Unit 14 Teacher Notes: Solution Chemistry

UNIT 14 NOTES: SOLUTIONS

STUDENT OBJECTIVES: Your fascinating teachers would like you amazing learners to be able to…

  1. describe the unique role of water in chemical and biological systems;
  2. develop and use general rules regarding solubility through investigations with aqueous solutions;
  3. calculate the concentration of solutions in units of molarity;
  4. use molarity to calculate the dilutions of solutions;
  5. distinguish between types of solutions such as electrolytes and nonelectrolytes and unsaturated, saturated, and supersaturated solutions;
  6. investigate factors that influence solubilities and rates of dissolution such as temperature, agitation, and surface area;

  1. Properties of Water
  1. Hydrogen Bonding
  • Water is polar which means the electrons are unevenly distributed and cause partial positive and partial negative charges within the molecule.
  • Water is found in groups of five molecules because of hydrogen bonding.
  • Hydrogen bonds are really just attractions between a positive and a negative charge.
  1. Cohesion/Surface Tension
  • Cohesion is an attraction between substances of the same kind.
  • Water molecules are attracted to each other because they are polar.
  • The cohesion of water molecules causes a condition called surface tension.
  • Surface tension prevents the surface of the water from stretching or breaking easily.
  1. Adhesion
  • Adhesion is an attraction between different substances.
  • Water molecules are attracted to many other polar substances.
  • Adhesion causes a behavior known as capillary action.

Cohesion causes water to form drops

Surface tension causes them to be round

Adhesion keeps the drops in place

  1. Density in Liquid & Solid States
  • As the temperature decreases, the forming of hydrogen bonds creates a crystalline structure which is very "open”.
  • This “openness” created actually lowers the density of water as it freezes.
  • The empty space between the molecules lowers the over all mass for a given volume.
  • Which state of water has the greatest density? SOLID
  1. The Universal Solvent
  • The polarity of water enables many substances to dissolve into it.
  • Ionic compounds and polar molecules dissolve best in water. (Ex: salt)
  • Nonpolar molecules do not dissolve well in water. (Ex: Oil)
  • The polar nature of water, which is very strong, causes water to be considered the greatest dissolving agent known to man.

Which of the following properties causes attraction between molecules of liquid water?

  1. Acidity
  2. Polarity
  3. Density
  4. Viscosity

Which state of water has the greatest density?

  1. Solid
  2. Liquid
  3. Gas

Fish survive through severe winters because of the property of water that allows water to–

  1. Form chemical bonds as it freezes, raising the water temperature below the ice.
  2. Increase in density while it freezes, dissolving more oxygen from the air.
  3. Expand when it freezes, creating a floating and insulating layer of ice.
  4. Precipitate vital nutrients when it freezes, increasing the food supply.


II.Pure Substances

A.A pure substance has a definite composition. Every sample of a pure substance has exactly the same properties and composition.

B.Pure substances can beelements(examples: Fe, C, N2) orcompounds(examples: H2O, C12H22O11, CuSO4).

1.Compounds can be separated chemically into elements.

2.Elementscannotbe separated.

III.Mixtures

Before we talk about solution formation, we need to recall some basics about the differences between heterogeneous mixtures and homogeneous mixtures (solutions).

A mixture is a blend oftwo or more kinds of matter, each of which retains its own identity and properties

And

A mixturecan be separated physicallyinto pure substances.

A.A homogeneous mixture has a uniformcomposition throughout.

1.Asolutionis a homogeneous mixture of two or more substances in a single phase.

**In this unit, we will be getting WAY more in-depth with HOMOGENEOUS MIXTURES – also known as solutions. It is important to remember that solutions only contain very small particles dissolved in another substance.

B.A heterogeneous mixture doesnot have auniformcomposition throughout.

1.A suspension is a heterogeneous mixture in which the particles are so large that they will settle out unless the mixture is constantly stirred or agitated. Example: soil suspended in water (mud).

a.Suspensions can be separated bysettling or filtration

IV.Solutions

A.Solution need to know facts:

1.Substances that are capable of being dissolved are called soluble.

