Chemical Formulas Unit-Day 4
Purpose: The purpose of this lesson involves reviewing nomenclature and molecular
formulas as well as introducing naming molecular compounds and identifying acids.
SOL’s Addressed:
• CH.3 The student will investigate and understand how conservation of energy
and matter is expressed in chemical formulas and balanced equations. Key
concepts include
a) nomenclature
c) writing chemical formulas
d) bonding types (ionic and covalent)
Safety Issues: There are no safety issues involved in this lesson.
Materials:
-Quiz
-Review sheet
Procedures:
Introduction to writing molecular formulas (20 minutes):
Students will now be taught the nonmetallic molecules and introduced to properties that
these compounds possess.
1. Molecules are composed of nonmetallic elements.
Example: CO2, CCl4, or SO2
2. Molecules are composed of neutral atoms. They do not give their electrons away as
ionic molecules tend to do. They share their electrons forming covalent bonds.
Example: Br2
3. The elements in the molecule can combine in more than one fashion. Not fixed as in
ionic compounds. Example: Carbon monoxide (CO) or Carbon Dioxide (CO2)
Example: Water (H2O) Hydrogen peroxide (H2O2)
4. Diatomic molecules are molecules composed of two of the same elements. These
include Br2, Cl2, F2, H2, I2, N2, O2.
Students will then be given a list of 10 compounds and they will need to be able to
identify whether they are molecular or ionic and why.
Nomenclature of Molecular Compounds and Identifying Acids (20 minutes):
1. The first word gives the name of the element that appears first in the compound. The
Greek prefix is used to show the number of atoms of that element in the formula. For the
cation, the prefix for one (mono) is understood.
Prefixes are:
1-mono
2-di
3-tri
4-tetra
5-penta
6-hexa
7-hepta
8-octo
9-nona
10-deca
2. The second word consists of the appropriate Greek prefix designating the # of atoms
the stem of the name of the second element
the suffix –ide
Give examples for students to practice:
Examples:
N2O5 Dinitrogen Pentaoxide
PCl3 Phosphorus trichloride
Cl2O7 Dichlorine heptaoxide
Introduction to different Formulas: (15 minutes)
Find Ratio of boys to girls in the classroom:
Suppose we have 6 boys for every 18 girls. What would the ratio be? 1:3 (1 boy for
every 3 girls) This in chemistry represents the empirical formula (the simplest formula
that will indicate the ratio of atoms present in a compound)
The actual number of boys versus girls will be indicated by the molecular formula.
H2O2 would be the molecular formula while HO would be the empirical formula.
Practice with the following:
What is the empirical formula for each of the following:
-C2H6
-C6H8O6
-CH4
N2O4
Formula Units: the formula of a compound represent definite amounts of that compound.
Use coefficients in front of the formula unit to represent more than one.
How many formula units are present in the following:
-3NaCl
-2NaCl
-6 Ba3(PO4)2
Acids: (15 minutes)
Compounds that produce Hydrogen Ions when dissolved in water.
Usually combinations of anions with as many Hydrogen ions (H+) necessary to
make the molecule neutral.
Go over naming conventions:
Binary Acids: HCl, HF, HI, HBr Hydro-stem-ic acid
Ternary Acids: H2SO4 Hydrogen sulfate going to take that sulfate (-ate ending) and
add –ic. Drop the hydrogen and get sulfuric acid.
HNO3 Nitric acid (hydrogen nitrate so drop the hydrogen and change the ate to
ic+acid)
H2CO3 Carbonic Acid
H3PO4 Phosphoric Acid
HNO2 Hydrogen nitrite going to change the –ite to –ous acid.
H2SO3 Sulfurous acid.
Review: (Rest of time)
Complete a review Worksheet for the Chemical Formulas Chapter.
Students will go to http://www.webteach.addr.com/CHSChemistry/resource.htm to
complete their online homework.
Assessment:
Students will be informally assessed on:
-Their lists differentiating between ionic compounds and molecular compounds.
-The practice problems for writing names of molecular compounds, recognizing a
molecular compound from ionic compounds and acids, writing empirical formulas, and
telling how many formula units are present.
-Students will also be able to complete a review worksheet for the upcoming test.
Formal Assessment:
-Students should complete their online homework either online or traditionally in order to
receive bonus points on their test.
-The upcoming test on chemical formulas. Material on test from this lesson will account
for about 20% of test material.