Chemical formulas (chemical university)

1.4 CHEMICAL FORMULAS

Formulas Describe Compounds
• •

A compound is a distinct substance that is composed of atoms of two or more elements Describe the compound by describing the number and type of each atom in the simplest unit of the compound
–

molecules or ions

• •

Each element is represented by its letter symbol The number of atoms of each element is written to the right of the element as a subscript
–

if there is only one atom, the 1 subscript is not written if more than one

•

Polyatomic ions are placed in parentheses
–

Tro: Chemistry: A Molecular Approach, 2/e

2

Representing Compounds with Chemical Formula
• •

Compounds are generally represented with a chemical formula The amount of information about the structure of the compound varies with the type of formula
–

all formula and models convey a limited amount of information – none are perfect representations

•

All chemical formulas tell what elements are in the compound
–

use the letter symbol of the element

Tro: Chemistry: A Molecular Approach, 2/e

3

Types of Formula (1): Empirical Formula
•

An empirical formula gives the relative number of atoms of each element in a compound
–

–

it does not describe how many atoms, the order of attachment, or the shape the formulas for ionic compounds are empirical

The empirical formula for the ionic compound fluorspar is CaCl2. This means that there is 1 Ca2+ ion for every 2 Cl− ions in the compound. The empirical formula for the molecular compound oxalic acid is CHO2. This means that there is 1 C atom and 1 H atom for every 2 O atoms in the molecule. The actual molecular formula is C2H2O4.
Tro: Chemistry: A Molecular Approach, 2/e

4

Types of Formula (2): Molecular Formula
•

A molecular formula gives the actual number of atoms of each element in a molecule of a compound
–

it does not describe the order of attachment, or the shape
The molecular formula is C2H2O4. This does not tell you that the carbon atoms are attached together in the center of the molecule, and that each is attached to two oxygen atoms.

Tro: Chemistry: A Molecular Approach, 2/e

5

Types of Formula (3): Structural Formula
•

A structural formula uses lines to represent covalent bonds and shows how atoms in a molecule are connected or bonded to each other
–

–

it does not directly describe the 3-dimensional shape, but an experienced chemist can make a good guess at it each line describes the number of electrons shared by the bonded atoms
• • •

single line = two shared electrons, a single covalent bond double line = four shared electrons, a double covalent bond triple line = six shared electrons, a triple covalent bond

Tro: Chemistry: A Molecular Approach, 2/e

6

Covalent Bonds
•

• •

A chemical bond in which 2 atoms share a single of electron to form one bond. Example: H and H Two nonmetal atoms form a covalent bond because they have less energy after they bonded. H + H H : H = HH = H2

hydrogen molecule
Material from

Double Covalent Bond
• •

2 pairs of electrons are shared between 2 atoms. Example: O2

 O +  double bond

 O  

 O::O 

 

Material from

Triple Covalent Bond
• •

3 pairs of electrons are shared between 2 atoms. Example: N2

 N  +

 N   triple bond

 N:::N



Material from

Structural Formula of Oxalic Acid
O H O C O C O H

Tro: Chemistry: A Molecular Approach, 2/e

10

Tro: Chemistry: A Molecular Approach, 2/e

11

Practice — Find the empirical formula for each of the following
The ionic compound that has two aluminum ions for every three oxide ions arabinose, C5H10O5 pyrimidine
H H C C N C N H

Al2O3

CH2O

C2H2N

ethylene glycolC H

CH3O

Tro: Chemistry: A Molecular Approach, 2/e

12

Compounds that Contain Ions
•

•

Compound must have no total charge, therefore we must balance the numbers of cations and anions in a compound to get zero charge If Na+ is combined with S2−, you will need two Na+ ions for every S2− ion to balance the charges, therefore the formula must be Na2S

Tro: Chemistry: A Molecular Approach, 2/e

13

Ex. 3.4: Write a formula for an ionic compound between calcium and oxygen
1. Write the symbol for the metal cation and its charge followed by the symbol for the nonmetal anion and its charge. Obtain charges from the element’s group number on the periodic table. 2. Adjust the subscript on each cation and anion to balance the overall charge.

