Unit 4 Chapter 7 Writing Formulas & Naming Compounds.

Unit 4Chapter 7

Writing Formulas & Naming Compounds

CHAPTER 7Chemical Formulas and Chemical Compounds

Chemical Formulas

• A chemical formula indicates the number of each kind of atom in a chemical compound.

• when there is no subscript next to an atom, the subscript is understood to be 1.

Examples:

octane — C8H18

aluminum sulfate — Al2(SO4)3

Chapter 7 – Section 1: Chemical Names and Formulas

there are 8 carbon atoms in the molecule.

there are 18 hydrogen atoms in the molecule.

there are 2 aluminum atoms in the formula unit.

Parentheses surround the polyatomic ion to identify it as a group. There are 3 SO4

- groups.

Chemical FormulasSample Problem

Count the number of atoms in the following chemical formulas:

Solution:a.Ca(OH)2

b.KClO3

c.NH4OH

d.Fe2(CrO4)3


1 Calcium, 2 Oxygens,

and 2 Hydrogens

1 Potassium, 1 Chlorine, and 3 Oxygens

1 Nitrogen, 5 Hydrogens, and 1 Oxygen

2 Irons, 3 Chromiums,

and 12 Oxygens

Cations

• Atoms with 1, 2, or 3 valenceelectrons tend to lose them to form positive ions, which are called a cations.


Anions

• Atoms with 5, 6, or 7 valence electrons tend to gain more in order to have an octet (8 electrons ) in their outer shell.

• Gaining extra electrons forms negative ions, called anions.


Monoatomic Ions

• Monoatomic Ions are ions formed from a single atom.

• Some main-group elements tend to form covalent bonds instead of ions (ex. C and Si.)


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

Nomenclature Flowchart

Compounds

Ionic Molecular

Binary PolyatomicIons

PrefixSystem

Acids

StockSystem

Simple (Main Group Elements)

Stock System (d-Block Elements)

BinaryAcids

Oxyacids


Compounds

Ionic

Binary


Writing Binary Ionic Compounds• Binary Compounds are composed of two elements.• Rules for writing binary ionic compounds:

1. Write the symbols for the ions, and their charges.Note: The cation is always written first.

2. Cross over the charges (use the absolute value of each ion’s charge as the subscript for the other ion.)

3. Simplify the numbers and remove the 1’s.Example: aluminum oxide

The correct formula for aluminum oxide is


Al3+

Al2O3

3O2–2

Naming Binary Ionic Compounds

• The name of the cation is given first, followed by the name of the anion.

• Monatomic cations are identified simply by the element’s name.

• For monatomic anions, the ending of the element’s name is dropped, and the ending -ide is added.

Examples:


Al2O3 KF

cation anion aluminum potassium

oxide fluoride

Binary Ionic CompoundsSample Problem

Write chemical formulas for :

a.Magnesium Iodide

b.Calcium Oxide

Write the correct names for:

a.Li2S

b.ZnCl2


Solution:

Hint: Always divide subscripts by their largest common factor .

Lithium

Zinc

Mg2+2I –

1

Ca2+2O 2–

2

MgI2

CaO

Lithium Sulfide

Zinc Chloride


Compounds

Ionic Molecular


PrefixSystem

Acids

StockSystem



BinaryAcids

Oxyacids


Compounds

Ionic

Binary


The Stock System• Most d-block elements (transition

metals) can form 2 or more ions with different charges.

• To name ions of these elements, scientists use the Stock system, designed by Alfred Stock in 1919.

• The system uses Roman numerals to indicate an ion’s charge.Example: Fe2+

Fe3+


iron(II)iron(III)

Visual Concept

Stock System NamingSample Problem A

Write the formula and give the name for the compound formed by the ions Cr3+ and F–.Solution:

Write the ions side by side, cation first.

Cross over the charges to give subscripts.

Chromium forms more than one ion, so its name must include the charge as a Roman numeral.


Cr3+3F –

1

CrF3

Chromium (III) Fluoride

2

I –

O 2–

Stock System NamingSample Problem B


a.Tin (IV) Iodide

b.Iron (III) Oxide


a.VF3

b.CuO


Solution:

Hint: “Uncross” subscripts to get the charges of the ions.

