Chemical equation - WAEC TUTORIALS
A chemical
equation is the symbolic representation of a chemical reaction in the form of
symbols and formulae, wherein the reactant entities are given on the left-hand
side
and the product entities on the right-hand side.
The coefficients next
to the symbols and formulae of entities are the absolute values of the
stoichiometric numbers.
It consists of
the chemical formulas of the reactants (the starting substances) and the
chemical formula of the products (substances formed in the chemical reaction).
The two are separated by an arrow symbol ( {\displaystyle \rightarrow }
\rightarrow , usually read as "yields") and each individual
substance's chemical formula is separated from others by a plus sign.
As an
example, the equation for the reaction of hydrochloric acid with sodium can be
denoted:
2 HCl + 2 Na
→ 2 NaCl + H2
This
equation would be read as "two HCl plus two Na yields two NaCl and H
two." But, for equations involving complex chemicals, rather than reading
the letter and its subscript, the chemical formulas are read using IUPAC
nomenclature. Using IUPAC nomenclature, this equation would be read as
"hydrochloric acid plus sodium yields sodium chloride and hydrogen
gas."
This
equation indicates that sodium and HCl react to form NaCl and H2. It also
indicates that two sodium molecules are required for every two hydrochloric
acid molecules and the reaction will form two sodium chloride molecules and one
diatomic molecule of hydrogen gas molecule for every two hydrochloric acid and
two sodium molecules that react. The stoichiometric coefficients (the numbers
in front of the chemical formulas) result from the law of conservation of mass
and the law of conservation of charge (see "Balancing Chemical
Equation" section below for more information).
Symbols are
used to differentiate between different types of reactions. To denote the type
of reaction:
- "" symbol is used to denote
a stoichiometric relation.
- "" symbol is used to denote
a net forward reaction.
- "" symbol is used to denote
a reaction in both directions.
- "" symbol is used to denote
an equilibrium.
The physical
state of chemicals is also very commonly stated in parentheses after the
chemical symbol, especially for ionic reactions. When stating physical state,
(s) denotes a solid, (l) denotes a liquid, (g) denotes a gas and (aq) denotes
an aqueous solution.
If the
reaction requires energy, it is indicated above the arrow. A capital Greek
letter delta () is put on the reaction arrow to
show that energy in the form of heat is added to the reaction. is used if the energy is added
in the form of light. Other symbols are used for other specific types of energy
or radiation.
Balancing
chemical equations
The law of conservation of mass dictates
that the quantity of each element does not change in a chemical reaction. Thus,
each side of the chemical equation must represent the same quantity of any
particular element. Likewise, the charge is conserved in a chemical reaction.
Therefore, the same charge must be present on both sides of the balanced
equation.
One balances
a chemical equation by changing the scalar number for each chemical formula.
Simple chemical equations can be balanced by inspection, that is, by trial and
error. Another technique involves solving a system of linear equations.
Balanced
equations are written with smallest whole-number coefficients. If there is no
coefficient before a chemical formula, the coefficient 1 is understood.
The method
of inspection can be outlined as putting a coefficient of 1 in front of the
most complex chemical formula and putting the other coefficients before
everything else such that both sides of the arrows have the same number of each
atom. If any fractional coefficient exists, multiply
every coefficient with the smallest number required to make them whole,
typically the denominator of the fractional coefficient for a
reaction with a single fractional coefficient.
As an
example, seen in the above image, the burning of methane would be balanced by
putting a coefficient of 1 before the CH4:
1 CH4 +
O2 → CO2 + H2O
Since there
is one carbon on each side of the arrow, the first atom (carbon) is balanced.
Looking at
the next atom (hydrogen), the right-hand side has two atoms, while the
left-hand side has four. To balance the hydrogens, 2 goes in front of the H2O,
which yields:
1 CH4 +
O2 → CO2 + 2 H2O
Inspection
of the last atom to be balanced (oxygen) shows that the right-hand side has
four atoms, while the left-hand side has two. It can be balanced by putting a 2
before O2, giving the balanced equation:
CH4 +
2 O2 → CO2 + 2 H2O
This equation
does not have any coefficients in front of CH4 and CO2,
since a coefficient of 1 is dropped.
Ionic
equations
An ionic
equation is a chemical equation in which electrolytes are
written as dissociated ions. Ionic equations are used for single and double displacement reactions that
occur in aqueous solutions. For
example, in the following precipitation reaction:
the full
ionic equation is:
In this
reaction, the Ca2+ and the NO3− ions
remain in solution and are not part of the reaction. That is, these ions are
identical on both the reactant and product side of the chemical equation.
Because such ions do not participate in the reaction, they are called spectator
ions. A net ionic equation is the full ionic equation from
which the spectator ions have been removed. The net ionic equation of the
proceeding reactions is:
or, in reduced balanced
form,
In a neutralization or acid/base reaction,
the net ionic equation will usually be:
H+(aq)
+ OH−(aq) → H2O(l)
There are a
few acid/base reactions that produce a precipitate in addition to the water
molecule shown above. An example is the reaction of barium
hydroxide with phosphoric
acid, which produces not only water but also the insoluble salt barium
phosphate. In this reaction, there are no spectator ions, so the net ionic
equation is the same as the full ionic equation.
Double
displacement reactions that feature a carbonate reacting
with an acid have the net ionic equation:
If every ion
is a "spectator ion" then there was no reaction, and the net ionic
equation is null.
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