Ca2+ O2−

CaO

3. Check that the sum of the charges of cations: +2 the cations equals the sum of the anions: −2 charges of the anions. The charges cancel.
Tro: Chemistry: A Molecular Approach, 2/e

14

Writing the Formula of an Ionic Compound

1. Write the symbol for the metal cation and its charge. 2. Write the symbol for the nonmetal anion and its 3. 4. 5.
charge. Charge (without sign) becomes subscript for other ion. Reduce subscripts to smallest whole number ratio. Check that the sum of the charges of the cations cancels the sum of the anions.

Tro: Chemistry: A Molecular Approach, 2/e

15

Ex. 3.3: Write the formula of a compound formed from aluminum and oxide ions
1.

2.

3.

4.

5.

Write the symbol for the metal cation and its charge Write the symbol for the nonmetal anion and its charge Charge (without sign) becomes subscript for other ion Reduce subscripts to smallest whole number ratio Check that the total charge of the cations cancels the total charge of the anions
16

Al3+ column 3A

O2− column 6A Al+3 O2−

Al2O3

Al = (2)∙(+3) = +6 O = (3)∙(−2) = −6

Tro: Chemistry: A Molecular Approach, 2/e

Example: Writing the chemical formula for a binary ionic compound containing variable charge metal manganese(IV) sulfide
1. 2. 3. Write the symbol for the cation and its charge Write the symbol for the anion and its charge Charge (without sign) becomes subscript for other ion Reduce subscripts to smallest whole number ratio Check that the total charge of the cations cancels the total charge of the anions
Mn4+ S2-

Mn4+ S2−

Mn2S4

4. 5.
17

MnS2

Mn = (1)∙(4+) = +4 S = (2)∙(2−) = −4

Tro: Chemistry: A Molecular Approach, 2/e

Practice — What are the formulas for compounds made from the following ions?
•

Potassium ion with a nitride ion Calcium ion with a bromide ion Aluminum ion with a sulfide ion

•

•

Tro: Chemistry: A Molecular Approach, 2/e

18

Practice — What are the formulas for compounds made from the following ions?
•

K+ with N3− Ca2+ with Br− Al3+ with S2−

K3N CaBr2 Al2S3

•

•

Tro: Chemistry: A Molecular Approach, 2/e

19

Practice — What are the formulas for compounds made from the following ions?
copper(II) ion with a nitride ion

Cu2+ with iron(III) ion with a bromide ion N3−

Cu3N2

Fe3+ with Br−

FeBr3

Tro: Chemistry: A Molecular Approach, 2/e

20

Example – Writing the formula for ionic compounds containing polyatomic ion Iron(III) phosphate
1. 2. 3. Write the symbol for the cation and its charge Write the symbol for the anion and its charge Charge (without sign) becomes subscript for other ion Reduce subscripts to smallest whole number ratio Check that the total charge of the cations cancels the total charge of the anions
Fe3+ PO43−

Fe3+ PO43−

Fe3(PO4)3

4. 5.
21

FePO4

Fe = (1)∙(3+) = +3 PO4 = (1)∙(3−) = −3

Tro: Chemistry: A Molecular Approach, 2/e

Practice — What are the formulas for compounds made from the following ions?
aluminum ion with a sulfate ion Al3+ with SO42− Al2(SO4)3 chromium(II) with hydrogen carbonate

Cr2+ with HCO3− Cr(HCO3)2

Tro: Chemistry: A Molecular Approach, 2/e

22

1.5 NAMING OF COMPOUNDS

• Consists of cation and anion. • Some have one or more nicknames that are only
learned by experience.
ü ü

Rules for naming Ionic Compounds

• Write systematic name by simply naming the ions
if cation is:
Ø Ø Ø
ü

NaCl = table salt, NaHCO3 = baking soda

metal with invariant charge = metal name metal with variable charge = metal name(charge) polyatomic ion = name of polyatomic ion nonmetal = stem of nonmetal name + ide polyatomic ion = name of polyatomic ion

if anion is:
Ø Ø

Tro: Chemistry: A Molecular Approach, 2/e

24

Naming Metal Cations
• •

Metals with variable Charges Metals with invariant charge ü metals whose ions can metals whose ions can only have one possible charge have more than one • possible charge Al3+, Groups 1A1+ & 2A2+, Ag1+, Zn2+, Sc3+ ü determine charge by cation name = metal name charge on anion and cation ü name = metal name with Roman numeral charge in parentheses
– –

