Objectives
 Write chemical equations to describe chemical
reactions
 Balance chemical equations
 Classify and identify chemical reactions
 Predict products from chemical reactions
I. Reactions and equations
 Chemical reactions:
 A chemical change has occurred
 Evidence of a chemical reaction
 Temperature change (energy
change)
 Change in color
 Evidence of chemical change
 Gas production (bubbles, fizzing, odor)
 Formation of a solid (precipitate)
 YouTube - ammonium dichromate volcano
 YouTube - Nitrogen Triiodide Detonation
 YouTube - Sodium and Water
 YouTube - Reaction of Sodium & Chlorine (with
subtitles)
 Representing chemical reactions:
 Reactants: starting substances (written on the left side)
 Products: substances that result from the reaction
(written on the right side)
 An arrow is written between reactants and products and
is read as “yields”
 Symbols used in chemical equations (p 203)
Symbol
Meaning

Yield (placed between reactants and products)
Separates reactants and products; indicates
reversible reaction
+
Separate two or more reactants or products
(s)
Substance is in a solid state
(l)
Substance is in a liquid state
(g)
Substance is in a gaseous state
(aq)
Substance is in an aqueous (water) solution

Heat is applied
Pt
A substance written above an arrow indicates it
is a catalyst needed for the reaction
 Word Equations
 Statements used to indicate reactants and products
 Lack important information
 Ex. Hydrogen (g) + oxygen (g)  water (l)
 Skeleton equations
 Chemical formulas are used to represent reactants and
products
 Ex. H2 (g) + O2 (g)
 H2O (l)
Learning Check:
 Write the skeleton equations for the following:
1. Hydrogen (g) + Bromine (l)  hydrogen bromide (g)
2.
Carbon monoxide (g) + oxygen (g)  carbon dioxide (g)
Practice:
 Write the skeleton equations for the following:
1. Hydrogen (g) + Bromine (l)  hydrogen bromide (g)
H2 (g) + Br2 (l)  HBr (g)
Carbon monoxide (g) + oxygen (g)  carbon dioxide (g)
CO (g) + O2 (g)  CO2 (g)
2.
CW p 284 # 3, p 980 9.1 #1,2
II. Balancing chemical equations
 Chemical equations:
 Must obey “law of conservation of matter”
 Must show that the number of atoms of each substance
is the same before and after the reaction.
 A chemical equation has to be BALANCED
 To balance chemical equations, COEFFICIENTS (whole
number)are written in front of a reactant or product.
 If coefficient is 1, it is not written
 Steps for balancing equations
1.
Write the skeleton equation for the reaction. Ex.
Hydrogen gas reacts with chlorine gas and yields
gaseous hydrogen chloride.
H2 (g) + Cl2 (g)  HCl(g)
2. Count the atoms of each substance for the reactants
and products.
REACTANTS
PRODUCTS
H:
H :
Cl :
Cl :
3. Place coefficients (ONLY IN FRONT OF A
REACTANT OR PRODUCT) to make number of
atoms of each element equal on both sides of the
equation. (#atoms= coefficient x subscript)
H2 (g) + Cl2 (g)  HCl(g)
REACTANTS
PRODUCTS
H:
H :
Cl :
Cl :
4. Write coefficients in lowest ratio possible
H2 (g) + Cl2 (g)  2 HCl(g)
5. Check your answer.
REACTANTS
PRODUCTS
H:
2
H :
Cl :
2
Cl : 2
2
Practice
1. Write a balanced chemical equation if sulfur trioxide
when heated yields sulfur dioxide and oxygen.
2. Write a balanced chemical equation for:
Iron (III) oxide iron (II) oxide + oxygen
3. Balance the following reaction
CaO + H2O  Ca(OH)2
4. Reactions for gummy bear sacrifice
KClO3  KCl + O2
CW p balancing equations handout