a useful rule-of-thumb.

• Liquid solubilities also generally follow this rule. Lipophilic plant oils, such as olive oil and palm oil, dissolve in non-polar gasoline (petrol), but polar liquids like water will not mix with gasoline.

TEMPERATURE & SOLUBILITY

Generally speaking,• solids become more

soluble as temperature increases

• Gases become less soluble as temperature increases

Gases are less soluble at high temperature than at low

temperature.

What affects the amount of solute that can dissolve?

What affects the rate of solubility?

Quantification of solubility• Solubility is commonly expressed as a concentration,

either molarity or molality, but also as a mole fraction.

• The maximum equilibrium amount of solute that can normally dissolve per amount of solvent is the solubility of that solute in that solvent. It is often expressed as a maximum concentration of a saturated solution. These maximum concentrations are often expressed as grams of solute per 100 ml of solvent.

• Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility. For salts, solubility in aqueous solutions or the maximum amount of salt that can be dissolved is the solubility constant. The solubility constant is a special case of an equilibrium constant. It describes the balance between dissolved salt and undissolved salt.

How to prepare a 1 molar NaCl solution

How to prepare a 1 molar NaCl solution

• To make 1.0 liter of a 1.0 M NaCl solution, you add 1.0 mole (58.44 g) of sodium chloride to a flask and then dilute to 1.0 liter of total volume.

• The pear-shaped flask is called a volumetric flask. The mark on the flask was placed there at the factory, providing a precise volume reading.

Making a solution by dilution of a more

concentrated solution • to make 5.00 L of a

1.50 M KCl solution from a 12.0 M stock solution.

• The dilution equation, M1V1 = M2V2 guides us as we determine how much of the stock solution to use.

• The solute-solvent forces make new structures (order) in the solution

• But the solute breaks up the structure (order) that the liquid solvent may already have.

Solubility of ionic compounds in water

Soluble

• Group I A and NH4+ compounds

• Nitrates

• acetates (ethanoates)

• chlorides, bromides and iodide (except Ag+, Pb2+, Cu+ and Hg2

2+)

• sulfates (except Ag+, Pb2+, Ba2+, Sr2+, and Ca2+)

Composite of NaCl crystal lattice next to beaker of

dissolving ions

• In an NaCl solution, the Na+ ions and the Cl- ions are dispersed in the water.

• Water's dipoles are able to overcome the attraction of the ions for one another in the crystal lattice.

Electrolyte and nonelectrolyte solutions

Electrolyte solutions• contain dissolved ions (charged particles)

and therefore conduct electricity. • Nonelectrolyte solutions contain dissolved

molecules (neutral particles) and do not conduct electricity.

• The negative end of water's dipole will attract the sodium ion; the positive end of water's dipole will attract the chloride ion.

• The sugar molecule contains many polar O-H bonds; these interact with water strongly enough for the sugar to dissolve, but will not conduct electricity (a nonelectrolyte).

Solubility of ionic compounds in water

Insoluble• carbonates (except Group IA, NH4

+ and uranyl compounds)

• sulfites (except Group IA and NH4+ compounds)

• phosphates (except Group IA and NH4+

compounds) • hydroxides and oxides (except Group IA, NH4

+, Ba2+, Sr2+, Ca2+ and Tl+)

• sulfides (except Group IA, Group IIA and NH4+

compounds)

Colligative properties

In chemistry, colligative properties are the properties of dilute solutions of non-volatile solutes whose values depend only on the concentration of solute particles, not on

the type of particles present.

The four colligative properties :

• 1. Vapor pressure: The change in vapor pressure where the solute is less volatile than the solvent.

• 2. Freezing-point depression: The presence of a solute decreases the freezing point as compared to a pure solvent. The exact change (ΔT) can be calculated as van 't Hoff factor (i) of the solute multiplied by its molality (m) multiplied by the freezing point depression constant of the solvent Kf):

ΔT = Kf i m.

• 3. Boiling-point elevation: Because of the lowered vapor pressure, the boiling point of a solution is elevated as compared to the pure solvent.

ΔT = Kbim

The Vapor Pressure of a Solution is Lower than that of

the Pure Solvent

The vapor pressure of a solvent depends on how pure it

is.

Impurities in a substance cause a change in its phase

diagram by making the liquid region bigger.

Phase Diagram for a Solution and the Pure Solvent Indicating the Freezing Point Depression

Phase Diagram for a Solvent and its Solution with a

Nonvolatile Solute

4. Osmotic pressure

• The presence of solute can cause pressure to be exerted across a permeable membrane according to an equation quite similar to the ideal gas law: π is the osmotic pressure, n is the number of moles of solute, R is the ideal gas constant, T is the absolute temperature in kelvins, and V is the volume:

π = (nRTi)/V or Π = TRi M

where M is molarity

Setup for Measuring the Osmotic Pressure of a Solution

OSMOSIS

• is the movement of solvent through a membrane to equalize the concentration on both sides.

• Osmotic pressure has a great effect on living CELLS, because their walls are a semipermeable membrane

OSMOSIS

Physiological effects of osmosis

Physiological effects of osmosis

• Drinking seawater promotes dehydration because seawater is a thirsty solution.

• As the seawater flows through the stomach and intestine it draws water out of bodily tissues.

• Solutes in solutions will always flow from a region of higher concentration to a region of lower concentration.

• Seawater has a lower concentration of water than pure water has (because of the salt!), so water molecules will migrate toward a sample of seawater.

• If the seawater is in a person's stomach or intestines, water will move toward the seawater from the body's tissues, resulting in dehydration.

• (a) crenation is caused by water movement out of a cell in a hypertonic solution.

• (b) hemolysis is caused by water movement into a cell in a hypotonic solution.

COLLOIDS

• colloid or colloidal dispersion • a substance with components of one or two phases• a heterogeneous mixture where very small particles

of one substance are distributed evenly throughout another substance

• particles are between 1 nm and 1000 nanometers in diameter

• Typical membranes restrict the passage of dispersed colloidal particles more than they restrict the passage of dissolved ions or molecules; i.e. ions or molecules may diffuse through a membrane through which dispersed colloidal particles will not.

Tyndall effect

• the effect of light scattering on particles in colloid systems, such as suspensions or emulsions.

• used to tell the difference between the different types of mixtures, namely solution, colloid, and suspension.

• the Tyndall effect is noticeable when car headlamps are used in fog.

• The light with shorter wavelengths scatters better, thus the color of scattered light has a bluish tint. This is also the reason why the sky looks blue: the light from the sun is scattered and we see the blue light because it scatters better.

Tyndall Effect

A supersaturated solution• is holding more than the maximum amount of

solute. • In some cases, such as the sodium acetate

solution pictured here, a supersaturated solution may be temporarily stable.

• Any disturbance however, such as dropping in a small piece of solid sodium acetate (a) will cause the solid to come out of solution (b) and (c).

• think of the disturbance as a "reminder" to the sodium acetate ions that they would be at lower energy if they were organized in a crystal lattice.

A supersaturated solution

Saturated concentrations (solubilities) depend strongly on

temperature.