Section 1: Reactions and Equations
Chemical reactions are represented by balanced chemical equations.
K
What I Know
W
What I Want to Find Out
L
What I Learned
Essential Questions
• What is evidence of chemical change?
• How are chemical reactions represented?
• Why do chemical equations need to be balanced and how is this
accomplished?
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Reactions and Equations
Vocabulary
Review
New
• chemical change
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chemical reaction
reactant
product
chemical equation
coefficient
Reactions and Equations
Chemical Reactions
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The process by which one or more substances are rearranged to form
different substances is called a chemical reaction. Evidence that a chemical
reaction may have occurred:
– Change in temperature
– Change in color
– Odor
– Gas bubbles
– Appearance of a solid
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Reactions and Equations
Representing Chemical Reactions
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Chemists use statements called equations
to represent chemical reactions.
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Reactants are the starting substances.
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Products are the substances formed in
the reaction.
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This table summarizes the symbols used
in chemical equations.
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Reactions and Equations
Representing Chemical Reactions
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In word equations, aluminum(s) + bromine(l) → aluminum bromide(s)
reads as “aluminum and bromine react to produce aluminum bromide”.
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Skeleton equations use symbols and formulas to represent the reactants
and products.
Al(s) + Br(l) → AlBr3(s)
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Both word and skeleton equations lack information about how many atoms
are involved in the reaction.
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Reactions and Equations
Representing Chemical Reactions
•
A chemical equation is a statement that uses chemical formulas to show
the identities and relative amounts of the substances involved in a
chemical reaction.
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Reactions and Equations
Balancing Chemical Equations
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This figure shows the balanced equation for the reaction between
aluminum and bromine.
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A coefficient in a chemical equation is the number written in front of a
reactant or product, describing the lowest whole-number ratio of the
amounts of all the reactants and products.
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Reactions and Equations
Interactive Table – Balancing Chemical Equations
Go to your ConnectED resources to play Interactive Table: Steps for
Balancing Equations.
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Reactions and Equations
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.
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.
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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)
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Count the atoms of each element in the
reactants.
1 Na, 1 O, 1 H, 1 Ca, 2 Br
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Count the atoms of each element in the
products.
1 Na, 2 O, 2 H, 1 Ca, 1 Br
Reactions and Equations
Writing a Balanced Chemical
Equation
SOLVE FOR THE UNKNOWN
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Insert the coefficient 2 in front of NaOH
to balance the hydroxide ions.
2NaOH + CaBr2 → Ca(OH)2 + NaBr
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Insert the coefficient 2 in front of NaBr
to balance the Na and Br atoms.
2NaOH + CaBr2 → Ca(OH)2 + 2NaBr
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Write the coefficients in their lowestpossible ratio.
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)
The ratio of the coefficients is 2:1:1:2.
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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.
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Reactions and Equations
Balancing Chemical Equations
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The most fundamental law in chemistry is the law of conservation of mass.
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Balanced equations show this law.
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Reactions and Equations
Review
Essential Questions
•
What is evidence of chemical change?
•
How are chemical reactions represented?
•
Why do chemical equations need to be balanced and how is this
accomplished?
Vocabulary
• chemical reaction
• reactant
Copyright © McGraw-Hill Education
• product
• chemical equation
• coefficient
Reactions and Equations