Chapter 13 & 14 Solutions Turn Outlines in to the Box, NOW! (Late work only worth 60%)

Chapter 13 & 14 Solutions

Turn Outlines in to the Box, NOW!

(Late work only worth 60%)

Let’s review the classification of matter.

Matter

Matter

Mixtures Substances

Matter

Mixtures Substances

Elements Compounds

Matter

Mixtures Substances

HomogeneousHeterogeneous

Elements Compounds

Matter

Mixtures Substances

HomogeneousHeterogeneous

Colloids

SuspensionsSolutions

Elements Compounds

What aren’t solutions?

1)Solutions are not colloids or suspensions.

a)Colloids have solute particles that are aggregates or large molecules between 1 nm and 1000 nm.

The particles (called the dispersed phase) cannot be separated from the solvent (called the dispersing medium) by filtration or settling. A colloid will scatter light (Tyndall effect).

Tyndall Effect

b)Suspensions have solute particles that are aggregates or molecules that are so large (>1000nm) that they settle and can be separated by a filter.

Although a suspension may scatter light, they are not transparent.

What are solutions?

1)Solutions are homogenous mixtures. This means solutions contain two or more substances, and are mixed enough to “look all the same”. Solutions do NOT have to be liquids, bronze is a solid solution and air is a gaseous solution.

A)All solutions have two parts. The substance doing the dissolving is the solvent and the substances being dissolved are the solutes. The solutes must have particles between 0.01-1nm. The solutes can be atoms, molecules, and ions. The solutes do not settle, scatter light, nor can be filtered.

If you don’t know which is which, the substance you have the most of overall is the solvent.

B)The most common solvent is water. It is so common and dissolves so many things that it is often called “the universal solvent” (the solute is hydrated.) This is not entirely true, as only polar solutes will dissolve in water (we say they are soluble or miscible). Nonpolar substances will not dissolve in water (we say they are insoluble or immiscible.) This is due differences in the types of intermolecular forces – H-bonding and dipole for polar, but only London forces for nonpolar.

If alcohol is the solvent, then the solution is often called a tincture.

In general it is said “like dissolves like” meaning polar solvents only dissolve polar solutes and nonpolar solvents only dissolve nonpolar solutes. Polar substances include salts, acids, bases, and nonsymmetrical molecules like water and NH3. Nonpolar substances include most carbohyrates, fats, oils, and other symmetrical molecules. The more similar two substances are the more miscible they will be. (Similar intermolecular forces!)

2)Even when things are soluble in water, or any other solvent, different solutes will dissolve to different amount. The actual amount (in grams) that will dissolve in 100 grams of a solvent is called the solubility. When discussing the solubility of solutes, solutions are classified into 3 types: saturated, unsaturated, and supersaturated.

A)A saturated solution has dissolved as much solute as it can. If you add sugar to iced tea and sugar is just sitting at the bottom of the glass your iced tea is saturated for sugar. (Even though it seems there is no change, there is an equilibrium occurring between dissolution and crystallization.)

B) An unsaturated solution can still dissolve more solute.

C)A supersaturated solution has been “tricked” into dissolving more solute than it should be able to. If any more solute is added to a supersaturated solution, all the extra solute will undissolve (precipitate). This is how sugar crystal candy (rock candy) is made.

D)Because the solubility of solutions is dependent on temperature, chemists represent the amounts that can be dissolved with a solubility curve:

1)Temperature is the independent variable (the one you control). This is shown on the bottom of the graph. You can warm up the solution with a stove or cool it down in a freezer.

2)The amount (in grams) of how much will dissolve at a particular temperature is the dependent variable (it depends on the temperature). This is shown on the left side of the graph. Notice this is for all the solutes in 100 grams of water.

A)If the temperature and amount of dissolved solute is on the line shown for that solute, then the solution is saturated. For example, 70 grams of NH3 in 100 grams of water will be a saturated solution at 10 °C.

B)If the temperature and amount of dissolved solute is above the line, then the solution is supersaturated. If the saturated solution of 70 grams of NH3 is very carefully heated, it will still have 70 grams of NH3 at 20 °C. As this amount is above the line, this would be a supersaturated solution.

C)If the temperature and amount of dissolved solute is below the line, then the solution is unsaturated. An unsaturated solution could still dissolve more solute if it is added.

For Review

3)Temperature is not the only factor that will affect how much solute can dissolve. Particle size, pressure, and agitation also have an effect.

A)Dissolving happens as the solvent touches the solute and starts to pull it into pieces. This happens only on the outside of the particles of the solute.

One big chunk of solute has less outside surface area than a lot of small pieces of solute. Thus, if the solute is crushed or ground it will dissolve faster. This only works for dissolving solids in liquids.

B)Gases are affected by pressure. With high pressure a lot of gas can be squeezed into a little bit of space. High pressures will help gases dissolve in a liquid, and low pressure will help remove the gas from a liquid. This only works for dissolving a gas in a liquid, and is why a carbonated beverage goes “flat” after it is opened (called Effervescence).

Henry’s Law shows that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the surface of the liquid.

P1=

S1

P2 S2

C) When a solid dissolves in a liquid, it takes time for the liquid molecules to grab pieces of the solid and float away. If the liquid is stirred, it speeds up the floating away processes, so more liquid can grab more solid in the same amount of time. Thus stirring or shaking will help a solid dissolve faster in a liquid.

