Chapter 3Molecules and Compounds

Molecules and Compounds -Chemical Formulas

1:1 1:2 1:3 2:3 1:4 etcCO H2O NH3 Al2O3 CH4

C + 4 H = CH4

Molecular model

Molecular Models

Ethanol

Molecular Models

Some compounds are IONIC - electrons are TRANSFERRED from a metal to a nonmetal; the compound is held together ELECTROSTATICALLY

Coulomb’s Law: force = k (n+)(n-)/d2

Some compounds are COVALENT - electrons are SHARED between two atoms

+

e-

+

e-

E. g. carbon dioxide

This commonly occurs for two or more NONMETALS

The Covalent Bond

Ions

• An atom or group of atoms with a net charge caused by the net loss or gain of electrons

• CATION = positively charged ion• ANION = negatively charged ion

Formation of Cations & Anions

Predicting Whether an Atom Will Form a Cation or Anion in Order to Make an Ionic Compound

Metals LOSE electrons to form CATIONS, nonmetals GAIN electrons to form ANIONS

Valence Electrons in Ionic Compounds

• The A-group (representative) elements follow the OCTET RULE; they obtain an inert gas valence (outer) shell that contains 8 electrons

• Metals - lose # electrons = group numbere.g. Ca Ca2+ + 2e- (Ar outer shell)

• Nonmetals - gain electrons = 8 - group #e.g. N + 3e- N3- (Ne outer shell)

Valence Electrons for Covalent Compounds

• Covalent compounds form between two or more nonmetals

• In this case the nonmetals can either LOSE all of their valence electrons,or -

• GAIN enough electrons to obtain an OCTET

Examples -

SO3 - oxygen (VIA) gains 8-6 = 2

O2- ion forms

- sulfur (VIA) loses all 6

S6+ ion formsNOTE: There is a rule that states that oxygen is ALWAYS -2. These rules are coming up!

Example 3.3 - Predicting Ion Charges When Forming Ionic Compounds

• Metals lose electrons, Nonmetals gain them

• Al = group IIIA metal, so LOSES 3 ELECTRONS

Al Al3+ + 3e-

• S = group VIA nonmetal, so GAINS 8 - 6 = 2 ELECTRONS

S + 2e- S2-

Polyatomic Ions• Contains 2 or more atoms COVALENTLY

bonded, and the complete unit contains a net

charge, e.g. nitrate, NO3-

Polyatomic Ion Examples

NO2-, CO3

2-, SO42-, PO4

3-

CO32- ion

NO2- ion

Ionic Compound = Metal + Nonmetal or a Metal + Polyatomic Ion

• Compound held together electrostatically• Very strong forces hold the lattice together, so

ionic cmpd’s have very high melting points

NaCl crystal latticem.p. = 800 oC

Predicting Formulas of Ionic Compounds

Balance positive and negative charges to produce a neutral molecule

Ca2+ + Cl-

Ca2+ + CO32-

Ca2+ + PO43-

Al3+ + O2-

Oxidation Numbers

• A number assigned to each element in a compound in order to keep track of the electrons during a reaction

Mg2+ = +2Cl- = -1O2- = -2N3- = -3

Rules for Assigning Oxidation Numbers(Chap. 5, p. 207)

Rules higher up take precedence over lower rules

1 The O.N. for an atom in its pure, uncombined state = 0.

2 The sum of the O.N.’s for a neutral molecule = 0. For a polyatomic ion, the sum = charge.

3 Group IA = +1Group IIA = +2

4 H = +1 UNLESS combined with IA or IIA, then = -1

5 Oxygen = -2

6 For binary ionic compounds only -Group VA = -3Group VIA = -2Group VIIA = -1

Rules cont’d

Examples

• P4

• Al2O3

• MnO4-

• NaH• Na2SO3

• Mg3N2

Chemical Nomenclature Examples (More Detail in Lab)

• Ionic Compounds

NaCl sodium chlorideAl2S3 aluminum sulfideFeSO4iron(II) sulfateKClO3 potassium chlorate

• Covalent Compounds

SO2 sulfur dioxideP2O5 diphosphorus pentaoxideN2O dinitrogen oxide

The Mole - The mole is the chemist’s counting unit

Avogadro’s Number (NA) = 6.022 X 1023

pair = 2 Dozen = 12

Gross = 144

Ream = 500

• By definition, 12C = 12.000 amu

• How many particles does it take to have 12.000 grams of 12C ?

