Download Chemical Reactions

Chemical
Reactions
Chapter 14 – 2
Chemical Formulas and
Equations
Essential Questions
• How are chemical equations
interpreted, written and
balanced?
• How does a balanced equation
show the law of conservation of
mass?
2
Definitions
• Chemical formula – chemical
symbols & numbers to represent
a substance
• Chemical equation – uses
chemical symbols and formulas
as a shortcut to describe a
chemical reaction
• Reactant – a substances or
molecule that participates in a
chemical reaction
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Definitions, continued
• Product – what is made during
the chemical reaction
• Law of conservation of mass (or
matter)- mass is neither created
or destroyed in ordinary chemical
or physical changes
• Subscript - number (representing
atoms) written below and to the
right of a chemical symbol
4
Physical Properties
 color
 melting point
 boiling point
 electrical conductivity
 specific heat
 density
 state (solid, liquid, or gas)
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Physical Changes
• melting
• boiling
• condensation
No change occurs in the identity
of the substance
Examples:
Ice , rain, and steam are all water
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Chemical Changes
Atoms in the reactants are
rearranged to form one or more
different substances
Old bonds are broken; new
bonds form
Examples:
Fe and O2 form rust (Fe2O3)
Ag and S form tarnish (Ag2S)
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Prefixes
• di – means two
• mono – means one; no prefix
means one atom
• Tri – means three
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Writing formulas for
Covalent Compounds
• Covalent compounds are two
nonmetals
• Use prefixes to represent a
number (tells how many atoms)
• Study table on page 393
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Writing formulas for Ionic
Compounds
• Ionic compounds are formed by a
metal and a nonmetal
• Charge must be zero
• Ex: magnesium (Mg) has two
valence electrons, so has a
change of +2; chlorine needs one
val. el. to make a full set, so it’s
charge is -1. Therefore you need
2 chlorine ions to equal zero:
MgCl2
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10
Writing a Chemical
Equation
Chemical symbols give a “before-andafter” picture of a chemical reaction
Reactants – what you start with
Product – what you get
Reactants
C
+
Products
O2
Carbon reacts with
oxygen
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CO2
to form
carbon
dioxide
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Law of Conservation of
Mass
• All chemical equations MUST be
balanced because…
• The Law of Conservation of
Mass (or Matter) states that
mass is neither created nor
destroyed in physical or chemical
changes
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12
Reading A Chemical
Equation
4 NH3 + 5 O2
4 NO + 6 H2O
Four molecules of NH3 react with
five molecules O2 to produce four
molecules NO and six molecules
of H2O
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Balanced Equations
Same numbers of each type of
atom on each side of the
equation
Al + S
2Al + 3S
Al2S3
Not Balanced
Al2S3
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Balanced
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Balancing Equations w/
Coefficients
Coefficients in front of formulas
balance each type of atom; they are
multiplied to any subscripts
4NH3 +
5O2
4NO + 6H2O
4N
=
4N
12 H
=
12 H
10 O
=
10 O
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Steps in Balancing an
Equation
Fe3O4 + H2
Fe + H2O
Fe: Fe3O4 + H2
3 Fe + H2O
O: Fe3O4 + H2
3 Fe + 4 H2O
H: Fe3O4 + 4 H2
3 Fe + 4 H2O
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Brain Check (Mini Quiz)
Fe3O4 + 4 H2
3 Fe + 4 H2O
1. Number of H atoms in 4 H2O
a) 2
b) 4
c) 8
2. Number of O atoms in 4 H2O
a) 2
b) 4
c) 8
3. Number of Fe atoms in Fe3O4
a) 1
b) 3
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c) 4
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