Notes #2:
Name
Writing Word and Skeleton Equations
Date
Balanced Chemical Equations and Coefficients
Block
A chemical reaction can be described by a sentence, a word equation, a skeleton equation, or a
balanced chemical equation. One or more substances, called reactants, react to form one or
more new substances, called products.
In any chemical equation, reactants are written to the LEFT of the reaction arrow, and
products are written to the RIGHT of the reaction arrow.
DO NOT CHANGE THE DIRECTION OF THE REACTION ARROW!!!!!!!
Sentences that describe a chemical reaction may give the reactants first, or the products may be
given first. You must be very careful when reading sentence descriptions of chemical reactions.
For example, in sentence form: Solid mercury (II) oxide reacts when heated and yields
oxygen gas and mercury metal.
This also could be written as: Oxygen gas and mercury metal are formed when solid
mercury(II) oxide reacts when heated.
The word equation for this reaction would be written:
mercury(II) oxide (s)

oxygen (g) +
mercury (l)
Note that the physical state of each reactant and product is given in parentheses after each
substance. (s means solid; g means gas; l means liquid; aq means aqueous solution)
Sentences and word equations are cumbersome and don’t give enough information about the
reaction. All we are given is WHAT the reactants and products were. Sometimes, we also are
told the conditions under which the reaction occurred; in the example above, the reactant was
heated.
A skeleton equation uses chemical formulas instead of compound and element names. The
physical state of each reactant and product is given. The skeleton equation would be written:
HgO(s) 
O2(g) +
Hg(l)
We know what reacted, what was formed, and what the physical state of each reactant and
product was: solid mercury(II) oxide reacted and the products were oxygen gas and liquid
mercury (remember that mercury is the only metal that is liquid at room temperature).
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A skeleton equation uses chemical formulas instead of words so we know how many atoms or
ions are in each of the reactants and products, but it does not indicate the relative amounts of the
reactants and products (how much of each reactant was used and how much of each product was
formed).
We need a balanced chemical equation in order to know how much of each reactant was
used and how much of each product was formed.
The balanced chemical equation would be written:
2 HgO(s) 
O2(g) +
2 Hg(l)
The balanced chemical equation uses chemical formulas, indicates the physical state of each
substance, and gives the relative amounts of each reactant and each product as indicated by the
COEFFICIENT placed in front of the formula for each substance.
In a balanced chemical equation, the number of atoms of an element present on the LEFT
of the reaction arrow is equal to the number of atoms of that element present on the
RIGHT of the reaction arrow. Also, the total mass of the reactants is equal to the total
mass of the products.
Do NOT try to balance the addition total of the coefficients on the left with the addition
total of the coefficients on the right. These rarely are equal. Coefficients are NOT
“counting” numbers.
NOTE: when NOT combined with any other element, certain
elements occur as DIATOMIC MOLECULES: hydrogen: H2;
nitrogen: N2; oxygen: O2; fluorine: F2; chlorine: Cl2; bromine:
Br2; iodine: I2.
These elements may be referred to as the “brincklehof” elements. If we use the element symbols
we can “spell” Br I N Cl H O F and pronounce this “word” as “brincklehof.”
Now then, what are coefficients and what DO the coefficients MEAN?
A coefficient is a SMALL WHOLE NUMBER that appears in front of a chemical formula in a
chemical equation.
When NO coefficient is written, but a chemical formula exists, the coefficient is assumed to be
“1.”
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Coefficients may represent any of the following:
a) number of atoms of an element
b) number of moles of an element or compound (we’ll explain “moles” in the next unit)
c) number of molecules or formula units of compounds
d) number of ions (if written in front of the formula of an ion as in 4 Na+1)
e) number of volumes of a gas assumed to be at STP: standard temperature and
pressure; the term “volumes” refers to liters unless otherwise noted
Coefficients may NEVER,
NEVER, NEVER,
NEVER represent the mass of any reactant or product.
For example, 2 H2(g) + O2(g)  2 H2O(g) may be read as:
a) 2 moles of hydrogen gas react with 1 mole oxygen gas to yield 2 moles water vapor; OR
b) 2 molecules hydrogen gas react with 1 molecule oxygen gas to yield 2 molecules water
vapor; OR
c) 2 volumes of hydrogen gas react with 1 volume of oxygen gas to yield 2 volumes of water
vapor.
But the above equation may NEVER be read as “2 grams of hydrogen gas react with 1 gram of
oxygen gas to produce 2 grams of water vapor.”
The equation
2 AgNO3(aq) + Cu(s)  Cu(NO3)2(aq) + 2 Ag(s)
may be read as:
a) 2 moles of aqueous silver nitrate react with 1 mole copper metal to yield
1 mole aqueous copper(II) nitrate and 2 moles silver metal; OR
b) 2 formula units of aqueous silver nitrate react with 1 atom of copper metal to yield
1 formula unit of aqueous copper(II) nitrate and 2 atoms of silver metal.
But the above equation may NEVER be read as “2 grams of aqueous silver nitrate react with 1
gram solid copper to produce 1 gram of aqueous copper(II) nitrate and 2 grams of solid silver.”
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