Tro: Chemistry: A Molecular Approach, 2/e

25

Naming Monatomic Nonmetal Anion
• Determine the charge from position on the •
Periodic Table To name anion, change ending on the element name to –ide

Tro: Chemistry: A Molecular Approach, 2/e

26

Naming Binary Ionic Compounds for Metals with Invariant Charge

• Contain metal cation + nonmetal anion • Metal listed first in formula and name
1. 2. 3. name metal cation first, name nonmetal anion second cation name is the metal name nonmetal anion named by changing the ending on the nonmetal name to -ide

Tro: Chemistry: A Molecular Approach, 2/e

27

with Invariant Charge Metal CsF
1. Identify cation and anion:
Cs = Cs+ because it is Group 1A F = F− because it is Group 7A

Example: Naming Binary Ionic

2. 3. 4.

Name the cation:
Cs+ = cesium

Name the anion:
F− = fluoride

Write the cation name first, then the anion name:
cesium fluoride

Tro: Chemistry: A Molecular Approach, 2/e

28

Naming Binary Ionic Compounds for Metals with Variable Charge

• Contain metal cation + nonmetal anion • Metal listed first in formula and name
1. 2. 3.

name metal cation first, name nonmetal anion second metal cation name is the metal name followed by a Roman numeral in parentheses to indicate its charge nonmetal anion named by changing the ending on the nonmetal name to -ide
determine charge from anion charge common ions Table 3.4

ü ü

Tro: Chemistry: A Molecular Approach, 2/e

Example: Naming binary ionic with variable charge metal
1. 2. 3. 4. Identify the cation and anion Name the cation Name the anion

CuF2

F = F− because it is Group 7 Cu = Cu2+ to balance the two (−) charges from 2 F− Cu2+ = copper(II) F− = fluoride

copper(II) fluoride

Write the cation name first, then the anion name

Tro: Chemistry: A Molecular Approach, 2/e

30

Name the following compounds
1. 2. 3. TiCl4 PbBr2 Fe2S3 titanium(IV) chloride lead(II) bromide iron(III) sulfide

Tro: Chemistry: A Molecular Approach, 2/e

31

Compounds Containing Polyatomic Ions
• Polyatomic ions are single ions that contain • • •
more than one atom Often identified by parentheses around ion in formula Name and charge of polyatomic ion do not change Name any ionic compound by naming cation first and then anion

Tro: Chemistry: A Molecular Approach, 2/e

32

Some Common Polyatomic Ions

Tro: Chemistry: A Molecular Approach, 2/e

33

Patterns for Polyatomic Ions
1. Elements in the same column form similar polyatomic ions

same number of O’s and same charge ClO3− = chlorate ∴ BrO3− = bromate

2.

CO32− = carbonate ∴ HCO3− = hydrogen carbonate
34

If the polyatomic ion starts with H, add hydrogen- prefix before name and add 1 to the charge

Tro: Chemistry: A Molecular Approach, 2/e

Periodic Pattern of Polyatomic Ions (-ate groups)

Tro: Chemistry: A Molecular Approach, 2/e

35

Patterns for Polyatomic Ions
• -ate ion
ü ü

• -ate ion + 1 O  same charge, per- prefix • -ate ion – 1 O  same charge, -ite suffix
ü

chlorate = ClO3−

perchlorate = ClO4− chlorite = ClO2−

• -ate ion – 2 O  same charge, hypo- prefix, -ite
suffix
ü

hypochlorite = ClO−

Tro: Chemistry: A Molecular Approach, 2/e

36

Example: Naming ionic compounds containing a polyatomic ion
1. 2. 3. 4.
Na = Na+ because in Group 1A SO4 = SO42− a polyatomic ion Na+ = sodium, metal with invariant charge SO42− = sulfate sodium sulfate
Tro: Chemistry: A Molecular Approach, 2/e

Na2SO4

Identify the ions

Name the cation Name the anion

Write the name of the cation followed by the name of the anion

37

Example: Naming ionic compounds containing a polyatomic ion
1. 2. 3. 4. Identify the ions

