Reaction stoichiometry studies the quantitative relationships between
reactants and products within a given chemical reaction.
LEARNING OBJECTIVES [ edit ]
Discuss the meaning and justification of stoichiometry in chemistry
Balance chemical equations using stoichiometry
KEY POINTS [ edit ]
Stoichiometry comes from the Greek "stoiechion" (element) and "metron" (to measure). As such,
stoichiometry deals with determining the amounts of reactants and products that are consumed
and produced within a given chemical reaction.
The stoichiometric coefficient of any species that does not participate in a given chemical reaction
is zero.
The principles of stoichiometry are based upon the law of conservation of mass. Matter can
neither be created nor destroyed, so the mass of every element present in the product(s) of a
chemical reaction must be equal to the mass of each and every element present in the reactant(s).
TERMS [ edit ]
stoichiometric number
Equal to the stoichiometric coefficient in balanced equation, but positive for products (because
they are produced) and negative for reactants (since they are consumed).
balanced equation
When the quantity of each individual element is equal on both sides of the equation.
stoichiometric ratio
A positive integer ratio that relates the number of moles of reactants and products involved in a
chemical reaction; this ratio can be determined from the coefficients of a balancedchemical
equation.
reaction stoichiometry
Describes the quantitative relationship between reactants and products within a given chemical
reaction.
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Stoichiometry is a branch
of chemistry that deals with the relative
quantities of reactants and products that
are consumed/produced within a
given chemical reaction. In order to make
any stoichiometric determinations,
however, we must first look to a
balanced chemical equation. In a balanced
chemical equation, we can easily
determine the stoichiometricratio between
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the number of moles of reactants and the number of moles of products, because this ratio will
always be a positiveinteger ratio. Consider the reaction of nitrogen gas and hydrogen gas to
form ammonia (NH3):
N 2 (g) + 3H 2 (g)
→
2N H 3 (g)
From the balanced equation, we can see that the stoichiometriccoefficient for nitrogen is 1,
while for hydrogen it is 3, and for ammonia it is 2. Therefore, the stoichiometric ratio,
oftentimes referred to simply as the "mole ratio" or "molar ratio," between N2(g), H2(g), and
NH3(g) is 1:3:2. In the special case where reactants are combined in their molar ratios (in this
case, 1 mole of N2(g) and 3 moles of H2(g)), they will react completely with each other, and no
reactant will be left over after the reaction has run to completion. However, in most realworld situations, reactants will not combine in such perfect stoichiometric amounts. In most
cases, one reactant will inevitably be the first to be completely consumed in the reaction,
causing the reaction to come to a halt. This reactant is known as the limiting reactant,
or limiting reagent.
From this brief description, we can see that stoichiometry has many important applications.
As we will see, through balancing chemical equations and determining the stoichiometric
coefficients, we will be able to determine the number of moles of product(s) that can be
produced in a given reaction, as well as the number of moles of reactant(s) that will be
consumed. Stoichiometry can also be used to make useful determinations about limiting
reactants, and to calculate the amount of excess reactant(s) left over after a given reaction has
run to completion.
The Basis of Stoichiometry
The science of stoichiometry is possible because it rests upon the law of conservation of mass.
Since matter can neither be created nor destroyed, nor can a chemical reaction transform
one element into another element, we can be sure that the mass of each individual element
present in the reactant(s) of a given reaction must necessarily be accounted for in the
product(s). This physical law is what makes all stoichiometric calculations possible. However,
we can only perform these calculations correctly if we have a balanced chemical equation
with which towork.
Interactive: Stoichiometry and Balancing Equations
To make hydrogen chloride or any other chemical there is only one ratio of reactants that works so that
all of the hydrogen and chlorine are used to make hydrogen chloride. Try several different ratios to see
which ones form a complete reaction with nothing left over. What is the simplest ratio of hydrogen to
chlorine for forming hydrogen chloride?
Balancing Equations
Before performing any stoichiometric calculation, we must first have a balanced chemical
equation. Take, for example, the reaction of hydrogen and oxygen gas to form liquid water:
H 2 (g) + O 2 (g)
→
H 2 O(l)
As it is written here, we should notice that our equation is not balanced, because we have two
oxygen atoms on the left side of the equation, but only one on the right. In order to balance
this, we need to add a stoichiometric coefficient of 2 in front of liquid water:
H 2 (g) + O 2 (g)
→
2H 2 O(l)
In doing this, however, our hydrogens have become unbalanced. To finish balancing the
equation, we must add a coefficient of 2 in front of hydrogen gas:
2H 2 (g) + O 2 (g)
→
2H 2 O(l)
As we can see, the stoichiometric coefficient for any given reactant/product is the number of
molecules that will participate in the reaction as written in the balanced equation. Keep in
mind, however, that in our calculations, we will often be working in moles, rather than in
molecules. In our example here, we can see that the stoichiometric coefficient of H2(g) is 2,
while for O2(g) it is 1, and for H2O(l) it is 2. Occasionally, you might come across the
term stoichiometric number, which is related to the stoichiometric coefficient, but is not the
same.
Electrolysis of water
Although this image illustrates the reverse reaction of 2H 2 (g) + O 2 (g)
→
2H 2 O(l)
, the stoichiometric
coefficients for each type of molecule are still the same. Water is 2, hydrogen gas is 2, and oxygen gas is
1.
For reactants, the stoichiometric number is the negative of the stoichiometric coefficient,
while for products, the stoichiometric number is simply equal to the stoichiometric
coefficient, remaining positive. Therefore, for our example here, the stoichiometric number
for H2(g) is -2, and for O2(g) it is -1. For H2O(l), however, it is +2. This is because in this
reaction, H2(g) and O2(g) are reactants that are consumed, whereas water is a product that is
produced.
Lastly, you might occasionally come across some chemical species that are present during a
reaction, but that are neither consumed nor produced in the reaction. A catalyst is the most
familiar example of this. For such species, their stoichiometric coefficients are always zero.
Example
In the equation H2(g) + Cl2(g) → 2 HCl(g), what is the molar ratio (stoichiometric ratio)
between H2(g) and HCl(g)?
In our balanced chemical equation, the coefficient for H2(g) is 1, and the coefficient for
HCl(g) is 2. The molar ratio between these two compounds is therefore 1:2. This tells us that
for every 1 mole of H2(g) that is consumed in the reaction, 2 moles of HCl(g) are produced.