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Dr Inam Ul Haq Jazbi 10. Inorganic SectionRules for writing Chemical Formulae of Compounds
To write chemical formulae of compounds, following steps are involved:
1. Every compound is supposed to consist of two types of radicals or ions namely cation and anion. First
of all name of types of radicals or ions are identified in the given compounds. Then symbols or
formulae of both radicals or ions are written side by side (showing cation first then anion) with their
real or formal charges (oxidation numbers) or valencies in superscripts (at the top or at the upper side
each radical).
2. Now the correct combination of ions is used to produce a compound with a net charge of zero. To do
so, the real or formal charges or valencies of corresponding cation and anion are cross multiplied
(criss-crossed). If the charges are same then they are cancelled out otherwise they become the
subscripts of radicals showing the ratios of atoms. Don't write the subscript '1'. Parentheses
surrounding the polyatomic ion and a subscript will be used when there is more than one polyatomic
ion in the formula.
Solved Examples
Example 1. Write down chemical formula of calcium oxide.
Step one Write the symbols for the elements in the compound. Note that the ending "ide" is used for
oxide to show that it is a negative ion of oxygen.
Calcium = Ca , Oxide = O
Step two Write down the charge or oxidation numbers of the elements or radicals involved as
superscripts to the right of the elemental symbols. (Note that when no number accompanies a
charge symbol, as in the case of fluoride, they charge value is understood to be "1").
Calcium = Ca2+ Oxide = O2-
Step three Use the correct combination of ions to produce a compound with a net charge of zero. In this
case, (2+) + (-2) = 0. So, one oxide ion will cancel out one calcium ion. Since it would take
one oxide ion (each with a charge of negative two) to cancel out one barium ion (with a charge
of plus two) we do not use a subscript of one after the symbol for calcium and oxygen to show
the ratio as when no number accompanies in subscript of elemental symbol the atomic ratio is
understood to be "1:1").
Ca2+ O2-
CaO
Example 2. Write the correct formula for Barium Fluoride.
Step one - Write the symbols for the elements in the compound. Note that the ending "ide" is used for
fluoride to show that it is a negative ion of fluorine.
Barium = Ba Fluoride = F
Step two - Look up the oxidation numbers of the elements involved and write them as superscripts to
the right of the elemental symbols. (Note that when no number accompanies a charge
symbol, as in the case of fluoride, they charge value is understood to be "1").
Barium = Ba2+ , Fluoride = F-
Step three - Use the correct combination of ions to produce a compound with a net charge of zero. In this
case, (2+) + 2(-1) = 0. So, two fluoride ions will cancel out one barium ion. Since it would
take two fluoride ions (each with a charge of negative one) to cancel out one barium ion
(with a charge of plus two) we use a subscript of two after the symbol for fluorine to show the
ratio.
Ba2+ F-
Ba2+ 2F-
BaF2
Conceptual Chemistry IX and X -1-
,Dr Inam Ul Haq Jazbi 10. Inorganic Section
Example 3. Write the proper formula for the ionic compound lithium bromide.
Step one Write the symbols for the elements in the compound. Note that the ending "ide" is used for
bromide to show that it is a negative ion of bromine.
Lithium = Li Bromide = Br
Step two Look up the oxidation numbers of the elements involved, and write them as superscripts to
the right of the elemental symbols. (Note that when no number accompanies a charge
symbol, they charge value is understood to be "1").
Lithium = Li+ Bromide = Br-
Step three Use the correct combination of ions to produce a compound with a net charge of zero. In this
case, (+1) + (-1) = 0. So, one lithium ion will cancel out the charge of one bromide ion. This
means that the two elements will combine in a one to one ratio, and no subscripts will be
needed.
Li+ Br- (LiBr)
Example 4; Show the correct formula for Zinc Nitrate.
Step one Write the symbols for the monatomic and polyatomic ions in the compound.
Zinc = Zn Nitrate = NO3
Step two Write down the charge or oxidation numbers of the elements or radicals involved as
superscripts to the right of the elemental or radical symbols. (Note that when no number
accompanies a charge symbol, as in the case of nitrate below, they charge value is
understood to be "1").