D)When a gas dissolves in a liquid, it is basically trapped by the liquid molecules. If the liquid is stirred, it removes the water molecules that are trapping the gas, and the gas will form bubbles and eventually leave the liquid. This is why shaking a carbonated beverage makes so many bubbles.

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature

Particle Size

Pressure

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature

Particle Size

Pressure

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size

Pressure

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size Make smaller pieces

Pressure

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size Make smaller pieces No effect

Pressure

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size Make smaller pieces No effect

Pressure No effect

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size Make smaller pieces No effect

Pressure No effect Increase pressure

Agitation

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size Make smaller pieces No effect

Pressure No effect Increase pressure

Agitation Agitate

Method Solid dissolved in liquid

Gas dissolved in liquid

Temperature Increase temperature Decrease temperature

Particle Size Make smaller pieces No effect

Pressure No effect Increase pressure

Agitation Agitate Do not agitate

Solution Concentration

1)Just knowing that a solution is unsaturated does not indicate much about the solution. Is there only a little bit of solute, or is there a lot and it is almost saturated? The only way to tell is to know the solutions concentration.

2)In general if there is a lot of solute in the solvent, the solution is called concentrated, and if there is only a little bit of solute in the solvent it is called dilute. But chemists need a more exact way to know how concentrated or dilute the solution is.

Chemists have many different ways to determine numbers for concentration. Each way is different and useful in different situations.

A)Percent by mass = mass of solute divided by the mass of solution × 100%

50 grams of Ca(OH)2 are dissolved in 300 grams of water. What is the percent by mass for Ca(OH)2?

B) Percent by volume = volume of solute divided by the volume of solution × 100%

C) Molarity = moles of solute divided by the liters of solution

What is the molarity of a 3.50 L of solution that contains 90.0 g of sodium chloride?

M = molL

Because

then M × L × M = m

where M = Molarity

L = liters of solution

M = molar mass of solute

m = mass of solute

n = mM

Often chemical are purchased in concentrated form to save on shipping costs, but they must be diluted before use in lab. If your liquid chemical is measured in molarity, it is simple to determine how much of the concentrated solution is required to make a particular dilute solution needed in lab.

M1V1 = M2V2

Where

M1 = concentrated molarity

V1 = amount needed of concentrated (x)

M2 = diluted molarity

V2 = final amount needed after dilution

What amount of 12.1 M HCl must be diluted to make 250 mL of a 1.50 M solution?

How much water would be added?

D) Mole fraction = moles of solute divided by the total moles of solute and solvent

50 grams of NaCl, 30 grams of KF, and 50 grams of water are mixed. What is the mole fraction of water?

Colligative Properties

1)Physical properties of solutions that are affected by how much solute is dissolved are called colligative properties. (Colligative means “depending on the collection”.) There are three colligative properties that we’ll be concerned with: electrolytes, boiling point elevation, and freezing point depression.

A) When an ionic solute dissolves it ionizes (breaks apart into positive and negative parts), and these parts will allow electricity to travel through the solution. For example: pure water will not conduct electricity, but if table salt is added, then the sodium and chlorine that break apart will conduct electricity.

NaCl → Na+ + Cl-

Solutions that conduct electricity are called electrolytes. The more of an ionizing solute that is dissolved, the better the conduction of electricity will be. Typical electrolytes are acids, bases, and salts.

On the other hand, molecular solutes (covalent molecules) don’t ionize (like sugar) will not conduct electricity in water. So sugar water will not conduct electricity. Solutions that don’t conduct electricity are called nonelectrolytes.

B)Boiling happens when the molecules of a liquid get enough energy to break through the liquid-gas barrier and escape as a gas. If a solute is added, it gets in the way of the liquid trying to escape, and thus it takes more energy (more heat) to get the liquid to boil. This is called boiling point elevation.

If pure water boils at 100 °C, and salt is added, the solution will no longer boil at 100 °C, but will boil at a higher temperature. Often the directions for cooking pasta require the addition of salt to the water. This causes the water to boil at a hotter temperature and thus the pasta cooks quicker (and the salt adds some flavor to the pasta). The more solute that is added, the higher the boiling point will be.

C)Similar to boiling point elevation is freezing point depression. Now the solute lowers the temperature at which the solution freezes. Pure water freezes at 0°C, but salt water freezes below that temperature. This is why “anti-freeze” is added to car engines and salt is added to icy roads. This is also why salt is added to the ice when making ice cream - some of the ice melts to make the rest of the ice colder - cold enough to freeze cream.

Solubility Rules

During a double replacement reaction it is often necessary for a solid (precipitate) to form to get the reaction to occur in the first place. This solid forms because the ions that make it up form a compound that is insoluble. Whether or not ions form a solid, or will dissolve in the first place has been studied and chemists have come up with the following guidelines, or Solubility Rules:

Examples

Is sodium iodide soluble?

Which part(s)?

Is lead(II) nitrate soluble?

Which part(s)?

What would sodium iodide look like?

What would lead(II) nitrate look like?

What would NaI and Pb(NO3)2 make?

Would a precipitate form?

Which product would it be?

Which part(s) make it insoluble?