Lesson 1
Reactions and Equations
Focus Question
How are chemical reactions modeled?
New Vocabulary
chemical reaction
reactant
product
chemical equation
coefficient
Review Vocabulary
chemical change: a process involving one or more
substances changing into a new substance
Chemical Reactions
• The process by which the atoms of one or
more substances are rearranged to form
different substances is called a chemical
reaction.
• A chemical reaction is another name for a
chemical change.
Chemical Reactions
Evidence of a Chemical Reaction
• A temperature change can indicate a chemical
reaction. Many reactions release energy in
the form of heat and light. Other chemical
reactions absorb heat.
• Color change can indicate a chemical reaction.
• Odor, gas bubbles, and the formation of a
solid are other indications of chemical
change.
Representing Chemical Reactions
• Equations are used to
represent chemical
reactions.
• Equations show a
reaction’s reactants
(starting substances) and
products (substances
formed).
• The table shows the
symbols used in chemical
equations.
Representing Chemical Reactions
Word Equations
aluminum(s) + bromine(l) → aluminum bromide(s)
• In this word equation, aluminum(s) + bromine(l)
→ aluminum bromide(s) reads as “aluminum and
bromine react to produce aluminum bromide.”
• Word equations lack information about the
number of atoms involved.
Representing Chemical Reactions
Skeleton Equations
Al(s) + Br(l) → AlBr3(s)
• Skeleton equations use symbols and formulas to
represent reactants and products.
• These equations also lack information about the
number of atoms involved.
Representing Chemical Reactions
Chemical Equations
A chemical equation uses chemical formulas to
show the identities and relative amounts of the
substances involved in a chemical reaction.
Balancing Chemical Equations
A coefficient in a chemical equation is the number
written in front of a reactant or product.
Balancing Chemical Equations
Steps for Balancing Equations
1. Write the skeleton equation for the reaction.
2. Count the atoms of the elements in the reactants.
3. Count the atoms of the elements in the products.
4. Change the coefficients to make the number of
each element equal on both sides of the equation.
This shows that atoms are conserved.
5. Write the coefficients in their lowest possible
ratio.
6. Check your work.
Writing a Balanced Chemical Equation
Use with Example Problem 1.
Problem
Write the balanced chemical equation for
the reaction in which aqueous sodium
hydroxide and aqueous calcium bromide
react to produce solid calcium hydroxide and
aqueous sodium bromide.
SOLVE FOR THE UNKNOWN
Write the skeleton equation for the chemical
reaction. Be sure to put the reactants on the
left side of the arrow and the products on the
right. Separate the substances with plus signs,
and indicate their physical states.
NaOH(aq) + CaBr2(aq) → Ca(OH)2(s) + NaBr(aq)
•
Response
ANALYZE THE PROBLEM
You are given the reactants and products in a
chemical reaction. Start with a skeleton equation,
and use the steps for balancing chemical
equations.
Count the atoms of each element in the
reactants.
1 Na, 1 O, 1 H, 1 Ca, 2 Br
•
Count the atoms of each element in the
products.
1 Na, 2 O, 2 H, 1 Ca, 1 Br
Writing a Balanced Chemical Equation
SOLVE FOR THE UNKNOWN
•
Insert the coefficient 2 in front of NaOH to
balance the hydroxide ions.
2NaOH + CaBr2 → Ca(OH)2 + NaBr
•
Insert the coefficient 2 in front of NaBr to
balance the Na and Br atoms.
2NaOH + CaBr2 → Ca(OH)2 + 2NaBr
•
Write the coefficients in their lowestpossible ratio.
The ratio of the coefficients is 2:1:1:2.
•
Check to make sure that the number of
atoms of each element is equal on both
sides of the equation.
Reactants: 2 Na, 2 OH, 1 Ca, 2 Br
Products: 2 Na, 2 OH, 1 Ca, 2 Br.
EVALUATE THE ANSWER
The chemical formulas for all substances are
written correctly. The number of atoms of
each element is equal on both sides of the
equation. The coefficients are written in the
lowest possible ratio. The balanced chemical
equation for the reaction is
2NaOH(aq) + CaBr2(aq) → Ca(OH)2(s) +
2NaBr(aq)
Quiz
1. Which of the following is not an example of a
chemical reaction?
A
rusting iron
B
burning wood
C
boiling water
D
rotting food
CORRECT
Quiz
2. Which of the following is not true about coefficients
in a balanced chemical equation?
A
They are numbers
written in front of
reactants or products.
C
They describe the
highest whole number
ratio of the amounts of
all reactants and
products.
CORRECT
B
They are usually whole
numbers.
D
They are not usually
written if the value is 1.
Quiz
3. Which observation is NOT physical evidence that a
chemical reaction has occurred?
A
There is an odor change.
B
A solid is no longer magnetic.
C
A solid melts.
D
A solid changes color.
CORRECT
Quiz
4. Which of the following is NOT true about balancing
chemical equations?
A
Subscripts should never
be changed to balance
the equation.
C
Coefficients are changed
to make the number of
each element equal on
both sides.
B
Coefficients are written
in their lowest possible
ratio.
D
Subscripts are changed
to make the number of
each element equal on
both sides.
CORRECT