UNIT 2:CHEMICAL REACTIONS
4.1: Intro to Chemical Reactions
1
CHEMICAL REACTION
 Process
where one or more substances
change into one or more new substances
Evidence of a Chemical Reaction
1. There is an unexpected colour change
2. Energy is released or absorbed, often in
the form or light of heat
3. A gas (bubbles) is produced
4. A precipitate forms (two liquids mixed
together form a solid and a liquid)
2
COUNTING ATOMS AND MOLECULES
# of atoms: subscript
◦ H2O = 2 hydrogen atoms,
1 oxygen atom
# of molecules: coefficient
◦ 2 H2O = 2 molecules,
4 hydrogen atoms,
2 oxygen atoms
Try: 3 Al2(CO3)3
◦ # molecules:
# of atoms:
3
SYMBOLS AND TERMS USED IN CHEMICAL
REACTIONS
1. Word Equations: use words
 Example:
Iron
+
sulfur
Reactants
(starting substances)

Yields/
produces
iron(II) sulfide
Products
(ending substances)
2. Chemical Equations: use chemical formulas
May be unbalanced (skeleton) or balanced
 Example:
Fe (s)
+
S
(s)

FeS
(s)
State
4
Symbols and Terms Used In
Chemical Reactions
State symbols:
(s) = Solid
(l) = Liquid
(g) = Gas
(aq) = Aqueous (dissolved in water)
5
BALANCING CHEMICAL EQUATIONS
 Follow

the Law of Conservation of Mass:
Mass reactants = mass products
 Add
coefficients in front of the formulas to
balance the equation!
** only whole numbers can be used
** you CANNOT change subscripts
** you must REDUCE the coefficients
6
BALANCING CHEMICAL EQUATIONS
TIPS!
 Balance any element that occurs more than once on
one side of the equation last (often H and O)


Keep polyatomic ions together as a group if it
remains unchanged on both sides of the reaction
ALWAYS do one last check to make sure everything
is balanced!
7
PRACTICE:
 Balance
the following skeleton equations:
1. N2 +
2. Fe +
H2
NH3
O2
Fe2O3
3.
Be +
Al2O3
4.
Mg +
HNO3
5.
CH4 +
O2
BeO +
H2 +
CO2 +
Al
Mg(NO3)2
H2O
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