Download chapter 7 - chemical formulas and chemical compounds

CHAPTER 7 - CHEMICAL FORMULAS AND CHEMICAL COMPOUNDS
7-1 Chemical Names and Formulas
Calcium carbonate = limestone
Sodium chloride = table salt
Dihydrogen monoxide = water
Chemical formula indicated the relative number of atoms of each kind in a chemical compound
_____________________________________________, the chemical formula reveals the number of atoms of each element
contained in a single molecule or the compound. (Hydrocarbons are molecular compounds composed solely of carbon and
hydrogen).
CH
8
18
8 carbon atoms
in a molecule of
octane
18 hydrogen atoms
in a molecule of
octane
______________________________________is a lattice of positive and negative ions held together by mutual attraction.
Chemical formula represents one formula unit - the simplest ratio of the compound’s positive ions (cations) and its negative
ions (anions).
Al (SO )
2
2 aluminum atoms
4 3
4 oxygen atoms in
sulfate ion
3 sulfate atoms, with a
total of 3 sulfur atoms
and 12 oxygen atoms
____________________- ions formed from a single atom
- naming cations: element’s name
- naming anions: ending of element’s name is dropped and an -ide is added
_____________________________-
compounds composed of two different elements
the total numbers of positive charges and negative charges must be equal
charges are not included in the formula
name cation first followed by anion
Use roman numeral numbers to distinguish the ions formed when there are more than one cation of an element.
They are place immediately after the name and enclosed in parentheses
Polyatomic ions - ____________________________-
polyatomic ions that contain oxygen
most common ion is given the ending -ate
ion with one less oxygen atom ending is ite
an anion with one less oxygen than the -ite anion is given the
prefix hypo- an anion with one more oxygen than the -ate anion is given the
prefix per-
Naming binary molecular compounds - are molecular compounds composed of individual covalently bonded
units for molecules
- use of prefixes:
1) less-electronegative element is given first. Given a prefix only if
it contributes more than one atom to a molecule of the
compound
2) second element is named by combining (a) a prefix indicating
the number of atoms contributed by the element, (b) the root
of the name of the second element, and (c) the ending –ide
(if only contains two elements)
3) the “o” or “a” at the end of a prefix is usually dropped when the
word following the prefix begins with another vowel
______________________________ and _____________________________are usually names second since they are more
electronegative.
The order of nonmetals in binary compound names and formulas is C, P, N, H, S, I, Br, Cl, O, F.
Covalent-network compounds (SiC - silicon carbide, SiO - silicon dioxide, Si N - trisilicon tetranitride) naming is similar
2
3
4
to naming molecular compounds
Acids:
1)
2)
2)
3)
molecular compounds
recognized by their properties in solutions of water
Binary acids - consist of two elements, usually hydrogen and one of the halogens (F, Cl, Br, I)
Oxyacids - contain hydrogen, oxygen, and a third element (usually a nonmetal)
Polyatomic ions are produced by the loss of hydrogen ins from oxyacids:
OXYACIDS
OXYANIONS
sulfuric acid
H SO
sulfate
SO
2
4
nitric acid
HNO
phosphoric acid
H PO
3
4
3
4
nitrate
NO
phosphate
PO
3
4
_____________________ - an ionic compound composed of a cation and the anion from an acid
- NaCl - anion from hydrochloric acid
- CaSO - anion from sulfuric acid
4
- salts that contain anions in which one or more hydrogen atoms from the acid are retained
- named by adding hydrogen or bi- to the anion name
- carbonic acid, H CO
2
3
- HCO3, hydrogen carbonate ion, bicarbonate ion
7-2 Oxidation Numbers
_______________________________ or __________________________________- numbers assigned to the atoms
composing the compound or ion due
to the distribution of electron among
the bonded atoms in a molecular
compound or a polyatomic ion
- useful in naming compounds, in
writing formulas, and in balancing
chemical equations
Assigning numbers for covalently bonded atoms:
1) atoms in a pure element have an oxidation number of 0
2) the more-electronegative element in a binary molecular compound is assigned the number equal to the
negative charge it would have as an anion.