Zinc = Zn2+ Nitrate = NO3-
Step three Use the correct combination of ions to produce a compound with a net charge of zero. In this
case, (2+) + 2(-1) = 0. So, two nitrate ions will cancel out one zinc ion. Since it would take
two nitrate ions (each with a charge of negative one) to cancel out one zinc ion (with a
charge of plus two) we use a subscript of two after the symbol for nitrate ion to show the
ratio. Parenthesis must be used if you need more than one of a polyatomic ion. We
need to show two nitrate ions in our formula. The subscript is put on the outside of the
parenthesis to show that the entire polyatomic ion is doubled. The correct use of parenthesis
will seem hard at first, but you must master this skill with practice!
Zn(NO3)2
Example 5; Show the correct formula for ferric sulphate.
Step one Write the symbols for the monatomic and polyatomic ions in the compound.
Ferric = Fe , Sulphate = SO4
Step two Write down the charge or oxidation numbers of the elements or radicals involved as
superscripts to the right of the elemental or radical symbols.
Ferric = Fe 3+ , Sulphate = SO42-
Step three Use the correct combination of ions to produce a compound with a net charge of zero. In this
case, 2(3+) + 3(2-) = 0. So, three sulphate ions will cancel out the charge of two ferric ions.
Since it would take three sulphate ions (each with a charge of negative two) to cancel out
two ferric ions (with a charge of plus three) we use a subscript of three after the symbol for
sulphate ion and a subscript of two after the symbol of ferric ion to show the ratio.
Parenthesis must be used if you need more than one of a polyatomic ion. We need to
show three sulphate ions in our formula. The subscript is put on the outside of the
parenthesis to show that the entire polyatomic ion is tripled. The correct use of parenthesis
will seem hard at first, but you must master this skill with practice!
Fe2(SO4)3
Conceptual Chemistry IX and X -2-
,Dr Inam Ul Haq Jazbi 10. Inorganic Section
Naming Binary Ionic Compounds
1. Naming Binary Ionic Compounds containing a metal and a nonmetal
A binary ionic compound is mostly a salt consisting of only two elements in which both elements are mostly
monoatomic ions, a cation and an anion. Binary compounds of a metal and a nonmetal contain a metallic
cation and a nonmetallic anion. When naming these compounds, its composition must be considered.
1. The cation (which is metallic) is listed first and the anion (which is generally nonmetallic) second.
2. The cation takes the name of its elemental form. For example, Li+ would be called "Lithium".
Recall that the alkali metals form only ions with a +1 charge, the alkaline earth metals form only ions
with a +2 charge, and aluminum forms only the ion Al3+. For these ions, the name of the element
followed by the term ion is an unambiguous name. For example, the sodium ion can only be Na+,
the calcium ion only Ca2+.
According to IUPAC rules, the names of all other metallic cations contain the name of the element
followed by its oxidation state (in parentheses) in that ion. This rule prevents ambiguity. The
name chromium ion does not say whether the ion is Cr2+ or Cr3+; the proper names for these ions
are chromium(II) and chromium(III) respectively.
Before leaving this group of compounds, we should mention again the second and less-preferred
method of naming cations of the same element in different oxidation states. This older method gives
the ending ous to the ion of lower oxidation state and the ending ic to the ion of higher oxidation
state. Often this system also uses the Latin root of the name of the element. Thus, in this system,
Fe2+ is ferrous and Fe3+ is ferric; Pb2+ is plumbous and Pb4+ is plumbic.
3. The anion name uses the first part of its elemental name or root name, and subsequently adding the
suffix "-ide" i.e. for the non-metal (the second element in the formula) write the name as it appears
on the Periodic Table and then replace the ending with ide. For example, Br– would be called
"Bromide", S2– would be called sulphide and O2– would be called oxide.
