Energy Changes &
Reactions
Unit 7 - Chapter 7.3
Chemical Bonds and Energy
Chemical reactions:
Breaking chemical bonds in the
reactants
Forming new chemical bonds in the
products.
Chemical energy: Energy stored in the
chemical bonds of a substance.
True for both reactants and products
Chemical Bonds and Energy
 Energy changes in reactions  determined by
changes in chemical bonding.
 Example: combustion of propane
C3H8 + 5O2  3CO2 + 4H2O
 Bonds in reactants are broken (C3H8 and O2)
Propane molecules have C-C and C-H single bonds
Oxygen molecules have O=O double bonds
 Bonds in products are formed (CO2 and H2O)
Carbon dioxide molecules have C=O double bonds
Water molecules have H-O single bonds
Chemical Bonds and Energy
Breaking Bonds
In combustion of propane, all chemical bonds
in both reactants (propane and oxygen) must
be broken.
Breaking chemical bonds requires energy.
A spark provides enough energy to get the
reaction started.
Chemical Bonds and Energy
Chemical Bonds and Energy
Forming Bonds: burning propane
C3H8 + 5O2  3CO2 + 4H2O
Each molecule of propane burned forms:
3 molecules of carbon dioxide
6 C=O (double) bonds formed
4 molecules of water.
8 O-H (single) bonds formed
Forming of chemical bonds releases energy.
Heat and light given off because new chemical
bonds formed
Chemical Bonds and Energy
Exothermic and Endothermic Reactions
During a chemical reaction, energy is
either released or absorbed.
Exothermic Reaction – Activity 3
Releases energy to its surroundings
“Exo” means Energy “exits” from the
reaction
Energy required to break reactant bonds 
less than energy released as products
formed
Chemical Bonds and Energy
 Exothermic reaction: chemical energy of reactants
is greater than chemical energy of products.
Chemical Bonds and Energy
 Chemical energy peaks before reactants change
into products.
 Peak Height = energy required to break the chemical
bonds of the reactants.
 Particles must collide with enough energy to break these
bonds, or the reaction will not occur.
Chemical Bonds and Energy
 Combustion is an extremely exothermic
reaction.
Example: 1 mole of propane + 5 moles of
oxygen  2220 kJ (kilojoules) of heat is
released.
C3H8 + 5O2
3CO2 + 4H2O + 2220 kJ
 1 joule = 1watt for 1 second
 100 watt light bulb for 10 seconds needs 1 kJ
 44g propane releases enough energy for >6 hrs
Chemical Bonds and Energy
Endothermic Reaction – Activity 1 & 2
Absorbs energy from surroundings
“Endo” means Energy goes “into” the
reaction
Energy required to break reactant
bonds  greater than energy released
by formation of products.
Chemical Bonds and Energy
Endothermic reaction - energy of products is
greater than energy of reactants.
Chemical Bonds and Energy
Endothermic Reaction
Example: Emergency Cold Packs
Liquid inside the cold pack is water.
In the water is another plastic bag or
tube containing ammonium-nitrate
fertilizer.
When you hit the cold pack, it breaks
the tube so that the water mixes with
the fertilizer.
This mixture creates an endothermic
reaction -- it absorbs heat.
The temperature of the solution falls to
about 35 F for 10 to 15 minutes.
Chemical Bonds and Energy
Conservation of Energy:
Total Energy is same before and after a
reaction
Exothermic reaction:
Chemical energy of reactants = heat +
chemical energy of products.
Endothermic reaction:
Chemical energy of reactants + heat =
chemical energy of products.