Transcript How to Write Chemical Formulas of Compounds

How to Write Chemical Formulas of
Compounds
In order to write the chemical formula of a
compound, the type of compound must be
identified based upon its chemical name.
The name of a compound references, in many
cases, a specific compound. The chemical
name may provide information necessary to
write the compound’s chemical formula.
Formulas of Ionic Compounds
Ionic compounds tend to be composed of the
following:
Metallic cation bonded to nonmetallic
anion
Metallic cation bonded to polyatomic anion
Polyatomic cation bonded to nonmetallic
anion
Polyatomic cation bonded to polyatomic
anion
Formulas of Ionic Compounds
All ionic compounds and their formulas are neutral
The total number of positive charges must
EQUAL the total number of negative
charges
In addition, ionic compounds are
represented by an empirical formula,
meaning, the simplest combining ratio
between the ions
Formulas of Ionic Compounds
In order to write the formula for an ionic
compound, the charges of the combining ions
(positive and negative) must be known.
These charges may be known through use of the
periodic table, use of the chemical name of the
compound, or by general knowledge
(memorization)
Formulas of Ionic Compounds
In some compounds, the metallic ion is identified
by the placement of the element on the periodic
table.
e.g. Alkali metals (Gp 1 or IA) form +1 ions
e.g. Alkaline earth metals (Gp 2 or IIA) form +2
ions
e.g. Gp 13 (IIIA) (first three elements) form +3 ions
Formulas of Ionic Compounds
Some “common” transition metals only have
one oxidation state or charge.
Zinc – always +2
Cadmium – always +2
Silver – always +1
Formulas of Ionic Compounds
In some compound names, the charge of the ion is
indicated by the name of the ion.
In the traditional system of naming, an “-ic” suffix
indicates the element’s ion is in its higher oxidation
state (possesses the higher oxidation number). If
the name has the “-ous” suffix, the element’s ion is
in its lower oxidation state.
Oxidation state/oxidation number – relates
to ionic charge for ionic compounds
Requires KNOWLEDGE of oxidation states of these
metals
Ions to Know for the Traditional
System
Transition Metals
Chromium
Cr2+, Cr3+
+
Manganese
Mn2+, Mn3+
Iron
Fe2+, Fe3+
(ferrum)
Cobalt
Co2+, Co3+
Gold
Au+, Au3+
(aurum)
Copper
Cu+, Cu2+
(cuprum)
Mercury
Hg22+, Hg2+
(hydragyrum)
Post-transition Metals
Tin
Sn2+, Sn4+
(stannum)
Lead
Pb2+, Pb4+
(plumbum)
Bismuth
Bi3+ Bi+5
Antimony
Sb+3 Sb+5
(stibium)
Formulas of Ionic Compounds
In some compound names, the charge of the ion
is indicated by the name of the ion.
In the Stock system, a Roman numeral inside
parentheses is used to indicate the oxidation
state (oxidation number) of the metal that
precedes it.
Formulas of Ionic Compounds
If the cation is a polyatomic ion, it’s formula and
charge must be recognized from its name in the
compound’s name.
Most common is ammonium; NH4+
Formulas of Ionic Compounds
If the anion is an element (nonmetal) its charge
may be obtained from the element’s location on
the periodic table.
e.g. Halogens (Gp 17 or VIIA) form -1 ions
e.g. Chalcogens (Gp 16 or VIA) form -2 ions
e.g. Nitrogen family (Gp 15 or VA) form -3 ions if
they form an ion
e.g. Carbon family (Gp 14 or IVA) form -4 ions if
they form an ion???
Formulas of Ionic Compounds
If the anion is a polyatomic anion, its formula
and charge must be KNOWN when it is
recognized by its name in the name of the
compound.
Polyatomic Oxyanions (-1)
Name
Formula
Name
Formula
Acetate
C2H3O21-
Iodite
IO2-1
Perbromate
BrO41-
Hypoiodite
IO-1
Bromate
BrO31-
Nitrate
NO3-1
Bromite
BrO21-
Nitrite
NO21-
Hypobromite
BrO1-
Permanganate
MnO41-
Perchlorate
ClO4-1
Cyanate
OCN-1
Chlorate
ClO3-1
Thiocyanate
SCN1-
Chlorite
ClO2-1
Bisulfate/hydrogen sulfate
HSO41-
Hypochlorite
ClO-1
Bisulfite/Hydrogen sulfite
HSO3-1
Periodate
IO4-1
Dihydrogen phosphate
H2PO4-1
Iodate
IO3-1
Bicarbonate/hydrogen carbonate
HCO3-1
Polyatomic Oxyanions (-2)
Name
Formula
Carbonate
CO32-
Selenate
SeO4-2
Chromate
CrO42-
Tellurate
TeO42-
Dichromate
Cr2O72-
Oxalate
C2O42-
Sulfate
SO4-2
Silicate
SiO3-2
Sulfite
SO3-2
Thiosulfate
S2O3-2
Hydrogen
phosphate
HPO42-
Polyatomic Oxyanions (-3)
Name
Formula
Borate
BO33-
Phosphate
PO43-
Phosphite
PO3-3
Arsenate
AsO4-3
Arsenite
AsO33-
Polyatomic Non-oxyanions
Name
Formula
Peroxide
O22-
Amide
NH2-1
Azide
N31-
Cyanide
CN-1
Hydroxide
OH-1
Formulas of Ionic Compounds
Methods the combining ions include trial and
error, the least common multiple, and the crossover method.
