Pre-AP Chemistry
Introduction to
Chemical Reactions
Chemical reactions
 Processes that change one or more
substances into different substances.
 Described by chemical equations.
 Represented by chemical symbols and
formulas to show amounts of
reactants (starting materials) and
products (end materials)
 Ex) 2 H2O(l) ---> 2 H2 (g) + O2 (g)
Chemical Reaction
Energy and reactions
 Energy must be added to form
chemical bonds. Breaking bonds
releases energy.
 Energy is conserved in chemical
reactions.
 Reactions that release energy are
exothermic.
 Reactions that absorb energy from
the surroundings are endothermic.
Evidence of a chemical reaction
 Substance produced different than
reactants
 Heat and/or light is released
 Production of a gas
 Color change
 Precipitate is formed
Reaction Types
 Synthesis- a reaction of at least two
substances that forms a new, more
complex compound
Ex) A + B ---> AB
Na + Cl---> NaCl
 Decomposition- a single compound
undergoes a reaction that produces
two or more simpler substances.
Ex) AB ---> A + B
2H2O2---> 2H2O + O2
Reaction types, cont.
 Single-displacement- one element
replaces a similar element in a
compound.
Ex) XA + B ---> AB + X
3Mg + Fe2O3 ---> 2Fe + 3MgO
Single-Displacement Reaction
Types of Reactions, cont.
 Double-displacement- the ions of
two compounds exchange places in
an aqueous solution to form two new
compounds. Acid-base neutralization
is an example
Ex) AX + BY ---> AY + BX
HCl + NaOH ---> NaCl + H2O
Types of Reactions, cont.
 Combustion- a substance combines
with oxygen, releasing a large
amount of energy in the form of light
and heat.
 Burning of propane and gasoline are
examples.
C3H8 + 5 O2 ---> 3 CO2 + 4 H2O