Vanadium (III)Copper (II)

Sn4+41

Fe3+32

SnI4

Fe2O3

Fluoride

Oxide

Be sure to verify the charge of the anion.

V3+

F3-

Cu+

O -2


Compounds

Ionic Molecular


PrefixSystem

Acids

StockSystem



BinaryAcids

Oxyacids


Compounds

Ionic

PolyatomicIons

Polyatomic Ions

• A polyatomic ion is a charged group of covalently bonded atoms.

• Common endings are -ate or -ite, but there are exceptions.

• For more than 1 polyatomic ion, use parentheses with the subscript on the outside.

Example: Al2(SO4)3


There are 3 sulfate ions in this compound

Common Polyatomic Ions

Visual Concept

(NO3)2


a.Calcium Hydroxide

b.Tin (IV) Sulfate


a.(NH4)3 PO4

b.Cu(NO3)2

Sn4+ SO4 2–

OH –

Polyatomic IonsSample Problem


Solution:

Hint: Remember to divide subscripts by their largest common factor .

Ammonium

Copper(II)

Ca2+21

42

Ca(OH)2

Sn(SO4)2

Phosphate

NitrateHint: “Uncross” subscripts to get the charges of the ions.

Cu2+ -


Compounds

Ionic Molecular


PrefixSystem

Acids

StockSystem



BinaryAcids

Oxyacids


Compounds

Molecular

PrefixSystem

The Prefix System

• Molecular compounds are composedof covalently-bonded molecules.

• The old prefix system is still used for molecular compounds.

• Name the prefix, then the element. Anions end in -ide.

• The prefix mono- usually isn’t used for cations.Examples: P4O10

CO


tetraphosphorus decoxidecarbon monoxide

The Prefix SystemSample Problem


a.dinitrogen trioxide

b.carbon tetrabromide


a.As2S3

b.PCl5


Solution:

diarsenic

phosphorus

N2O3

CBr4

trisulfide

pentachloride

StockSystem


Compounds

Ionic Molecular


PrefixSystem

Acids



BinaryAcids

Oxyacids


Compounds

Acids

BinaryAcids

Oxyacids

Acids

• An acid is a certain typeof molecular compound.All acids start with H (e.g. HCl, H2SO4).

• Acids can be divided into two categories:1. Binary acids are acids that consist of H and a

non-metal. (e.g. HCl.)2. Oxyacids are acids that contain H and a

polyatomic ion that includes O (e.g. H2SO4.)


Binary Acids

• General rules for naming a binary acid:1. Begin with the prefix hydro-.2. Name the anion, but change the ending

to –ic.3. Add acid to the name.Examples:HCl, hydrochloric acid.HBr, hydrobromic acid.H2S, hydrosulfuric acid.


Oxyacids

• General rules for naming an oxyacid :1. Name the polyatomic ion.2. Replace -ate with -ic or -ite with -ous3. Add acid to the name.Examples:H2SO4, sulfuric acid.

H2SO3, sulfurous acid.

HNO3, nitric acid.

HNO2, nitrous acid.


sulfate

Naming AcidsSample Problem

Write the correct name for each of the following:

a.HF

b.HNO2

c.H2S

d.H2SO4

e.H3PO4

Type of Acid:binary acid

oxyacid

Name:hydro nitrite


fluorineic acid

binary acid hydro

sulfuric acid

ous acid

oxyacid uric acidoxyacid phosphateoric acid

StockSystem


Compounds

Ionic Molecular


PrefixSystem

Acids

StockSystem



BinaryAcids

Oxyacids


Compounds

Molecular

Oxidation Numbers

• In order to indicate the general distribution of electrons among covalently bonded atoms, oxidation numbers are assigned to the atoms.

• Unlike ionic charges, oxidation #’s do not represent actual electronsgained or lost.

• Many elements can have differentoxidation #’s depending on whatthey’re combined with.

Chapter 7 – Section 2: Oxidation Numbers

The 9 Oxidation #’s of Nitrogen

Rules for Assigning Oxidation Numbers

1. The sum of the oxidation numbers for a neutral compound equals zero.

2. The sum of the oxidation numbers for an ion equals the charge of the ion.

3. Atoms in a pure element are zero.

4. The most electronegative element in a compound is assigned a negative number equal to the charge it would have as an anion.

