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Chemical
Reactions
Mr. Matthew Totaro
Legacy High School
Honors Chemistry
Chemical Reactions
• Reactions involve
chemical changes in
matter resulting in new
substances.
• Reactions involve
rearrangement and
exchange of atoms to
produce new molecules.
 Elements are not
changed into new
elements during a
reaction.
Thermite Reaction
2
Example of a Chemical Reaction
Reactants

Products
3
Evidence for a Chemical Reaction
• Look for evidence of a new substance.
• Visual clues (permanent).
Color change.
Precipitate formation.
Solid that forms when liquid solutions are mixed.
Gas bubbles.
Large energy changes.
Container becomes very hot or cold (transfer of heat).
Emission of light.
• Other clues.
New odor.
Whooshing sound from a tube.
Permanent new state.
4
Evidence of a Chemical Change
Release or Absorption of Heat
Color Change
Formation of a Gas
Emission of Light
Formation of Solid Precipitate 5
Evidence of Chemical Change,
Continued
• In order to be
ABSOLUTELY
SURE that a chemical
reaction has taken place,
you need to go down to the
molecular level and analyze
the structures of the
molecules at the beginning
and at the end.
Is boiling water
a chemical change?
6
Practice—Decide Whether Each of the
Following Involve a Chemical Reaction.
• Photosynthesis
•
•
•
•
•
Yes, CO2 and H2O combine into carbohydrates
Heating sugar until it turns black Yes, sugar decomposing
Heating ice until it turns liquid
No, molecules still same
Yes, food decomposing and combining
Digestion of food
with stomach acid
Dissolving sugar in water
No, molecules still same
Burning of alcohol in a flambé dessert
Yes, alcohol combining with O2 to make CO2 and H2O
7
Chemical Equations
Concise representation of a chemical reaction
The Combustion of Methane
• Methane gas burns to produce carbon
dioxide gas and gaseous water.
Whenever something burns it combines with
O2(g).
9
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
Reactants appear on the
left side of the equation.
CO2 (g) + 2 H2O (g)
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
Products appear on the right
side of the equation.
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (g)
The states of the reactants and products are
written in parentheses to the right of each
compound.
Anatomy of a Chemical Equation
CH4 (g) + 2 O2 (g)
Coefficients are inserted to
balance the equation.
CO2 (g) + 2 H2O (g)
Subscripts and Coefficients Give
Different Information
• Subscripts tell the number of atoms of
each element in a molecule
Subscripts and Coefficients Give
Different Information
• Coefficients tell the number of molecules
Symbols Used in Equations
• Symbols used to indicate state after
chemical.
(g) = gas; (l) = liquid; (s) = solid.
(aq) = aqueous = dissolved in water.
• Energy symbols used above the arrow for
decomposition reactions.
 ∆ = heat.
 hv = light.
shock = mechanical.
elec = electrical.
17
Law of Conservation of Mass
“We may lay it down as an
incontestable axiom that, in all the
operations of art and nature,
nothing is created; an equal amount
of matter exists both before and
after the experiment. Upon this
principle, the whole art of
performing chemical experiments
depends.”
--Antoine Lavoisier, 1789
Conservation of Mass in a
Reaction
• In a chemical reaction, matter cannot be
created or destroyed.
Therefore, the total mass cannot change.
And the total mass of the reactants will be
the same as the total mass of the products.
• In a chemical reaction, all the atoms
present at the beginning are still present
at the end.
If all the atoms are still there, then the mass
will not change.
19
Balancing Example
• When magnesium metal burns in air, it
produces a white, powdery compound
magnesium oxide.
Mg(s) + O2(g) → MgO(s)
20
Another Balancing Example
•
Under appropriate conditions at 1000°C, ammonia gas
reacts with oxygen gas to produce gaseous nitrogen
monoxide and steam
NH3(g) + O2(g) → NO(g) + H2O(g)
21
Reaction
Types
Synthesis (Combination)
Reactions
• the combination of 2 or more substances
to form a compound
• only one product
A + B  AB
Synthesis (Combination)
Reactions
• Two or more
substances
react to form
one product
• Examples:
N2 (g) + 3 H2 (g)  2 NH3 (g)
C3H6 (g) + Br2 (l)  C3H6Br2 (l)
2 Mg (s) + O2 (g)  2 MgO (s)
2 Mg (s) + O2 (g)  2 MgO (s)
Decomposition Reactions
• a compound breaks down into 2 or more
simpler substances
• only one reactant
AB  A + B
Decomposition Reactions
• One substance breaks
down into two or more
substances
• Examples:
CaCO3 (s)  CaO (s) + CO2 (g)
2 KClO3 (s)  2 KCl (s) + O2 (g)
2 NaN3 (s)  2 Na (s) + 3 N2 (g)
Combustion Reactions
• the burning of any substance in O2 to
produce heat
A + O2  B
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Combustion Reactions
• Rapid reactions that
produce a flame
• Most often involve
hydrocarbons
reacting with oxygen
in the air
• Examples:
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (g)
Whoosh Bottle Demo is
a Combustion Reaction
30
Single Replacement
• one element replaces another in a
compound
metal replaces metal (+)
nonmetal replaces nonmetal (-)
A + BC  B + AC
C. Johannesson
Single Replacement
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
C. Johannesson
Double Replacement
• ions in two compounds “change partners”
• cation of one compound combines with
anion of the other
AB + CD  AD + CB
C. Johannesson
Double Replacement
Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)
C. Johannesson
Practice—Classify the Following Reactions as
Synthesis, Decomposition, Single Displacement,
or Double Displacement.
3 Mg(s) + 2 FeCl3(aq)  3 MgCl2(aq) + 2 Fe(s)
CO2(g) + H2O(l)  H2CO3(aq)
3 KOH(aq) + H3PO4(aq)  K3PO4(aq) + 3 H2O(l)

CaCO3 ( s ) 

CaO(s )  CO 2 ( g )
35
Practice—Classify the Following Reactions as
Synthesis, Decomposition, Single Displacement,
or Double Displacement, Continued.
3 Mg(s) + 2 FeCl3(aq)  3 MgCl2(aq) + 2 Fe(s)
Single displacement.
CO2(g) + H2O(l)  H2CO3(aq)
Synthesis.
3 KOH(aq) + H3PO4(aq)  K3PO4(aq) + 3 H2O(l)
Double displacement.

CaCO3 ( s ) 

CaO(s )  CO 2 ( g )
Decomposition.
36
Reaction
Prediction
Synthesis Reactions
A + B  AB
Two substances combine to form a single compound.
Al (s) + O2 (g) 
2 elements combine to form a single compound
Decomposition Reactions
AB  A + B
One compound breaks down into multiple substances
(usually because of the presence of heat).
K2CO3 (s)  
∆
Metal carbonate + heat → metal oxide + carbon dioxide gas
Combustion Reactions
A + O2  CO2 + H2O
CH4 (s) + O2 (g) 
One substance reacts with Oxygen to form Carbon Dioxide and Water
Single Replacement Reactions
A + BC  B + AC
Cu (s) + AgNO3 (aq) 
A more active metal replaces a metal in a compound to form a
new compound and an element
Double Replacement Reactions
AB + CD  AD + CB
AgNO3 (aq) + KCl (aq) 
Two compounds exchange partners to form two new compounds