Fe(NO3)3

NO3 = NO3− a polyatomic ion Fe = Fe3+ to balance the charge of the 3 NO3− Fe3+ = iron(III), metal with variable charge NO3− = nitrate iron(III) nitrate

Name the cation Name the anion

Write the name of the cation followed by the name of the anion

Tro: Chemistry: A Molecular Approach, 2/e

38

Name the Following Compounds
1. 2. 3. NH4Cl Ca(C2H3O2)2 Cu(NO3)2 ammonium chloride calcium acetate copper(II) nitrate

Tro: Chemistry: A Molecular Approach, 2/e

39

Hydrates
• Hydrates are ionic compounds containing a specific number of • • In formula, attached waters follow ∙ ü CoCl2∙6H2O • In name attached waters indicated by prefix+hydrate after name
of ionic compound
ü ü

waters for each formula unit Water of hydration often “driven off” by heating

CoCl2∙6H2O = cobalt(II) chloride hexahydrate CaSO4∙½H2O = calcium sulfate hemihydrate

Tro: Chemistry: A Molecular Approach, 2/e

40

Cobalt(II) chloride hexahydrate

Tro: Chemistry: A Molecular Approach, 2/e

41

Practice
What is the formula of magnesium sulfate heptahydrate? What is the name of NiCl2•6H2O?
Mg2+ + SO42− MgSO4 MgSO47H2O

Cl−  Ni2+ nickel(II) chloride nickel(II) chloride hexahydrate

Tro: Chemistry: A Molecular Approach, 2/e

42

Writing Names of Binary Molecular Compounds of Two Nonmetals
1. 2. 3.
a) b)

Write name of first element in formula

Writes name the second element in the formula with an -ide suffix
a)

element furthest left and down on the Periodic Table use the full name of the element

Use a prefix in front of each name to indicate the number of atoms
a) Never use the prefix mono- on the first element

as if it were an anion, however, remember these compounds do not contain ions!

Tro: Chemistry: A Molecular Approach, 2/e

43

Subscript – Prefixes
• 1 = monoü

• • • •

not used on first nonmetal

• 6 = hexa• • • •
7 = hepta8 = octa9 = nona10 = deca-

2 = di3 = tri4 = tetra5 = penta-

• Drop last “a” if name begins with a vowel

Tro: Chemistry: A Molecular Approach, 2/e

44

Example: Naming a binary molecular compound BF3
1. 2. 3. 4.
boron

Name the first element Name the second element with an –ide

fluorine  fluoride

monoboron, trifluoride
a)

Add a prefix to each name to indicate the subscript Write the first element with prefix, then the second element with prefix
drop prefix mono from first element

boron trifluoride
Tro: Chemistry: A Molecular Approach, 2/e

45

Name the Following
NO2 PCl5 I2F7 nitrogen dioxide phosphorus pentachloride diiodine heptafluoride

Tro: Chemistry: A Molecular Approach, 2/e

46

Write Formulas for the Following
dinitrogen tetroxide sulfur hexafluoride diarsenic trisulfide N2O 4 SF6 As2S 3

Tro: Chemistry: A Molecular Approach, 2/e

47

Name the Following
H2S HClO3 HNO2 hydrosulfuric acid chloric acid nitrous acid

Tro: Chemistry: A Molecular Approach, 2/e

48

Example: Binary Acids hydrosulfuric acid
1. 2. 3. 4. 5. Write the symbol for the cation and its charge Write the symbol for the anion and its charge Charge (without sign) becomes subscript for other ion Add (aq) to indicate dissolved in water Check that the total charge of the cations cancels the total charge of the anions
49 H+ S2− H+ S2− in all acids the cation is H+ hydro means binary H2S

H2S(aq) H = (2)∙(1+) = +2 S = (1)∙(2−) = −2

Tro: Chemistry: A Molecular Approach, 2/e

Example: Oxyacids carbonic acid
1. 2. 3. 4. 5. Write the symbol for the H+ cation and its charge no hydro means CO32− polyatomic ion Write the symbol for the anion and its charge -ic means -ate ion Charge (without sign) H+ CO32− H2CO3 becomes subscript for other ion Add (aq) to indicate dissolved H2CO3(aq) in water Check that the total charge of H = (2)∙(1+) = +2 CO3 = (1)∙(2−) = −2 the cations cancels the total charge of the anions
50 in all acids the cation is H+

