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Chapter 7
Pg. 192-219
Describing Reactions
Chapter 7 Section 1
Pg. 192-198
Chemical Equations
• Reactants- the substances that undergo
change in a chemical reaction
• Products- the new substances formed as a
result of that change
• Chemical equation- a representation of a
chemical reaction in which the reactants and
products are expressed as formulas.
Chemical Equations
• Example:
Conservation of Mass
• The law of conservation of mass states that
mass in neither created nor destroyed in a
chemical reaction.
• During chemical reactions, the mass of the
products is always equal to the mass of the
reactants.
• Number of atoms on the left side of the
equation equals the number of atoms on the
right.
Balancing Equations
• In order to show that mass is conserved
during a reaction, a chemical equation must
be balanced.
• Balance a chemical equation by changing the
coefficients (the number that appears before
the formulas)
• Never change the subscripts in a formula
when balancing equations.
Balancing Equations
• Step 1: count the number of atoms of each element
on each side of the equation
• Step 2: change one or more coefficients until the
equation is balanced
CH4 + O2
CO2 + H2O
unbalanced
C=1
C=1
H=4
H=2
O=2
O=3
balanced
Balancing Equations
•
H2 +
O2 
H2O
•
H2 +
N2 
NH3
•
Al2O3 
Al +
O2
Counting with Moles
• Because chemical reactions often involve large
numbers of small particles, chemists use a
counting unit called the mole to measure
amounts of a substance.
• A mole (mol) is an amount of a substance that
contains approximately 6.02 x 1023 particles
(atoms, molecules, or ions) of that substance.
This number is known as Avogadro’s number.
Counting with Moles
• Molar Mass
– The mass of one mole of a substance
– The molar mass is the same as its atomic mass
expressed in grams. (Ex: C is 12amu, so molar
mass of C is 12 g)
– A CO2 molecules is composed of one C atom (12
amu) and 2 O atoms (2 x 16amu = 32amu). So CO2
has a molar mass of 44 grams. (add the two amu
totals together)
Counting with Moles
• Mole-Mass Conversions
– Once you know the molar mass, you can convert
moles of that substance into mass or vice versa.
– Example: the molar mass of CO2 is 44 grams,
which means that one mole of CO2 has a mass of
grams. This relationship yields the following
conversion factors.
44 g CO2
1mol CO2
1mol CO2
44g CO2
Counting with Moles
• Mole-Mass Conversion (continued)
– Suppose you have 55 grams of CO2. To calculate
how many moles of CO2 you have, multiply the
mass by the conversion factor.
55g CO2 X 1mol CO2 = 1.25 mol CO2
44g CO2
You can check your answer by using the other conversion factor
1.25mol CO2 X 44g CO2 = 55g CO2
1mol CO2