Unit 9
Compounds – Molecules
INTRODUCTION
Compounds are the results of combinations of elements. These new substances have unique properties compared to the elements that make them up.
Compounds are by far the most numerous substances known to man.
Aspirin, table salt, and the oxygen in the air we breathe are all compounds.
OBJECTIVES
1. The student will identify or list the general characteristics of compounds.
2. The student will define the terms used in this unit and give examples of each.
3. The student will identify the different types of compounds and interpret their formulas.
DISCUSSION
A. Compounds and their General Characteristics
A compound is a combination of two or more elements. The elements involved need not be different.
A molecule is the combination of two or more atoms and is the smallest part of a compound that exhibits all its properties.
In comparison, an atom is the smallest part of an element.
The following table summarizes these relationships.
Substance Smallest Part Element Atom Compound Molecule Compounds have the following properties.
1. They are combinations of two or more elements.
2. The elements combine in simple whole number ratios.
3. The smallest part of a compound is a molecule.
4. All molecules of a compound are exactly alike.
5. The properties of a compound are completely different from those of the elements that make them up.
These properties will be the basis for our dealings with compounds or their smallest parts, molecules.
B. A chemical formula describes which elements are part of the compound and how many atoms of each.
The elements are represented by their symbols, and the quantity of each is given by a simple whole number written as a subscript following the symbol for the element.
If only one element of a type appears in the formula, the number 1 is not written.
We could also say that a chemical formula tells us the number of atoms of each element in a molecule of a compound. This is a more practical definition.
Example Problem (1)
Hydrogen Peroxide has the chemical formula H2O2. Interpret it.
Solution
The formula for Hydrogen Peroxide, H2O2 describes a molecule of this compound as having two atoms of Hydrogen (H) and two atoms of Oxygen (O) per molecule.
Or we might say that the compound Hydrogen Peroxide contains two atoms of Hydrogen for every two atoms of Oxygen.
Example Problem (2)
Interpret the chemical formula for Sulfuric acid H2SO4 . Solution
Sulfuric acid contains Hydrogen, Sulfur, and Oxygen in an atomic ratio of 2:1:4.
That is, one molecule of Sulfuric acid contains two atoms of Hydrogen, one atom of Sulfur, and four atoms of Oxygen.
Note that there is no number written for Sulfur, this means there is only one.
C. Empirical and Molecular Formulas
Chemical formulas can be either Empirical or Molecular.
An empirical formula is the simplest whole number ratio between atoms in a molecule.
A molecular formula is the actual ratio of atoms in a molecule of a compound.
In some cases, the empirical and molecular formulas are the same, but the most useful is the molecular formula because it provides the exact makeup of the molecule.
The molecular formula for Hydrogen Peroxide is:
H2O2
dividing both subscripts by their lowest common denominator, which is 2, yields the empirical formula:
H O
this formula tells us the ratio of atoms in the molecule, but tells us nothing about the actual number of atoms in the molecule.
The molecular formula for Sulfuric acid is:
H2SO4
The subscripts 2, 1, and 4 have no common denominator. This molecular formula is also the empirical formula.
Example Problem (3)
Determine which of the following formulas are empirical, which are molecular. For the molecular formulas determine the empirical formula.
C6H6 , C7H8O2 , H3PO4 , C2H6O2 Solution
C6H6 is a molecular formula because the subscripts 6 and 6 have a lowest common denominator which is 6.
The empirical formula is CH .
C7H8O2 is an empirical formula because there is no lowest common denominator for the subscripts 7, 8, and 2.
H3PO4 is an empirical formula because there is no lowest common denominator for the subscripts 3, 1, and 4.
C2H6O2 is a molecular formula because the subscripts 2, 6, and 2 have a lowest common denominator, which is 2.
The empirical formula is CH3O .
The empirical formulas C7H8O2 and H3PO4 may also be molecular formulas. In this case, they are.
The information needed to determine whether or not they are is the molecular weight.
D. Molecular Weight
The Molecular Weight (MW) of a molecule is the sum of the atomic weights for every atom in a molecule.
Molecular weights are based on the same scale as the atomic weights of atoms. The units are atomic mass units (amus).
The molecular weight for Hydrogen Peroxide (H2O2) is the sum of two atomic weights of Hydrogen plus two atomic weights of Oxygen.
MW (H2O2) = 2 AW (H) + 2 AW (O)
= 2 (1) + 2 (16)
= 2 + 32
MW (H2O2) = 34 amu
The molecular weight of Sulfuric acid (H2SO4) is 98 amu.
MW (H2SO4) = 2 AW (H) + AW (S) + 4 AW (O)
= 2 (1) + 32 + 4 (16)
= 2 + 32 + 64 MW (H2SO4) = 98 amu
Example Problem (4)
Calculate the molecular weight of Sodium Benzoate, a common preservative. The molecular formula for Sodium Benzoate is
C7H5O2Na Solution
The molecular weight of Sodium Benzoate is the sum of seven atomic weights of Carbon, five of Hydrogen, two of Oxygen, and one of Sodium.
MW (C7H5O2Na) = 7 AW (C) + 5 AW (H) + 2 AW (O) + AW (Na)
= 7 (12) + 5 (1) + 2 (16) + 23
= 84 + 5 + 32 + 23 MW (C7H5O2Na) = 144 amu
The molecular weight may be used to determine whether a formula is an empirical formula or a molecular formula.
Only the sum of the atomic weights of the atoms in the molecular formula will give the correct molecular weight.
Example Problem (5)
Benzene, a known carcinogen (cancer causer) has a molecular weight of 78 amu. Which of the following formulas is its molecular formula?
CH, C3H3, C6H6
Solution
CH has a “molecular weight” of 13 amu.
C3H3 has a “molecular weight” of 39 amu.
C6H6 has the correct molecular weight of 78 amu.
CH is the empirical formula, while C3H3 can only be reduced to the empirical formula.
E. Some Types of Compounds
There are literally millions of compounds, most of them quite complex.
Diatomic compounds are generally those compounds made of the same two elements. Examples are:
H2 Hydrogen N2 Nitrogen O2 Oxygen F2 Fluorine Cl2 Chlorine
Br2 Bromine
I2 Iodine
These elements bear the same names as the elements that make them up. They are the fashion in which the pure element is found naturally, but they are still considered compounds.
Binary compounds are those compounds made of two different elements. Examples are:
NaCl Sodium Chloride HBr Hydrogen Bromide
H2O Water (Hydrogen Oxide) Na2S Sodium Sulfide
Ternary compounds are those compounds made of three different elements. Examples are:
H2SO4 Sulfuric Acid H3PO4 Phosphoric Acid
NaBrO3 Sodium Bromate
KMnO4 Potassium Permanganate Na2C2O4 Sodium Oxalate
There are many other compounds of each type and many other classes of compounds.
PROBLEMS
1. List the general properties of compounds.
2. Define the terms.
a. Empirical formula b. Molecular formula c. Molecular weight
d. Diatomic compound
e. Binary compound
f. Ternary compound
3. Interpret the following chemical formulas.
a. HClO4 b. C6H5CO2Na
4. Which of the following is an empirical formula?
a. C7H14N2 b. C7H21O3
c. H2O d. C2H6
5. Calculate the molecular weight for the compounds.
a. HNO3
b. C22H40O11
c. Na2Cr2O7
