Introduction to
Chemical
Reactions and
Equations
1
What are the clues that a
chemical reaction has
occurred?
2
A gas bubbles off.
3
A new solid material is formed (a
precipitate.)
4
Color change.
5
Heat and/or light are emitted.
6
The key to chemical reactions!
The Law of Conservation of Mass
In a chemical reaction, no atoms are created or
destroyed. The atoms in the reacting molecules
simply re-arrange to form new molecules.
Since atoms aren’t created or destroyed, no mass
is created or destroyed either.
Mass is “conserved.”
7
Count the atoms, before & after
2H2 + O2
2H2O
8
In the reaction
N2 + 3H2
2NH3
• If 14 grams of N2 reacted with 3 grams of H2,
how many grams of ammonia would be
produced?
In the reaction
2NaCl
2Na + Cl2
• If 58.5 grams of NaCl is decomposed, and 23
grams of Na is formed, how many grams of
Cl2 must also be formed?
9
What else is conserved?
• In N2 + 3H2
2NH3 besides mass (grams), what
else is being conserved?
o Atoms?
o Molecules?
o Moles?
10
Coefficients
• In a chemical reaction, if more than 1 molecule of a
substance reacts or is produced, the number of
molecules is shown with a coefficient.
• 6CO2 + 6H2O
C6H12O6 + 6O2
• Coefficients multiply subscripts through the whole
molecule that follows, so 6CO2 shows that there are
6 Carbon atoms and 12 Oxygen atoms.
11
How many?
• How many of each atom are shown
by
4Al2(SO4)3
• How many of each atom are shown
by
3(NH4)3PO4
12
Terminology
• In a chemical reaction, the materials to the left of
the arrow are the reactants. Reactants are
consumed in the reaction.
• The arrow is like an equal sign in math. It can be
read “react to produce” or “produces” or “forms”.
• The materials to the right of the arrow are the
products. They form as the reaction proceeds.
• H2O + SO3
H2SO4
13
Phase Notation
• A chemical equation can also show the physical
state of the materials in the reaction. This is done
with phase notations:
• (s) solid (l) liquid
(g) gas
• (aq) aqueous or dissolved in water
• (ppt) or ( ) means “a precipitate forms”
• ( ) means “a gas bubbles off”
•
means “is heated”
• Phase notations are written to the lower right of
each formula. Clues to phase notations are often
given in equations written in words.
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An example
• When pellets of sodium hydroxide are dropped into
an aqueous solution of iron(III) chloride, iron(III)
hydroxide precipitates and sodium chloride stays
dissolved.
• 3NaOH(s) + FeCl3(aq)
Fe(OH)3 (ppt) + 3NaCl(aq)
15
Now onward to
“balancing chemical
equations”
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