Tro: Chemistry: A Molecular Approach, 2/e

Example: Oxyacids sulfurous acid
1. 2. 3. Write the symbol for the cation and its charge Write the symbol for the anion and its charge Charge (without sign) becomes subscript for other ion Add (aq) to indicate dissolved in water Check that the total charge of the cations cancels the total charge of the anions
H+ SO32− in all acids the cation is H+ no hydro means polyatomic ion -ous means -ite ion H+ SO32− H2SO3(aq) H = (2)∙(1+) = +2 SO3 = (1)∙(2−) = −2 H2SO3

4. 5.

Tro: Chemistry: A Molecular Approach, 2/e

51

1.6 BALANCING CHEMICAL EQUATIONS

Chemical Equations
• •

Shorthand way of describing a reaction Provides information about the reaction
– – –

–

formulas of reactants and products states of reactants and products relative numbers of reactant and product molecules that are required can be used to determine weights of reactants used and products that can be made

Tro: Chemistry: A Molecular Approach, 2/e

53

Combustion of Methane, Balanced
•

To show the reaction obeys the Law of Conservation of Mass the equation must be balanced
–

we adjust the numbers of molecules so there are equal numbers of atoms of each element on both sides of the arrow

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
O O + O O O O C

O

H H

C

H H

+

+

H + O H

H

H

1C + 4H + 4O
Tro: Chemistry: A Molecular Approach, 2/e

1C + 4H + 4O 54

Chemical Equations
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
•

•

•

CH4 and O2 are the reactants, and CO2 and H2O are the products The (g) after the formulas tells us the state of the chemical The number in front of each substance tells us the numbers of those molecules in the reaction
–

called the coefficients

Tro: Chemistry: A Molecular Approach, 2/e

55

Chemical Equations
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
•

This equation is balanced, meaning that there are equal numbers of atoms of each element on the reactant and product sides
–

to obtain the number of atoms of an element, multiply the subscript by the coefficient

1C1 4H4 4O2+2

Tro: Chemistry: A Molecular Approach, 2/e

56

Symbols Used in Equations
•

Symbols used to indicate state after chemical
– –

(g) = gas; (l) = liquid; (s) = solid (aq) = aqueous = dissolved in water

Tro: Chemistry: A Molecular Approach, 2/e

57

Example 3.22: Write a balanced equation for the combustion of butane, C4H10
Write a skeletal equation Balance atoms in complex substances first

C4H10(l) + O2(g)  CO2(g) + H2O(g) 4C1x4 C4H10(l) + O2(g)  4 CO2(g) + H2O(g) 10  H  2 x 5 C4H10(l) + O2(g)  4 CO2(g) + 5 H2O(g) 13/2 x 2  O  13 C4H10(l) + 13/2 O2(g)  4 CO2(g) + 5 H2O(g)
{C4H10(l) + 13/2 O2(g)  4 CO2(g) + 5 H2O(g)}x 2 2 C4H10(l) + 13 O2(g)  8 CO2(g) + 10 H2O(g)

Balance free elements by adjusting coefficient in front of free element If fractional coefficients, multiply thru by denominator Check
Tro: Chemistry: A Molecular Approach, 2/e

8  C  8; 20  H  20; 26  O  26

58

Practice
when aluminum metal reacts with air, it produces a white, powdery compound, aluminum oxide
reacting with air means reacting with O2 aluminum(s) + oxygen(g) → aluminum oxide(s)
–

Al(s) + O2(g) → Al2O3(s)
4 Al(s) + 3 O2(g)  2 Al2O3(s)

Tro: Chemistry: A Molecular Approach, 2/e

59

Practice
Acetic acid reacts with the metal aluminum to make aqueous aluminum acetate and gaseous hydrogen

acids are always aqueous – metals are solid except for mercury Al(s) + HC2H3O2(aq) → Al(C2H3O2)3(aq) + H2(g)
–

2 Al(s) + 6 HC2H3O2(aq) → 2 Al(C2H3O2)3(aq) + 3 H2(g)

Tro: Chemistry: A Molecular Approach, 2/e

60