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meaning. For example, the equation describing carbon reacting
with oxygen to yield carbon dioxide becomes C means One atom of carbon

C + O 2 → CO 2
O means One atom of oxygen
(balanced)
One molecule of oxygen consisting
The new, added meaning is that one atom of carbon (C) reacts O 2 means of two atoms of oxygen
with one molecule of oxygen (O 2 ) to yield one molecule of
carbon dioxide (CO 2 ). Note that the equation also shows one One molecule of carbon monoxide
atom of carbon and two atoms of oxygen (recall that oxygen CO means consisting of one atom of carbon
attached to one atom of oxygen
occurs as a diatomic molecule) as reactants on the left side
and one atom of carbon and two atoms of oxygen as products One molecule of carbon dioxide
on the right side. Since the same number of each kind of atom CO 2 means consisting of one atom of carbon
attached to two atoms of oxygen
appears on both sides of the equation, the equation is said to
be balanced. Three molecules of carbon dioxide,
You would not want to use a charcoal grill in a closed room each consisting of one atom of
3 CO 2 means carbon attached to two atoms of
because there might not be enough oxygen. An insufficient
oxygen
supply of oxygen produces a completely different product, the
poisonous gas carbon monoxide (CO). An equation for this
FIGURE 10.5 The meaning of subscripts and coefficients used
reaction is
with a chemical formula. The subscripts tell you how many atoms
C + O 2 → CO of a particular element are in a compound. The coefficient tells you
about the quantity, or number, of molecules of the compound.
(not balanced)
As it stands, this equation describes a reaction that violates the bon monoxide, and 3 CO means three molecules of carbon
law of conservation of mass, that matter is neither created nor monoxide. If there is no coefficient, 1 is understood, as with
destroyed in a chemical reaction. From the point of view of an subscripts. The meaning of coefficients and subscripts is il-
equation, this law states that
lustrated in Figure 10.5.
mass of reactants = mass of products Placing a coefficient of 2 in front of the C and a coefficient
of 2 in front of the CO in the equation will result in the same
Mass of reactants here means all that you start with, including numbers of each kind of atom on both sides:
some that might not react. Thus, elements are neither created
nor destroyed, and this means the elements present and their 2 C + O 2 → 2 CO
mass. In any chemical reaction, the kind and mass of the reac- Reactants: 2 C Products: 2 C
tive elements are identical to the kind and mass of the product
elements. 2 O 2 O
The law of conservation of mass means that atoms are
conserved, that is, neither created nor destroyed in a chemi- The equation is now balanced.
Suppose your barbecue grill burns natural gas, not char-
cal reaction. A chemical reaction is the making or breaking coal. Natural gas is mostly methane, CH 4 . Methane burns
of chemical bonds between atoms or groups of atoms, and by reacting with oxygen (O 2 ) to produce carbon dioxide
atoms are not lost or destroyed in the process, nor are they (CO 2 ) and water vapor (H 2 O). A balanced chemical equation
changed to a different kind. The equation for the formation for this reaction can be written by following a procedure of
of carbon monoxide has two oxygen atoms in the reactants four steps.
(O 2 ) but only one in the product (CO). An atom of oxygen
has disappeared somewhere, and that violates the law of con- Step 1: Write the correct formulas for the reactants and
servation of mass. You cannot fix the equation by changing products in an unbalanced equation. The reactants
the CO to a CO 2 , because this would change the identity of and products could have been identified by chemical
the compounds. Carbon monoxide is a poisonous gas that is experiments, or they could have been predicted
different from carbon dioxide, a relatively harmless product from what is known about chemical properties.
of burning and respiration. You cannot change the subscript in This will be discussed in greater detail later. For
a formula because that would change the formula. A different now, assume that the reactants and products are
formula means a different composition and thus a different known and are given in words. For the burning of
compound. methane, the unbalanced, but otherwise correct,
You cannot change the subscripts of a formula, but you formula equation is
can place a number called a coefficient in front of the formula. CH 4 + O 2 → CO 2 + H 2 O
Changing a coefficient changes the amount of a substance,
not the identity. Thus, 2 CO means two molecules of car- (not balanced)

256 CHAPTER 10 Chemical Reactions 10-6