Chemical Formulas and Chemical Compounds

Chemical Formulas and Chemical Compounds

Chapter 7

Chemistry chapter 7 2

Chemical Formulas

• Combinations of symbols are used to represent compounds of two or more elements.

• Also indicate the ratio of the number of atoms of each type of element in the compound.• H2O – means that there are 2 hydrogen atoms

for every oxygen atom.• No subscript on O – means there is 1


Chemical Formulas

• Show either one molecule or one formula unit


Organic Compounds

• Written differently than other formulas• The shorthand shows how the atoms are

joined, not just the number present.• Example –

• CH3COOH, not C2H4O2


Ions

• Ion – charged atom or group of atoms• Monatomic Ions – single atom• Polyatomic Ions – more than one atom


Monatomic Ions

• Can be anions or cations• Transition elements can form more than one

kind of ion• See table 7-1 on page 205• You must memorize this table.


Naming monatomic ions

• Cations• Element’s name• Roman numerals are used when there are

multiple ions• Anions

• Drop the element name ending• Add -ide


Binary compounds

• Contain two different elements• When we write chemical formula for a

compound, the charges must add up to zero.• Write the positive ion first.


Example

• Write a formula for a compound of tin (II) and Iodine.

• Tin (II) is 2+• Iodine is 1-• We need two iodines to cancel out the

charge on the tin (II).• SnI2


Nomenclature

• Naming system• Works for most compounds


Naming binary compounds

• Write the name of the positive cation first.• Add the name of the negative anion

• AlN – Aluminum nitride• KCl – potassium chloride


The stock system

• Elements with more than one possible charge

• Cu2S – copper (I) sulfide• CuS – copper (II) sulfide• Note – in an older naming system the above

could be written as cuprous sulfide and cupric sulfide


Oxyanions

• Polyatomic ions that contain oxygen• When there are two or more oxyanions

formed from the same two elements, the most common has the ending –ate• The ion with one less oxygen than –ate ends in

–ite• The ion with one less oxygen than –ite adds the

prefix hypo-• The ion with one more oxygen than –ate adds

the prefix per-


Compounds with polyatomic ions

• See table 7-2 on page 210• They are written like binary compounds.

• Except the ending isn’t changed to end in –ide• CuSO4 – copper (II) sulfate• Sn(SO4)2 – tin (IV) sulfate


Discuss

• Practice problems 7-1, 7-2, and 7-3 on pages 207, 209, and 211

• Practice


Polyatomic ions you must memorize

• Ammonium• Acetate• Chlorate• Chlorite• Hydroxide• Hypochlorite• Nitrate• Nitrite

• Perchlorate• Permanganate• Carbonate• Peroxide• Sulfate• Sulfite• Phosphate


Naming binary molecular compounds

• Two systems – one will be covered in section 7-2

• Older system• Prefixes used – see table 7-3 on page 212• CO – carbon monoxide• CO2 – carbon dioxide• SO2 – sulfur dioxide• SO3 sulfur trioxide


Rules

• List the less-electronegative element first.• Only has a prefix if there is more than one.

• The second element• Has a prefix• Root of the element name• -ide ending

• If the word begins with a vowel, drop the o or a at the end of the prefix (monoxide, not monooxide)

• Order: C, P, N, H, S, I, Br, Cl, O, F


Examples

• PF5

• Phosphorus pentafluoride• N2O5

• Dinitrogen pentoxide• OF2

• Oxygen difluoride


Acids

• Have a different naming rules.• Some common ones are listed in table 7-5

on page 214• You should know

• Hydrochloric acid (HCl)• Sulfuric acid (H2SO4)• Acetic acid (CH3COOH) (vinegar)


Salts

• An ionic compound composed of a cation and the anion from an acid

• Sometimes the salt keeps one or more hydrogen atoms from the acid• The prefix bi- or the word hydrogen is added to

the anion name• HCO3

-

• Hydrogen carbonate ion or bicarbonate ion


Discuss

• Sample problem 7-4 on page 213• Practice


Discuss

• www.dhmo.org/facts.html

http://www.dhmo.org/facts.html


Oxidation numbers

• Also called oxidation states• Assigned to atoms in molecules• Indicate the general distribution of electrons

among the bonded atoms• Sort of like ionic charge


Pure elements

• Have oxidation numbers of zero• Single atoms – Na• Molecules of a pure substance

• O2

• P4

• S8


Like charges on ions

• Shared electrons are assumed to belong to the more-electronegative atom

• The more electronegative element gets a number equal to the negative charge it would have as an anion.

• The less electronegative element gets a number equal to the positive charge it would have as a cation.


Fluorine

• Oxidation number of -1• The most electronegative element


Oxygen

• Usually -2• In peroxides, -1

• H2O2

• In compounds with halogens, +2• OF2


Hydrogen

• +1 with more electronegative elements• -1 with metals


Sum of oxidation numbers

• In a neutral compound must be zero• In a polyatomic ion must equal the charge

on the ion


Ion

• Can be assigned an oxidation number equal to the charge on the ion


Example

• Assign oxidation numbers to each atom in the following compound:

• KClO4• O is -2, which gives -8, since there are 4.• The charge on perchlorate is 1-, so Cl must be

+7• K must be +1 to cancel out the 1-

• +1, +7, -2


Example


• SO32-

• O is -2, which gives -6, since there are 3.• The charge on sulfite is 2-, so S must be +4