Formulas of Ionic Compounds
Trial and Error
e.g. magnesium and bromite
Mg2+ and BrO2Start with one to one (1:1)
MgBrO2
(1 x +2) + (1 x -1) = +1 net charge not
correct so increase number of anions
Mg(BrO2)2
(1 x +2) + (2 x -1) = 0 net charge CORRECT
Formulas of Ionic Compounds
LCM – Least Common Multiple
e.g. magnesium and bromite
Mg2+ and BrO2The LCM of the absolute value of the charges would be the
LCM of 2 and 1 which is 2. Divide the LCM by the absolute
value of each ion’s charge to determine the subscript for the
ion.
Mg
2 ÷ 2 = 1 the subscript for Mg
BrO2 2 ÷ 1 = 2 the subscript for bromite
Correct formula is Mg(BrO2)2
Formulas of Ionic Compounds
The Crossover method – cross the absolute
value of the cation’s charge to the subscript of
the anion and cross the absolute value of the
anion’s charge to the subscript of the cation
e.g. magnesium and bromite
Mg2+ BrO2-1
Correct formula
Mg(BrO2)2
Formulas of Ionic Compounds
Important Notes:
NEVER EXPRESS a SUBSCRIPT of ONE!!
Polyatomic ions with a subscript equal to or greater than
two are placed INSIDE PARENTHESES and the subscript
placed outside!
When using the crossover method – be careful – reduce
subscripts when possible (exception – mercurous ion)
Writing Formulas for molecular
Compounds
A molecular compound is either going to be organic
(involve carbon) or be composed of two nonmetals.
Organic compounds have their own nomenclature –
see later slide.
Molecular compounds composed of two nonmetals
may be named traditionally or at times with a Stock
system name.
Writing Formulas for molecular
Compounds
Molecular compounds of two nonmetals named
traditionally have a numerical prefix attached to
the element name that indicates the subscript of
that element. If no prefix is present on the first,
it is assumed to be “mono-”, representing a
subscript of one that is NEVER expressed.
Writing Formulas for molecular
Compounds
Molecular compounds of two nonmetals named
with the Stock system will use the Roman
numeral in parentheses to indicate the
“apparent” positive charge on the more positive
element. The apparent charge on the more
negative element is assumed to be similar to its
ionic charge. They are combined to create a
neutral compound where apparent positive
charge in the formula equals apparent negative
charge in the formula.
Writing Formulas for molecular
Compounds - Organic
The organic compounds use a prefix to refer to
the number of carbon atoms present in the
carbon chain.
Prefix
# carbons
Prefix
# carbons
Meth
1
Hex
6
Eth
2
Hept
7
Prop
3
Oct
8
But
4
Non
9
Pent
5
Dec
10
Writing Formulas for molecular
Compounds - Organic
The classes of organic compounds determine the
suffix attached to each prefix and the carbon to
hydrogen ratio. For straight chained hydrocarbons:
-ane ----> alkane ------> CnH2n+2
-ene ----> alkene ------> CnH2n
-yne ----> alkyne ------> CnH2n-2
Alcohols have a suffix of –anol attached to the
prefix
Writing Formulas for molecular
Compounds - Organic
For the alkenes (double bond), alkynes (triple
bond), and the alcohols (-OH), a number
followed by a dash may precede the
compound’s name to indicate the carbon
nearest to the terminal carbon where the
double bond, triple bond, or hydroxyl group
(-OH) is attached.
Acids
Acids are either oxyacids or non-oxyacids/binary acids.
These inorganic acids are usually solvated (aq).
Oxyacids contain hydrogen combined with an
polyatomic oxyanion.
Non-oxyacids contain hydrogen combined with a
non-oxy polyatomic anion
Binary acids contain hydrogen bonded with a
nonmetallic element.
Acids
The formula of an oxyacid is formed by combining
the hydrogen cation (H+) with the appropriate
oxyanion to form a neutral combination.
The “-ic” suffix acids are formed from “- ate”
ending ions.
the “-ous” suffix acids are formed from the
“-ite” ending ions.
The prefixes “per-” and “hypo-” are retained if
present.
Acids
The binary or non-oxy acids are always identified
by the “hydro-” prefix in their name.
Their formulas involve combining the
hydrogen cation (H+) with the appropriate
anion or nonmetallic anion to form a
neutral combination.
Hydrated Compounds
The name of a hydrated compound (hydrate)
consists of the name of an anhydrous salt
followed by a numerical prefix attached to the
word hydrate.
Anhydrous formula – this is the formula of
a salt or ionic compound.
Prefixes: mono-, di-, tri-, tetra-, penta-, hexahepta-, octa-, nona-, deca- …….
Hydrated Compounds
To write the formula of a hydrate, follow the
instructions for ionic compounds to write the
formula of the anhydrous salt. Place a raised dot
(•), a numerical coefficient (except one; 1) that
refers to the numerical prefix attached to the
word hydrate, and the formula for water (H2O).