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Chapter 9
Chemical Reactions
9.1: Reactions & Equations
Objectives
• Recognize evidence of chemical change
• Represent chemical reactions with equations
• Balance chemical equations
What is a chemical reaction?
• It is the process by which the atoms of one
or more substances are rearranged to form
new, different substances.
• A chemical reaction is a chemical change!
The 4 signs
There are 4 kinds of observations that indicate
a chemical reaction may have taken place.
1.A change in energy: A
temperature change is often
used to determine if
energy was released
or absorbed. In addition,
release of light also indicates
an energy change.
The 4 signs
• 2. A color change
The 4 signs
3. Production of a gas may be observed as
bubbling or visible fumes. Detection of an
odor also indicates a gas was produced.
The 4 signs
4. Formation of a precipitate (or appearance
of a solid) in a solution that was
previously clear.
Practice
• Recall burning magnesium as part of a lab
activity. What evidence was there that a
chemical reaction occurred?
Review
• Equations represent chemical reactions.
• Reactants (on the
left) are the starting
substances.
• Products (on the
right) are the
substances formed
during the reaction.
Review
• An arrow shows the way the reaction
progresses; It is found between the reactants
and products and is usually read as “yields”.
• “+” signs are placed between each reactant
and product; They are read as “and”.
Review
• Symbols are often used after each reactant
and product to indicate their state.
– (s) for solid
– (l) for liquid
- (g) for gas
- (aq) for water solution
Review
• Coefficients are
whole numbers
written in front
of formulas in the
equation. They
indicate the
number of atoms,
molecules, or ions
of that substance
that are involved in the reaction.
Practice
• Identify the reactants and products and, using
words not formulas, interpret the following
equations.
 Mg (s) + H2SO4 (aq)  H2 (g) + MgSO4 (aq)
 C(s) + 2ZnO (s)  2Zn (s) + CO2 (g)
Types of Equations
• Word Equation: words are used to summarize
the reaction.
– Ex.: iron + chlorine
– Ex.: carbon + sulfur
iron (III) chloride
carbon disulfide
• Skeleton equation: Chemical formulas are
substituted for words.
– Ex.: Fe + Cl2
– Ex.: C + S
FeCl3
CS2
Skeleton Equations
• Skeleton equations are also called
unbalanced equations because they do not
obey the Law of Conservation of Mass.
– Fe + Cl2
+
FeCl3
1 iron atom 2 chlorine atoms
1 iron atom
3 chlorine atoms
Practice Problems
• Write skeleton equations for the following:
1 - hydrogen + bromine
hydrogen bromide
2 - carbon monoxide + oxygen carbon dioxide
3 - potassium chlorate
potassium chloride +
oxygen
4 – Manganese metal reacts with sulfuric acid to
produce manganese (II) sulfate and hydrogen
gas.
5 – Silver nitride is produced when silver reacts
with atmospheric nitrogen.
Types of Equations
• Balanced chemical equation: This equation
shows that the number of atoms of each
reactant and each product is equal on both
sides of the arrow.
– 2Fe + 3Cl2
2FeCl3
+
2 iron atoms
2 iron atoms 6 chlorine atoms
6 chlorine atoms
Balanced Chemical Equation
• This is defined as a statement that uses
chemical formulas to show the identities
and relative amounts of the substances
involved in a reaction.
• Balanced chemical equations obey the
Law of Conservation of Mass.
Steps to Balancing Equations
1 - Write a skeleton equation.
Problem: Hydrogen and chlorine combine
to form hydrogen chloride
H2 + Cl2
HCl
*Recall there are 7 elements that
exist as diatomic molecules.
Steps to Balancing Equations
2 - Determine the number and kinds of atoms
in the reactants.
3 - Determine the number and kinds of atoms
in the products.
H2 + Cl2
HCl
2H
1H
2 Cl
1 Cl
Steps to Balancing Equations
4 - Change the coefficients (never the
subscripts) until the number of atoms of
each element is equal on both sides of the
equation.
H2 + Cl2
2HCl
2H
2 1H
2 Cl
2 1 Cl
Steps to Balancing Equations
5 - Make sure the coefficients are in the
lowest possible ratio. Reduce if necessary.
6 - Go back and add the atoms up again.
Make sure the number of atoms of each
element is equal on both sides of the arrow.
If you cannot balance the equation, go
back and make sure your formulas are
written correctly!!
Practice Problems
1 - Sodium hydroxide and calcium bromide
react to produce solid calcium hydroxide
and sodium bromide.
2 - Carbon disulfide reacts with oxygen gas to
produce carbon dioxide and sulfur dioxide.
3 - Zinc and sulfuric acid react to produce
hydrogen gas and zinc sulfate.
4 - Iron (III) chloride reacts with sodium
hydroxide to produce iron (III) hydroxide
and sodium chloride.