• NA = 6.022 X 1023 (as determined by experiment)

Where Does Avogadro’s Number Come From?

Significance of the Mole

Mass in amu’s

Mass in grams/mole

NA of carbon atoms weighs

NA of iron atoms weighs

Molar Mass - the mass in grams of one mole of any element

• Molar mass of sodium (Na) = mass of 1 mol of Na atoms= 22.99 g/mol= mass of 6.022 X 1023 Na atoms

• Molar mass of lead (Pb) = mass of 1 mol of Pb atoms= 207.2 g/mol= mass of 6.022 X 1023 Pb atoms

Mass Moles Conversion

Moles to Mass

moles • grams = grams 1 mole

Molar mass

Mass to Moles

grams • 1 mole = moles grams

1 / Molar mass

Example 3.6 - Mass to Moles

How many moles are represented by 125 g of silicon, an element used in semiconductors?

Example 3.7 - Moles to Mass

What mass, in grams, is equivalent to 2.50 mol of lead (Pb) ?

Mole Calculation Using DensityThe graduated cylinder in the photograph contains 25.0 cm3 of Hg. If the density of Hg = 13.534 g/cm3 at 25 oC, how many moles of Hg are in the cylinder? How many atoms of Hg are there?

Molar Mass of a CompoundSum up the molar masses of each

atom in the compound

HC2H3O2

Example 3.9 - Molar Mass & Moles

You have 16.5 g of the common compound oxalic acid, H2C2O4. Calculate -

1. The number of moles2. The number of molecules3. The number of C atoms4. The mass of one molecule

Other Fun Stuff1 molecule contains - 2 carbon atoms 1 oxygen atom 6 hydrogen atoms

1 mole contains - 2 moles of carbon atoms 1 mole of oxygen atoms 6 moles hydrogen atoms

46.07 g contains - 2(12.01) = 24.02 g of carbon 1(16.00) = 16.00 g of oxygen 6(1.008) = 6.05 g of hydrogen

C2H5OHMW = 46.07

Conversion factors for C2H5OH -

• 2(12.01) g C/ 46.07 g C2H5OH

OR

24.02 g C/ 46.07 g C2H5OH

• 6 moles H/ mole C2H5OH

• 1 mole oxygen/ 2 moles C

More Problems -How many grams of Na are there in 200. g of Na2CO3 ?

How many moles of oxygen are there in 25.0 mol of SO2 ?

More Problems -How many aluminum atoms are there in 150. g of Al2O3 ?

How many oxygen atoms are there in 500. mL of a 30.0 % solution of H2SO4 with a density of 1.250 g/cm3 ? (MW = 98.1)

Percent Composition from a Known Formula

NH3 MW = 17.03 g/mol

% N =

% H =

Empirical & Molecular Formulas

Empirical = simplest ratio of atoms in the molecules

Molecular = actual ratio

Calculating Empirical FormulasFormulas of unknown compounds are determined from the percent composition of each element by mass.

Assume 100 g and divide by atomic weight

Divide by fewest number of moles

Calculating Molecular FormulasThe molecular weight must be known. It is obtained from a separate experiment

Benzene empirical formula = CHformula weight = 12.01 + 1.008 = 13.018

If the MW = 78.11, then what is the molecular formula?

Example 3.10Eugenol is the active component of oil of cloves. It has a MW of 164.2 g/mol and is 73.14 %C and 7.37 %H; the remainder is oxygen. What are the empirical and molecular formulas?

Another example -Vanillin is a common flavoring agent. It has a molar mass of 152 g/mol and is 63.15 %C and 5.30 %H; the rest is oxygen. What are the empirical and molecular formulas?