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 Element Compound Smallest Part Atom 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 H 2 O 2. Interpret it. The formula for Hydrogen Peroxide, H 2 O 2 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 H 2 SO 4. 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: H 2 O 2 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: H 2 SO 4 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. D. Molecular Weight C 6 H 6, C 7 H 8 O 2, H 3 PO 4, C 2 H 6 O 2 C 6 H 6 is a molecular formula because the subscripts 6 and 6 have a lowest common denominator which is 6. The empirical formula is CH. C 7 H 8 O 2 is an empirical formula because there is no lowest common denominator for the subscripts 7, 8, and 2. H 3 PO 4 is an empirical formula because there is no lowest common denominator for the subscripts 3, 1, and 4. C 2 H 6 O 2 is a molecular formula because the subscripts 2, 6, and 2 have a lowest common denominator, which is 2. The empirical formula is CH 3 O. The empirical formulas C 7 H 8 O 2 and H 3 PO 4 may also be molecular formulas. In this case, they are. The information needed to determine whether or not they are is the 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 (H 2 O 2 ) is the sum of two atomic weights of Hydrogen plus two atomic weights of Oxygen. MW (H 2 O 2 ) MW (H 2 O 2 ) = 2 AW (H) + 2 AW (O) = 2 (1) + 2 (16) = 2 + 32 = 34 amu The molecular weight of Sulfuric acid (H 2 SO 4 ) is 98 amu. MW (H 2 SO 4 ) = 2 AW (H) + AW (S) + 4 AW (O) = 2 (1) + 32 + 4 (16) = 2 + 32 + 64 MW (H 2 SO 4 ) = 98 amu
Example Problem (4) Calculate the molecular weight of Sodium Benzoate, a common preservative. The molecular formula for Sodium Benzoate is C 7 H 5 O 2 Na 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 (C 7 H 5 O 2 Na) MW (C 7 H 5 O 2 Na) = 7 AW (C) + 5 AW (H) + 2 AW (O) + AW (Na) = 7 (12) + 5 (1) + 2 (16) + 23 = 84 + 5 + 32 + 23 = 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, C 3 H 3, C 6 H 6 CH has a molecular weight of 13 amu. C 3 H 3 has a molecular weight of 39 amu. C 6 H 6 has the correct molecular weight of 78 amu. CH is the empirical formula, while C 3 H 3 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: H 2 N 2 O 2 F 2 Cl 2 Br 2 I 2 Hydrogen Nitrogen Oxygen Fluorine Chlorine Bromine 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 H 2 O Water (Hydrogen Oxide) Na 2 S Sodium Sulfide Ternary compounds are those compounds made of three different elements. Examples are: H 2 SO 4 H 3 PO 4 NaBrO 3 KMnO 4 Na 2 C 2 O 4 Sulfuric Acid Phosphoric Acid Sodium Bromate Potassium Permanganate 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. HClO 4 b. C 6 H 5 CO 2 Na 4. Which of the following is an empirical formula? a. C 7 H 14 N 2 b. C 7 H 21 O 3 c. H 2 O d. C 2 H 6 5. Calculate the molecular weight for the compounds. a. HNO 3 b. C 22 H 40 O 11 c. Na 2 Cr 2 O 7