6.3
Figure 1 Wood disappears to almost
nothing when it burns. How does this
reaction obey the law of conservation
of mass?
law of conservation of mass the
statement that, in any given chemical
reaction, the total mass of the reactants
equals the total mass of the products
Conserving Mass in Chemical Reactions
Think again about Investigation 6.2: Is Mass Gained or Lost During
a Chemical Reaction? In Part A, you saw that when two solutions are
combined to form a product in a sealed container, the total mass of reactants
equals the total mass of products. No mass is gained; none is lost. In other
words, mass is conserved during the chemical reaction.
Is mass conserved in other chemical reactions as well? The answer to this
questionmaynotbeimmediatelyobvious.Afterall,everydayexperience
may suggest that mass does change during a chemical reaction. For
example,acampfireburnsdowntoapileoffluffyashesthathavemuch
less mass than the original wood (Figure 1). One of the first scientists to
study this question was the eighteenth-century French chemist Antoine
Lavoisier. Until this time, chemists had not considered the possibility that
some reactions could involve gases. Gases, like other forms of matter, have
mass. In his experiments, Lavoisier used equipment designed to trap all the
reactants and products, including any gases produced during the reactions.
Lavoisier found that mass was always conserved if all the reactants and
products were considered.
Lavoisier’s conclusion has been supported by the work of many other
scientists and is now considered to be a scientific law. It is known as the
law of conservation of mass:
In any given chemical reaction, the total mass of the reactants equals
the total mass of the products.
The law of conservation of mass makes sense when you consider what
is occurring at the atomic level. (Recall the Bohr–Rutherford model
inSection5.4.)Experimentshaveshownthat,duringachemical
reaction, the atoms in reactant molecules are rearranged to form
products. Therefore, all the atoms that existed in the reactants
are still present in the products of the reaction. Atoms cannot
be created or destroyed. That explains why the total mass of
reactants is equal to the total mass of products.
Reactions, Equations, and
the Conservation of Mass
Figure 2 There are two (red) oxygen
atoms and one (black) carbon atom on
either side of the scale. Similarly, there
are two oxygen atoms and one carbon
atom on either side of the arrow in the
chemical equation.
In Section 6.1, you learned that chemical equations can
provide a lot of information about chemical reaction. Equations
give the formulas and physical state of the reactants and products.
To be a completely accurate description of the chemical reaction, an
equation must also follow the law of conservation of mass. It must show
an equal number of each kind of atom on both sides of the equation. This
indicates that there are equal numbers of each kind of atom before and after
the chemical reaction takes place.
Let’s consider the reaction in which carbon and oxygen react to form
carbon dioxide (Figure 2):
C(s) + O2(g) → CO2(g)
230
Chapter 6 • Chemicals and Their Reactions
NEL
Note that the same numbers and kinds of atoms are present on both
sides of the equation. The product (carbon dioxide) contains exactly the
same numbers and kinds of atoms as the reactants (carbon and oxygen).
Therefore, the chemical equation obeys the law of conservation of mass.
Let’s look at another chemical equation:
LeaRning Tip
Diatomic Molecules
Remember that some elements exist
as diatomic molecules. Refer back to
Table 1 in Section 5.10.
H2(g) + Cl2(g) → HCl(g)
This equation does not accurately describe the reaction between hydrogen
and chlorine. Can you tell why? Count the atoms on each side of the arrow.
In this reaction, two atoms of hydrogen react with two atoms of chlorine.
(Remember that hydrogen and chlorine both exist as diatomic molecules.)
The product, however, contains only one atom of hydrogen and one of
chlorine (Figure 3(a)). One atom of hydrogen and one atom of chlorine
remain unaccounted for. An equation in which the reactants and products are
not balanced is sometimes called a “skeleton equation.” Since atoms cannot
just vanish, we can assume that two molecules of HCl(g) are produced in this
reaction. Laboratory investigations indicate that this is the case (Figure 3(b)).
To show that two hydrogen chloride molecules are produced in this
reaction, the coefficient “2” is placed before HCl(g) in the chemical equation:
H2(g) + Cl2(g) → 2 HCl(g)
The coefficient in a chemical equation applies to all the atoms in the
molecule. “2 HCl” means that there are two molecules of hydrogen chloride,
each containing one hydrogen atom and one chlorine atom.
Now the chemical equation obeys the law of conservation of mass.
Chemical equations must always be balanced, with the same kinds and same
numbers of atoms on both sides of the arrow.