2.Substances that are incapable of being dissolved are calledinsoluble.

As a reminder, when you have an insoluble solid that collects at the bottom of a solution, we call that solid a______PRECIPITATE______.

3.The dissolving medium in a solution is called the solvent.(what is doing the dissolving)……. usually water, but not always!!!!

4.The substance dissolved in a solution is called the solute. (what is being dissolved)

Example 11-1: In salt water, what is the solute and what is the solvent?

Salt-solute, water-solvent

5.. Solutions may exist as solids, liquids or gases.

**All solution are not necessarily liquids!!!

Solute / Solvent / Example
gas / gas / air (O2 in N2)
gas / liquid / carbonated water (CO2 in H2O)
liquid / liquid / vinegar (acetic acid in H2O)
liquid / solid / dental amalgam (Hg in Ag)
solid / liquid / seawater (NaCl in H2O)
solid / solid / 14-karat gold (Ag in Au)

6.Miscibleliquids are able to dissolve freely in one another in any proportion.

7.Immiscible liquids are not soluble in each other(forms layers).

Miscible and immiscible refer toliquid-liquidsolutions, only.

Example 11-2:Determine whether the following are miscible or immiscible.

  1. Rubbing alcohol and water are miscible.
  2. Oil and water are immiscible..

Think quick: How do you determine which layers will be on top of others??????

density

B.“Like dissolves like”is a rule that predicts whether one substance will dissolve in another.

1.Polar and ionic solutes dissolve inpolarsolvents.

2.Non-polar solutes dissolve in non-polar solvents.

3.Most compounds are either polar orionic. Water is apolarsolvent. Therefore water is universal solvent.

C.Rate of dissolutionrefers to how quickly a solute dissolves.

Increasing the frequency of collisions between solvent molecules and solute molecules increases the rate of dissolution.

D.Solubilityrefers to the amount of solute that can be dissolved at a specific temperature.

Dissolution Rate of a Solid / Solubility of a
/ Solid / Gas
Heating Solution / Increase ↑ / Increase ↑ / Decrease ↓
Agitating Solution
(stir or shake) / Increase ↑ / No effect / Decrease ↓
Increase surface area of solute (crush/grind) / Increase ↑ / No effect / No effect
Increasing Surface Pressure of Solution / No effect / No effect / Increase ↑

1.A saturated solution contains themaximumamount of dissolved solute at a given temperature.

2.An unsaturatedsolution containslessdissolved solute than the maximumamount.

3.A supersaturatedsolution containsmoredissolved solute than the maximum amount.

Supersaturated solutions contain more solute than normal for that temperature and so are unstable. This unstable state can be disrupted by adding more solute into the solvent. When you do so, you will decrease your solubility down to normal saturated conditions and all of the extra solute that was being dissolved will precipitate (fall) to the bottom of your container.

For example, adding more sugar to sweet tea can actually cause your tea to become less sweet because all of the extra sugar that was being dissolved will be disrupted and fall to the bottom of your glass.

V.SOLUBILITY CURVES--shows the dependence of solubility on temperature

Below are solubility curves showing the solubility of several different compounds at different temperatures.The solubility chart with the solid compounds is expressed as grams of solute dissolved in 100 grams of water. This can be different depending on your chart. Make sure to be aware of what your chart shows.

SOLUBILITY OF SOLID IONIC SALTS (and NH3)SOLUBILITY OF GASES

Example 11-3: What do you notice in general about the solubility of solids with increasing temperature?

increases

Example 11-4: What do you notice in general about the solubility of gases with increasing temperature?

decreases

USING SOLUBILITY CURVES…

  1. Any amount of solute below the line indicates the solution is unsaturated at a certain temperature.
  1. Any amount of solute above the line in which all of the solute has dissolved shows the solution is supersaturated.
  1. If the amount of solute is above the line but has not all dissolved, the solution is saturated and the extra grams of solute have precipitated (settled) to the bottom. (# g precipitated = total # g in solution – # g of a saturated solution at that temperature)

Example 11-5: According to the graph shown above, which substance maintains almost constant solubility over the given temperature range?