• +4, -2


You try


• CO2

• O is -2, which gives -4, since there are 2.• The charge is 0, so C must be +4

• +4, -2


You try


• NO3-

• O is -2, which gives -6, since there are 3.• The charge is 1-, so N must be +5

• +5, -2


More oxidation numbers

• See Appendix Table A-15• There is also a pattern on the periodic table• Group 1 is usually +1• Group 2 is usually +2• Group 13 is usually +3• Group 14 is usually +2 or +4• Group 15 is usually -3• Group 16 is usually -2• Group 17 is usually -1


The stock system

• Can be used instead of prefixes for molecular compounds

• Use the oxidation number• SO2

• Sulfur dioxide• Sulfur (IV) oxide

• SO3• Sulfur trioxide• Sulfur (VI) oxide


Discuss

• Name each of the following binary molecular compounds according to the stock system

• CI4

• SO3

• As2S3

• NCl3


Formula mass

• The sum of the average atomic masses of all the atoms in a formula

• For ionic compounds or molecules• Can also be called molecular mass for

molecules


Example

• Find the formula mass of Na2SO3

• 126.05 amu


Example

• Find the formula mass of HClO3

• 84.46 amu


You try

• Find the formula mass of MnO4-

• 118.94 amu


You try

• Find the formula mass of C2H6O• 46.08 amu


Molar Mass

• Chapter 3• The mass in grams of one mole (6.022 x

1023 particles) of a substance• Example: H2O

• The mass of two moles of hydrogen atoms and one mole of oxygen atoms


Example

• Find the molar mass of K2SO4

• 174.27 g/mol


You try

• Find the molar mass of (NH4)2CrO4

• 152.10 g/mol


Formula mass and molar mass

• Numerically equal• Only the units are different


Discuss

• How many moles of atoms of each element are there in one mole of ammonium carbonate, (NH4)2CO3

• 2 mol N, 8 mol H, 1 mol C, 3 mol O• Determine both the formula mass and the

molar mass of ammonium carbonate• 96.11 amu, 96.11 g/mol


Converting with molar mass

• Relate mass in grams to number of moles• Relate mass in grams to number of particles


Example

• What is the mass in grams of 3.04 mol of ammonia vapor, NH3?

• 51.8 g


You try

• What is the mass in grams of 0.257 mol of calcium nitrate, Ca(NO3)2?

• 42.2 g


Example

• How many moles of SO2 are in 3.82 g?• 0.0596 mol


You try

• How many moles of Cl2 are there in 77.1 g?• 1.09 mol


Example

• How many molecules are there in 77.1 g Cl2?

• 6.55 x 1023 molecules


You try

• How many molecules are in 4.15 x 10-3 g of C6H12O6?

• 1.39 x 1019 molecules


Percentage composition

• Percentage by mass of each element in a compound

• Example: gum


Example

• Find the percentage composition of sodium nitrate NaNO3.

• 27.05% Na• 16.48% N• 56.47% O


You try

• Find the percentage composition of silver sulfate, Ag2SO4.

• 69.19% Ag• 10.29% S• 20.53% O


Discuss

• Zinc chloride, ZnCl2 is 52.02% chlorine by mass. What mass of chlorine is contained in 80.3 g of ZnCl2?

• How many moles of Cl is this?• 41.8 g• 1.18 mol


Empirical formula

• The symbols for the elements combined in a compound

• Subscripts show the smallest whole-number mole ratio of the atoms

• Determined from the percent composition of a substance


Empirical formula

• Usually the same as an ionic compound’s formula unit

• Not always the same as the molecular formula• Diborane’s molecular formula is B2H6

• The empirical formula is BH3


Example

• A compound is analyzed and found to contain 36.70% potassium, 33.27% chlorine, and 30.03% oxygen. What is the empirical formula of the compound?

• KClO2


You try

• Determine the empirical formula of the compound that contains 17.15% carbon, 1.44% hydrogen, and 81.41% fluorine.

• CHF3


Example

• A 60.0 g sample of tetraethylead, a gasoline additive, is found to contain 38.43 g lead, 17.83 g carbon, and 3.74 g hydrogen. Find its empirical formula

• PbC8H20


You try

• A 170.00 g sample of an unidentified compound contains 29.84 g sodium, 67.49 g chromium, and 72.67 g oxygen. What is its empirical formula?

• Na2Cr2O7


Discuss

• Find the empirical formula of a compound that contains 53.70% iron and 46.30% sulfur.

• Fe2S3


Molecular formula

• Show how many atoms are in each molecule

• Related to empirical formula

• x is the whole number the subscripts must be multiplied by• It might be 1

formulamolecular formula empirical x


Mass relationship

mass formulamolecular mass formula empirical x


Example

• The empirical formula for trichloroisocyanuric acid is OCNCl. The molar mass of this compound is 232.41 g/mol. What is its molecular formula?

• O3C3N3Cl3


Example

• Determine the molecular formula of a compound with an empirical formula of NH2 and a formula mass of 32.06 amu.

• N2H4


You try

• Determine the molecular formula of the compound with an empirical formula of CH and a formula mass of 78.110 amu.

• C6H6


Example

• If 4.04 g of N combine with 11.46 g of O to produce a compound with a formula mass of 108.0 amu, what is the molecular formula of this compound?

• N2O5


You try

• The molar mass of a compound is 92 g/mol. Analysis of a sample of the compound indicates that it contains 0.606 g N and 1.390 g O. Find its molecular formula.

• N2O4

Date post:	16-Feb-2016
Category:	Documents
Upload:	creola
View:	75 times
Download:	0 times

Chemical Formulas and Chemical Compounds

Documents