5. Hydrogen is always either +1 or -1.


Assigning Oxidation NumbersSample Problem

Assign oxidation numbers to each atom in the following compounds or ions:

a.UF6

b.H2SO4

c. ClO3-

Solution:F = -1, U = +6

O = -2, H = +1, S = +6


O = -2, Cl = +5

1- -1 x 6 = -6 +

06

6+

-2 x 4 = -8

+ 06

2-+1 x 2 = +2

1+ 6+

-2 x 3 = -6 +

-15

2-5+

Using Oxidation Numbers in Naming

• The Stock System is actually based on oxidation numbers.

• It can be used as an alternative to the prefix system for naming molecular compounds.


Prefix system Stock system

PCl3 phosphorus trichloride phosphorus(III) chloride

PCl5 phosphorus pentachloride phosphorus(V) chloride

N2O dinitrogen monoxide nitrogen(I) oxide

NO nitrogen monoxide nitrogen(II) oxide

Mo2O3 dimolybdenum trioxide molybdenum(III) oxide

Using Oxidation Numbers in NamingSample Problem

Write the correct prefix system name and the correct Stock system name for each of the following:

a.As2S3

b.SO3

Prefix System:diarsenic trisulfide

sulfur trioxide


Stock System:2-

-2 x 3 = -6 + 0

+3 x 2 = +6

3+arsenic (III) sulfide

-2 x 3 = -6 + 0

+6

2-6+sulfur (VI) oxide

Formula Masses

• The formula mass of any compound is the sum of the masses of all the atoms in its formula.example: Formula mass of water, H2O:

H2 = 1.0 amu x 2 = 2.0 amu. O = + 16.0 amu. 18.0 amu

• A compound’s molar mass is numerically equal to its formula mass. Only the units are different. (Ex: Molar mass of H2O = 18.0 g.)

Chapter 7 – Section 3: Using Chemical Formulas

Molar MassesSample Problem

Determine the molar mass of each of the following compounds:

a.Al2S3

a.Ba(OH)2

Solution:Al2 = 27.0 x 2 = 54.0 g


S3 = 32.1 x 3 = + 96.3 g

150.3 g

Ba = 137.3 g O2 = 16.0 x 2 = 32.0 g H2 = 1.0 x 2 = + 2.0 g

171.3 g

Molar Mass as a Conversion Factor

• The molar mass of a compound can be used as a conversion factor to convert between moles and grams for a given substance.

Example:• What is the mass of 2.5 moles of H2O?

molar mass of H2O = 18.0 g/mol


given conversion factor

2.5 mol H2O g H2Omol H2O

45 g H2O=x118.0

2

Molar Mass as a Conversion FactorSample Problem

Calculate the moles in 1170 g of copper (II) nitrate.Solution:

Given Conversion factor

1170 g Cu(NO3)2mol Cu(NO3)2

g Cu(NO3)2

6.24 mol Cu(NO3)2

=x187.5

1


Cu = 63.5 g

O6 = 16.0 x 6 = + 96.0 g

187.5 g

(NO3)Cu2+

1. Determine the correct formula:2. Calculate the molar mass:

-Cu(NO3)2

N2 = 14.0 x 2 = 28.0 g 3. Convert from g to mol:

Percent Composition

• The percentage by mass of each elementin a compound is known as the percent composition of the compound.


% of element = mass of element in compoundmolar mass of compound

x 100

Visual Concept

Percent CompositionSample Problem

Find the percentage composition of copper(I) sulfide, Cu2S.

1.Find the molar mass of Cu2S:

2.Find the percentage by mass of each element:

Solution: Cu2 = 63.5 x 2 = 127.0 g


S = + 32.1 g

159.1 g

% Cu = 127.0 g159.1 g x 100 = 79.8% Cu

% S = 32.1 g159.1 g x 100 = 20.2% S

Unit 4 Chapter 7 Writing Formulas & Naming Compounds.

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Transcript of Unit 4 Chapter 7 Writing Formulas & Naming Compounds.