Unit 2:
Chemical Reactions
4.1: Intro to Chemical Reactions
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 of heat or light
3. A gas (bubbles) is produced
4. A precipitate forms (two liquids mixed
together form a solid and a liquid)
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 =
3
◦ # atoms:
6 aluminum, 9 carbon, 27 oxygen
Symbols and Terms Used
In Chemical Reactions
1. Word Equations: use words
◦ Example:
Iron
+
sulfur
Reactants
(starting substances)

iron(II) sulfide
Yields/
produces
Products
(ending substances)
2. Chemical Equations: use chemical formulas
May be unbalanced (skeleton) or balanced
 Example:
Fe (s)
+
S
(s)
State

FeS
(s)
Symbols and Terms Used
In Chemical Reactions
State symbols:
(s) = Solid
(l) = Liquid
(g) = Gas
(aq) = Aqueous (dissolved in water)
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
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!
Practice:
Balance the following skeleton equations:
1. N2 +
2. Fe +
H2
NH3
O2
Fe2O3
3.
Be +
Al2O3
BeO +
4.
Mg +
HNO3
H2 +
5.
CH4 +
O2
CO2 +
Al
Mg(NO3)2
H2O