Examples
+ –
LiF (which is composed of Li cation and F anion) = Lithium fluorine = Lithium fluoride
2+ 2–
BaO (which is composed of Ba cation and O anion) = Barium oxygen = Barium oxide
+ 2−
Na2S (which is composed of Na cation and S anion) = Sodium sulphur = Sodium sulphide
3+ 3–
AlN (which is composed of Al cation and N anion) = Aluminium nitrogen = Aluminium nitride
2+ –
MgCl2 (which is composed of Mg cation and Cl anion) = Magnesium chlorine = Magnesium chloride
3+ 3–
CrP (which is composed of Cr cation and P anion) = Chromium phosphorus = Chromium phosphide
2+ –
Nil2 (which is composed of Ni cation and I anion) = Nickel iodine = nickel(II) iodide
2+ 2–
ZnO (which is composed of Zn cation and O anion) = Zinc oxygen = zinc(II) oxide
2+ –
CaBr2 (which is composed of Ca cation and Br anion) = Calcium bromine = calcium bromide
2. Naming Binary Molecular compounds containing two non-metals but not hydrogen
Binary compounds of two non-metals, neither of which is hydrogen, are molecular rather than ionic. They
do not contain cations and anions. Carbon dioxide (CO2) and phosphorus trichloride (PCl3) are examples of
such compounds. They are named using prefixes like di, tri, tetra, penta, hexa etc. to state how many
atoms of an element are in one molecule of the compound.
The name of the second element is modified to the root of its name followed by the ending ide. In both the
formula and the name of these compounds, the most non-metallic element comes first. The
prefix mono is often omitted for the first element but never omitted for the second. Thus,
CO is carbon monoxide
SF6 is sulfur hexafluoride
N2O is dinitrogen monoxide
Conceptual Chemistry IX and X -3-
, Dr Inam Ul Haq Jazbi 10. Inorganic Section
3. Binary Acids
The binary compound formed when a halogen or any element, except oxygen (from group VIA of the
periodic table), combines with hydrogen can be named as were the binary non-metallic compounds.
However, when these compounds are dissolved in water, the solution contains hydrogen ions. Because this
property identifies an acid, these compounds must also be named as acids. Therefore, these compounds
have two sets of names, one for the pure state and one for the compound dissolved in water.
Two points should be noted:
(1) The acid name has the prefix hydro and the suffix ic. For binary acids, add the prefix –hydro to the
root name of the non-metallic anion and change the suffix of the anion name from –ide to –ic. e.g.
The stem for chlorine is chlor. Cl– …… chloride ion, HCl ….. hydrochloric acid
(2) These formulas are always written with hydrogen first. Other non-metals like C, Si, N, P, As etc.
form compounds with hydrogen, but they are not acids; their formulas are written with hydrogen
last. Methane, CH4, ammonia, NH3, and arsine, AsH3, are some examples.
Formula of binary acids Name in pure state Name in water or aqueous solution
HF Hydrogen fluoride Hydrofluoric acid
HCl Hydrogen chloride Hydrochloric acid
HBr Hydrogen bromide Hydrobromic acid
HI Hydrogen iodide Hydroiodic acid
H2S Hydrogen bromide Hydrosulphuric acid
HCN Hydrogen cyanide Hydrocyanic acid
4. Pseudo-Binary Compounds
Several polyatomic ions act so much like monatomic ions that they are classified as such. These ions are
called pseudo-binary ions. They include the ammonium ion, NH4+, the hydroxide ion, OH-, the cyanide ion,
CN-, and others. Compounds containing these ions are pseudo-binary compounds.
The properties of the ammonium ion are much like those of the alkali-metal ions. Compounds containing
the hydroxide ion are bases. A general definition of a base is that its aqueous solution contains more
hydroxide than hydrogen ions. The cyanide ion behaves very much like a halogen ion. Many compounds
containing the cyanide ion are extremely toxic.
NaOH = Sodium hydroxide
KCN = Potassium cyanide
NH4Cl = Ammonium chloride
If a compound contains two or more different electropositive elements, it is also named as a binary
compound. Suppose you have a compound such as Na2S, sodium sulfide, in which a potassium replaces a
sodium yielding KNaS. This is called potassium sodium sulfide, potassium being named first because it
is more electropositive. The compound NaHS would be sodium hydrogen sulfide for the same reason.
Conceptual Chemistry IX and X -4-
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