Chemical Reactions
Basics of Chemical Reactions:
1. Chemical Reactions – occur when substances undergo chemical changes to form new
substances.
2. Signs that a chemical reaction has taken place:
A. Production of a gas/bubbling
B. Change in color
C. Energy released as sound, heat or light
D. Disappearance of a substance
E. Appearance of a new substance
3. Reactants are the substances that undergo the chemical change (left side of arrow).
Products are the substances that are formed as a result of the chemical change
(right side of arrow).
Reactant + reactant  product + product
Energy in Reactions
1. Chemical Energy is the energy stored within the bonds that holds atoms together.
2. In many reactions, energy is needed to start the reaction – activation energy.
Ex. Why shouldn’t there be any sparks when you are pumping gas?
3. Law of conservation of energy – total energy before the reaction is equal to the total
energy after the reaction. Heat can be absorbed from or released to the
environment.
4. Exothermic reaction – releases heat to the environment as the reaction takes place.
Examples: hand warmers, explosions, burning, rusting, glow-sticks
5. Endothermic reaction – absorbs heat as the reaction takes place (takes the heat from the
surroundings, so the surroundings get cold).
Examples: cold packs, photosynthesis, melting
Balancing chemical equations:
Law of conservation of matter – Matter cannot be created or destroyed. Therefore,
you need to have the same type and number of atoms on both sides of the arrow.
+
Note: You cannot change any subscripts when balancing chemical equations, you can only
add coefficients in front of the element or
compound.
1. Write the correct formulas for any compounds and the symbols for any single elements.
2. Be sure that any single diatomic element has a subscript 2 behind it.
Diatomic elements:
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H2
O2
N2
Cl2
F2
Br2
I2
3. Count the number of each type of atom on each side of the arrow. If the number is
equal, that element is balanced. If not, then you must add coefficients until the
numbers are balanced.
Example:
Aluminum plus Oxygen produces Aluminum oxide.
Step 1
Al + O  Al2O3
Step 2
Al + O2  Al2O3
Step 3
Al
+
O2
1 aluminum
2 oxygen

Al2O3
2 aluminum
3 oxygen
Not balanced!
Al
+
3O2
1 aluminum
6 oxygen

2Al2O3
4 aluminum
6 oxygen
Not balanced!
4Al
+
3O2 
4 aluminum
6 oxygen
2Al2O3
4 aluminum
6 oxygen
Balanced!