Chemical Reactions
Chemical Equations
 Reactants are the substances entering a reaction
 Written on left
 Products are the result
 Written on right
 A chemical equation uses symbols and formulas to
represent identities and amounts of products and
reactants in a chemical equation
Indicators of Chemical Reaction
 Change in temperature
 Change in color
 Smell
 Formation of a precipitate
 Formation of bubbles
Diatomic Molecules
 Always occur in 2’s when alone
 Br I N Cl H O F
Equations
 Word equations: use words and symbols to describe
a reaction
 Formula equations: use formulas and symbols to
describe a reaction
 Write a word & formula equation for the following:
during electrolysis, water decomposes into hydrogen
and oxygen gas
Symbols
Description
Symbol
Yields

Reversible reaction

Solid
(s)
Liquid
(l)
Gas
(g)
Aqueous-dissolved in water
(aq)
Reactants are heated
Pressure is 2 atm
Pressure is higher than normal
Temp is 25 degrees C
Catalyst MnO2 was used
25 C 
Balancing Equations
 Law of Conservation of Mass must be satisfied
 Equal # of atoms on both sides
 Balance by adjusting coefficients
 Do not alter subscripts
 Balance O’s and H’s last
Counting Atoms
 Distribute coefficient to every atom
 2 H2O - 4 H’s and 2 O’s
 3 Ca (NO3)2 – 3 Ca’s, 6 N’s, 18 O’s
Example
 __ Al2O3  ___ Al + ___ O2
 1. Count atoms- 2 Al’s on left and 1 on right
 2. Balance by placing 2 in front of Al on right
__ Al2O3  2 Al + ___ O2
 3. Count atoms- 3 O’s on left and 2 O’s on right
 4. Balance by placing 2 in front of compound on left
and 3 in front of the O’s on the right to make 6 O’s on
each side
Example
2 Al2O3  2 Al + 3 O2
5. Now Al’s are not balanced. Make the one on right a 4
2 Al2O3  4 Al + 3 O2
Equation is now balanced!
Reaction Energy
 1. A catalyst is used to speed up or slow down a
reaction, but it does not react
 2. Activation energy is the amount of energy needed
for a reaction to start
 3. ∆H is the symbol used to represent a change in
energy


+ ∆ = exothermic (heat is released)
- ∆ = endothermic (heat is absorbed)
Energy Diagrams
 Exothermic- energy is released and is lower than starting

*catalyst lowers the hump (activation energy)
 Endothermic- energy is absorbed and is higher than starting
Reaction Types: Single Replacement
 One element replaces another
 Reactants: element + compound
 A + BX  AX + B
Reaction Types: Decomposition
 One compound splits into 2 pieces
 Can be elements or simpler compounds
 AB  A + B
Reaction Types: Synthesis
 Reverse of decomposition reaction
 2 pieces join together to produce one, more complex
compound

Pieces can be elements or simpler compounds
 A + B  AB
Reaction Types: Double Replacement
 Cations and anions of 2 different compounds switch
places
 AB + XY  AY + XB
 A, X= cations
 Y, B= anions
Reaction Types: Combustion
 Reaction of O2 gas with anything, normally a
hydrocarbon (compound with H + C)
 CxHy + O2  CO2 + H2O