CHE 111 - Module 3

CHAPTER 3

LECTURE NOTES

STOICHIOMETRY

• Stoichiometry is the study of the quantitative relationships between the amounts of reactants and products in chemical reactions.

• We use BALANCED equations to understand stoichiometric relationships of the elements and compounds within a chemical reaction.

The Balanced Equation

2Al(s) + 3Br2(l) Al2Br6(s)

2mol of Al : 3mol of Br2 : 1mol of Al2Br6

Therefore the ratio of Al to Br2 to Al2Br6

is 2:3:1 for the chemical reaction to occur.

A Closer Look at the Equation

2Al(s) + 3Br2(l) Al2Br6(s)

• The chemicals on the left are the reactants and the right are the products.

• The coefficient in front of the chemical denotes the stoichiometric relationship.

Numerical Subscripts

2Al(s) + 3Br2(l) Al2Br6(s)

• The numerical subscript represents the

number of atoms present in the molecule

– ex. Br2 means that an atom of Br is bonded to another atom of Br

– Therefore: Br-Br = Br2

Denoting the Phase of Matter

Phase of Matter Symbol

Solid (s)

Liquid (l)

Gas (g)

Aqueous (aq)

2Al(s) + 3Br2(l) Al2Br6(s)

The subscript letters in parenthesis denote the phase of matter that the chemical is in.

Formulas and Models of Ethanol

• Molecular Formulas C2H6O

• Condensed Formulas C2H5OH

H H

• Structural Formulas H-C-C-O-H

H H

• Molecular Models (classroom models)

Molecular Models

• Cache program - models - organic models - ethanol

• CD-ROM screen 3.4

• Model of ice

Ions and Ionic Compounds

• Ions are atoms or groups of atoms that have lost or gained electrons resulting in an overall positive or negative charges.

• Ionic compounds are compounds formed by the combination of (+) and (-) ions.

(+) ions are called cations(-) ions are called anions

Formation of Ions

Formation of a cation by a loss of electrons

Li atom Li+ + 1 e- released

(3p and 3e-) (3p and 2e-)

Formation of an anion by gaining electrons

F atom + 1 e- added F

(9p and 9e-) (9p and 10e-)

Ions and the Periodic Table

Family 1A 2A 3A 4A 5A 6A 7A 8A

IonicCharge

+1 +2 +3 -3 -2 -1

Metals of group 1A, 2A & 3A form+1, +2, and +3 ions; and non-metalsof group 5A, 6A, and 7A form -3, -2,and -1 respectively.

Polyatomic Ions

• Table 3.1 - page 89

• CD-ROM Screen 3.6

• Hand out

• Flash Cards

Common Polyatomic Ions

• carbonate ion CO3-2

• sulfate ion SO4-2

• sulfite ion SO3-2

• hydroxide OH-

• phosphate PO4-3

• permanganate MnO4-

• chromate CrO4-2

• dichromate Cr2O7-2

• ammonium NH4+

• oxalate C2O4-2

• bicarbonate HCO3-

• cyanide ion CN-

• acetate C2H3O2-

• peroxide O2-2

• thiosulfate S2O3-2

• bisulfite HSO3-

Oxoanions

A polyatomic anion containing oxygen is called an oxoanion and is named as follows:

• Greater # of O atoms has the suffix -ate.

• Lesser # of O atoms has the suffix -ite.

Ex.NO3

- is called nitrate ion

NO2- is called nitrite ion

Naming Oxoanions

More than 2 ions in an oxoanion grouping

are named as follows:

• Largest # of O atoms has a prefix of per- and a suffix of -ate

• Next larger # of O atoms has a suffix -ate

• Smaller # of O atoms has a suffix -ite

• Smallest # of O atoms has a prefix of hypo- and a suffix of -ite

Naming Oxoanions

Ex.

ClO4- is called perchlorate

ClO3- is called chlorate

ClO2- is called chlorite

ClO- is called hypochlorite

Ionic Compounds

Ca+2 + 2Cl- CaCl2

Each ion comes together based on charge to form an overall neutral ionic compound.

3Ca+2 + 2PO4-3 Ca3(PO4)2

The cation and the polyatomic ion come together based on charge to form an overall neutral ionic compound.

Naming Ionic Compounds

• Naming Positive Ions – Cations• Cations are named first in the compound

and as follows:– Monatomic cations are mostly metals and are

named directly as they are on the periodic table.– Transition metals are named according to their

ionic charge– Polyatomic cation, NH4

+ is named ammonium directly

Naming Ionic Compounds

• Naming Negative Ions – Anions

• Anions are named lastly and have specific naming rules as follows:– Monatomic ions are named with an –ide after

its atomic name– Polyatomic ions are named as memorized

dropping the word ion.

Cr +2 Cr +3 Cr +6 Chromium II,III&VI

Mn +2 Mn +3 Mn +4 Mn +6 Mn +7

Fe +2 Fe +3 Fe +6 Iron II, III & VI

Co +2 Co +3 Cobalt II & III

Ni +2 Ni +3 Nickel II & III

Cu +1 Cu +2 Copper I & II

Pb +2 Pb +4 Lead II & IV

Sn +2 Sn +4 Tin II & IV

Au +1 Au +3 Gold I & III

Hg +1 Hg +2 Mercury I & II

Zn +2 Zinc

Ag +1 Silver

Naming Molecular Compounds

• 1 mono• 2 di• 3 tri• 4 tetra• 5 penta• 6 hexa• 7 hepta• 8 octa

Formula & Molecular Weights

• Review of spectra lab - MW calculations

• CD-ROM Screen 3.14

• Definition: The total mass of the formula unit or molecule with consideration to the mass of each component element that makes up the overall unit.

Calculating Formula & MW

• Remember we said that:

1 mole C = 12.011g C = 6.02x1023 atoms C

• If we add up the number atoms present of each element in a molecule or formula unit and multiply each by its atomic weight on the periodic table,

• Then the resultant sum of each element added together will give you the formula or molecular weight.

Example of MW Calculation

• Determine the MW of H20

– 1 O @ 15.999g/mole– 2 H @ 1.008g/mole

• Therefore 2 x 1.008 = 2.016g/mole

• and 1 x 15.999 = 15.999g/mole

Total molar mass = 18.015g/mole

Determine the MW of ethanol: C2H5OH

Converting Mass to Moles

• Question: How many moles of H2O are in 42.0g of water?

• Answer: First you determine the MW of water as we did on the previous slide, then you convert

42.0g H2O x 1 mole H2O =

18.016g H2O

Percent Composition

• Calculate the percent composition of NH3

– First determine the atomic weights of each N and H from the periodic table

– Then calculate the MW of the ammonia molecule

– Take the mass of each element and divide by the MW and multiply 100%

• CD-ROM Screens 3.14 and 3.16

Hydrated Compounds

• Definition: Ionic compound that has water molecules incorporated within its crystal structure

• Ex. CuCl2•2H2O

– Where we name this compound copper(II) chloride dihydrate

– When calculating MW, we calculate the two waters into the overall mass of the compound