The less-electronegative atom is assigned the number equal to the positive charge it would have as a
cation
3) F has an oxidation number of -1 in all of its compounds because it is the most electronegative element
4) O has an oxidation number of -2 in almost all compounds. Exceptions: when in peroxides, H O ,
2
2
and then it is -1; when in compounds with halogens such as OF is +2
2
5) H has an oxidation number of +1 in all compounds containing elements that are more-electronegative;
has oxidation number of -1 in compounds with metals
6) algebraic sum of oxidation numbers of all atoms in a neutral compound is equal to 0
7) algebraic sum of the oxidation number of all atoms in a polyatomic ion is equal to the charge of the ion
8) oxidation numbers can be assigned to atoms in ionic compounds
- monatomic ion has an oxidation number equal to the charge of the ion
Assigning oxidation numbers when they are not known:
1) UF : place oxidation numbers above elements
6
UF
F always has an oxidation number of -1
6
multiply known oxidation numbers by appropriate number of atoms and place totals
underneath corresponding elements
UF
6
UF is molecular, so the sum of the oxidation numbers must equal zero. The total of positive
6
oxidation numbers is therefore +6
UF
6
divide the total calculated oxidation number by the appropriate number of atoms. There is
only one uranium atom in the molecule, so it must have an oxidation number of +6
UF
6
2) H SO : O and S are each more electronegative than H, so H has an oxidation number of +1. O is
2
4
not combined with a halogen nor is H SO a peroxide. Oxidation number of O is -2.
2
4
The sum of the oxidation numbers must equal zero, and there is only one S atom in each
molecule of H SO . Each S atom therefore must have an oxidation number of
2
4
(+2) + (-8) = +6
3) ClO : assign oxidation numbers as above
3
ClO
3
The total of the oxidation numbers should equal the overall charge of the anion, 1-. The oxidation
number of a single O atom in the ion is -2. The total oxidation number due to the atoms is -6. For the
chlorate ion to have a 1- charge, chlorine must be assigned an oxidation number of +5.
ClO
3
Nonmetals can have more than one oxidation number
7-3 Using Chemical Formulas
Can calculate the formula mass, the molar mass, and the percentage composition by mass of a compound
___________________________________- sum of the average atomic masses of all the atoms represented in its formula
_________________________- calculated by summing the masses of the elements present in a mole of the molecules, or
formula units that make up the compound
23
- is equal to the mass in grams of one mole, or approximately 6.02 x 10 particles
- is numerically equal to its formula mass
- convert amount of a compound in moles to a mass in grams:
amount in moles x molar mass (g/mol) = mass in grams
Convert known mass of a compound in grams to an amount in moles:
mass in grams x
1
molar mass (g/mol)
=
amount in moles
__________________________- percentage by mass of each element in a compound
mass of element in sample of compound
x 100 =
% element in compound
mass of sample of compound
7-4 Determining Chemical Formulas
_______________________- consists of the symbols for the elements combined in a compound, with
subscripts showing the smallest whole-number mole ratio of the different atoms
in the compound
- ionic compounds - formula unit is the compound’s empirical formula
- molecular compound - empirical formula does not indicate the actual numbers of
atoms present in each molecule
- calculate: convert percentage composition to a mass composition
mass composition is converted to a composition in moles by dividing
by the appropriate molar mass
now have a ratio
to get smallest whole number ratio: divide each number of moles by the
smallest number in the existing ratio
- if mass composition is known instead of percentage composition, determine
empirical formula: convert mass composition to composition in moles
calculate the smallest whole-number mole ratio of atoms
Molecular formula - is actual formula of a molecular compound
- relationship of molecular formula to empirical formula is
x(empirical formula) = molecular formula mass
(x = is a whole-number multiple indicating the factor by which the
subscripts in the empirical formula must be multiplied to obtain the
molecular formula)
- molecular formula mass = molar mass
- empirical formula mass = adding the masses of each of the atoms indicated
in the empirical formula