NaCl

Example 11-6: Which substance decreases in solubility with increasing temperature?

Ce2(SO4)3

Example 11-7: What is the solubility of KClO3 at 60°C?

21 g KclO3/100 g H20/100 g H

2O

Example 11-8: A solution containing 50 g CaCl2 in 100 g H2O is prepared at 10°C. Is the solution saturated, unsaturated or supersaturated?

Unsaturated

Example 11-9: A supersaturated solution at 10°C contains 40 g KCl in 100 g H2O. At what temperature was the solution saturated, before being cooled to 10°C?

40 degrees C

Example 11-10: If the supersaturated solution from the previous example is disturbed, how many grams of KCl will suddenly crystallize?

10 g

VI.ELECTROLYTES – When will solutions conduct electricity?

An electrolyte is a substance that dissolves in water to make a solution that can conductelectric current.

A.Any soluble, ionic compound is an electrolyte.

B.Some highly polar covalent molecules, such as strong acids, are also electrolytes. Use the following to determine if a substance is an electrolyte:

1.Is the substance ionic? (metal bonded with non-metal, or contains a polyatomic ion)

2.Is the substance soluble in water?

C.Dissociationreactions show theseparationof ions that occur when an ionic compound dissolves in water.

Here’s a picture of how NaCl gets dissociated/dissolved by water:

NaCl + H2O ----> Na+(aq) + Cl-(aq)

More dissociation examples:

1.

Calcium chloride is a soluble ionic compound (an electrolyte). Its dissociation reaction is written above. One formula unit of CaCl2 produces a total of three ions in solution (1 Ca2+ ion and 2 Cl- ions).

2. H2SO4 (l)  2H+1 (aq) + SO4–2 (aq)

3. NaOH (l)  Na+1 (aq) + OH–1 (aq)

Example 11-11: Determine if lithium phosphate is an electrolyte. If it is, write the dissociation reaction and count the number of ions produced. Hint: Use the 2 questions in section B directly above!!!

Li3PO4ionic, soluble, electrolyte

4 ions

i3PO4ionic, soluble, electrolyte ions

Example 11-12: Determine if barium sulfate is an electrolyte. If it is, write the dissociation reaction and count the number of ions produced. Hint: Use the 2 questions in section B directly above!!

BaSO4ionic, not soluble, not an electrolyte

NO REACTION

electrolytREACTION

Example 11-13: Determine if aluminum nitrate is an electrolyte. If it is, write the dissociation reaction and count the number of ions produced. Hint: Use the 2 questions in section B directly above!!!

Al(NO3)3ionic, soluble, electrolyte

4 ions

NO3)3ionic, soluble, electrolyteons

Example 11-14: Determine if iodine is an electrolyte. If it is, write the dissociation reaction and count the number of ions produced. Hint: Use the 2 questions in section B directly above!!!

I2non-polar covalent, not soluble, not an electrolyte

NO REACTION

I2n-polar covalent, not soluble, not n electrolyte

NO REACTION

VII.Solution Concentration

A.Concentration is a measure of the amount of solute in a given amountof solvent or solution.

1.Molarity (M) is the number of moles of solute in one liter of solution. (any solvent, not just water).

or

**Molarity is designated by using the capital “M” right after a number. MOLAR refers to molarity.

**As you increase molarity, the solution is more CONCENTRATED……low molarity means you have a dilute solution, and high molarity means you have a CONCENTRATED solution

Example 11-15: What is the molarity of a solution that contains 1.54 mol sodium chloride in 3.5 L solution?

Example 11-16: How many moles of HCl are required to prepare 0.8 L of a 0.5 M HCl solution?

mol = (0.5 M)(0.8 L) = 0.4 mol HCl

Example 11-17: If 35.8 grams of lithium hydroxide are dissolved in enough water to make 750 ml of solution, what is the molar concentration (molarity) of the solution?

General ChemistryPage 1 of 14