January 11, 2015
Unit 4: Chemical Reactions
What is a chemical reaction?
• the process by which the atoms of one or more
substances are rearranged to form different
substances
• chemical change
What chemical reactions are important in
your life?
Chemical change: new
substances are formed
Physical change:
substance does not
change identity
January 11, 2015
Evidence of Chemical Reaction (or chemical
change)
1.
2.
3.
4.
5.
6.
Temperature change
Light
Color change
Production of gasses
Production of solids (precipitate)
Odor change
What evidence of a chemical
reaction do you see in a fire?
What evidence of a chemical
reaction do you see when you
roast a marshmallow?
January 11, 2015
Representing Chemical Reactions
• As chemists, we can describe chemical reactions in
different ways.
> In a sentence
> Word equations
> Skeleton equation
> Chemical equations
January 11, 2015
Representing Chemical Reactions
• Reactants are the starting substances of a reaction.
• Products are the substances formed during the
reaction.
reactant 1 + reactant 2
When you have more
than one reactant or
product, use a plus
sign
product 1 + product 2
This arrow separates the
reactants from the products
(kind of like the equal sign in
math)
January 11, 2015
1. In a sentence
"Solid iron and chlorine gas react to produce solid iron
(III) chloride."
2. Word Equation
iron(s) + chlorine(g)
iron (III) chloride(s)
3. Skeleton Equation: uses chemical formulas
Fe(s) + Cl2(g)
The letters in parentheses
shows you the state of
matter:
FeCl3(s)
(s) = solid state
(l) = liquid state
(g) = gas state
(aq) = water solution
January 11, 2015
Example 1:
HON17: Always diatomic
Write skeleton equations for these word equations.
1. hydrogen(g) + bromine(g)
2. carbon monoxide(g) + oxygen(g)
3. potassium chlorate(s)
hydrogen bromide(g)
carbon dioxide(g)
potassium chloride(s) + oxygen(g)
January 11, 2015
4. Chemical equation: Balanced equation that shows
the reactants and products, and the relative amounts
involved in the reaction.
> Conservation of mass (mass of reactants = mass
of products)
> Need to have same # of atoms of each element in
the reactants and products.
January 11, 2015
How to balance chemical equations
1.
2.
Write the unbalanced (skeleton) equation.
Count the atoms of the elements in the reactants
and products.
3. Use coefficients to make the # of atoms of each
element equal on both sides of the equation.
> Coefficients:numbers written in front of a
reactant or product. Tells us the number of
particles of the substance involved in the reaction.
4.
5.
Make sure coefficients are in their lowest
possible ratio.
Check your work!
unbalanced equation: Fe(s) + Cl2(g)
chemical equation:
Fe(s) +
Cl2(g)
FeCl3(s)
FeCl3(s)
January 11, 2015
Example 2: Write the balanced chemical
equation for each.
1. Hydrogen gas and chlorine gas combine to form
hydrochloric acid (HCl)
2. __CS2(l) + __O2(g)
__CO2(g) + __SO2(g)
January 11, 2015
Example 2 (cont): Balance the following
H2(g) +
CS2(l) +
Cl2(g)
O2(g)
HCl(g)
CO2(g) +
SO2(g)
January 11, 2015
Example 2 (cont): Balance the following
CaO(s) + H2O(l)
NaOH(aq) +
CaBr2(aq)
Ca(OH)2(s)
Ca(OH)2(s) +
NaBr(aq)
*When balancing equations with Polyatomic Ions
that appear in the reactant and product, keep it
together as one "unit" and balance.
January 11, 2015