LeaRning Tip
Coefficients versus Subscripts
Do not confuse coefficients with
subscripts in chemical formulas.
Coefficients give the ratio of reactants
and products in a reaction. Subscripts
give the ratio of elements in a chemical
formula; they cannot change in a given
chemical. Thus, chemical equations can
be balanced only by using coefficients.
Reading Tip
Adjusting Inferences
Sometimes as you continue reading,
you come across information that
conflicts with an inference that you have
already made. For example, you read
that an equation must obey the law of
conservation of mass. Later you read a
chemical equation that appears to break
this fundamental law. You infer that
exceptions are possible. Later you read
that the equation as stated is incorrect.
You revise your inference and conclude
that the law of conservation of mass is
unbreakable.
(a)
(b)
NEL
Figure 3 In (a), there are two (white)
hydrogen atoms and two (green)
chlorine atoms on the left side of the
scale, but only one hydrogen atom and
one chlorine atom on the right side.
In (b), there are two hydrogen atoms
and two chlorine atoms on each side of
the scale. Reactants and products are
balanced.
6.3 Conserving Mass in Chemical Reactions
231
T RY THIS
MOdeLLing BaLanCed CHeMiCaL eQUaTiOnS
SKILLS: Observing, Predicting, Analyzing
In this activity, you will use molecular models to visualize the law
of conservation of mass.
Equipment and Materials: molecular model kit
1. Build one molecule of hydrogen, H2. Build one molecule
of bromine, Br2. Predict how many molecules of hydrogen
bromide, HBr, can be made from these two models. Verify your
prediction by making the “product” from the “reactants.”
2. Build two molecules of hydrogen and one molecule of
oxygen. Predict how many molecules of water, H2O, can
be made from them. Verify your prediction by making
the product.
in
3. Build two molecules of hydrogen peroxide, H2O2. Predict how
many molecules of oxygen and water can be made from
them. Verify your prediction.
4. Build two molecules of ammonia, NH3. Imagine that this is the
product of a reaction between hydrogen and nitrogen. Predict
how many molecules of hydrogen and nitrogen are required to
make two molecules of ammonia. Check your prediction.
A. Write the word equations and chemical equations for each of
these four reactions. K/U
B. Explain how the results of this activity illustrate the law of
conservation of mass. T/I
SUMMARY
•Th
elawofconservationofmassstatesthatin
any given chemical reaction, the total mass of the
reactants equals the total mass of the products.
•Chemicalequationsobeythelawofconservation
of mass. They show that all the atoms in the
reactants are still present in the products.
CHECK
•Coeffi
cientsareaddedbeforechemicalformulas
in a chemical equation to ensure that the
numbers of atoms on each side of the arrow are
equal (balanced).
YOUR LeaRning
1. (a) The idea that gases have mass can be difficult
to accept. How has this reading helped your
understanding of this concept?
(b) Identify what remains unclear. Discuss this with
your teacher. C
2. (a) State the law of conservation of mass.
(b) Explain the law of conservation of mass by referring
to the atoms involved in a chemical reaction.
(c) Which best represents the law of conservation of
mass: a skeleton equation or a balanced chemical
equation? Explain. K/U
3. Are the following situations exceptions to the law of
conservation of mass? Justify your answer in each
case. K/U
(a) The mass of a hamburger decreases as it is barbecued.
(b) A tree’s mass is continually increasing as the tree grows.
(c) The mass of a copper penny increases if it is heated in
a Bunsen burner flame.
4. You might have noticed that new copper roofs turn
green over time. This occurs because copper reacts with
substances in the air to form a hard, protective coating.
Will the mass of the new copper roof increase or decrease
over time? Explain. Does this prediction violate the law of
conservation of mass? Explain. T/I A
5. Design an experiment involving the reaction of vinegar and
baking soda to test the law of conservation of mass. T/ I
6. A 20 g sample of compound A is mixed with 45 g of
compound B. A chemical reaction occurs in which a gas is
produced. Once the reaction is complete, the final mixture
has a mass of 55 g. T/I
(a) What is the mass of the gas?
(b) What assumption did you make in (a)?
7. Soon after learning about the work of Lavoisier, John
Dalton proposed that atoms are never created or
destroyed in chemical reactions, only rearranged. Explain
how this statement applies to the law of conservation of
mass. A
(d) You are often lighter in the morning than you were
when you went to bed.
232
Chapter 6 • Chemicals and Their Reactions
NEL