PS-4.7   Summarize characteristics of balanced chemical equations (including conservation of mass and changes in energy in the form of heat-that is, exothermic or endothermic reactions)
Key Concepts:
Chemical equation:  balanced equation, conservation of mass
Energy change: exothermic, endothermic  
    Both physical and chemical changes can be endothermic or exothermic. Endothermic physical changes are usually associated with a phase change. The substances undergoing the change take in heat from their surroundings. The heat produces an increase in the potential energy of the atoms or molecules. Examples of endothermic physical changes are boiling, evaporation, and melting. Diagram "A" below illustrates an endothermic physical change.  Note that the heat is absorbed and this absorption of heat results in a phase change, evaporation.
    Diagram "B" illustrates an exothermic physical change. Note that the heat is released and this release of heat results in a phase change, condensation.

                           LIQUID                             GAS
Diagram A: Boiling or evaporation of water. Heat is absorbed during an endothermic physical change. The absorbed heat produces an increase in  Potential energy. As potential energy increases the distance between the molecules increases.

            GAS                                                    LIQUID
Diagram B: Condensation of water vapor. Heat is released during an exothermic physical change. This allows a decrease in the potential energy and the molecules move closer together.
   The two diagrams below illustrate endothermic and exothermic chemical changes. Diagram "C" is a graph of the energy changes that result from a reaction between Barium Hydroxide and Ammonium Thiocyanate. The reaction looks like this.
Diagram C:
Ba(OH)2.8H2O(s ) + 2NH4SCN(s ) --> Ba(SCN)2(s ) + 10H2O(l ) + 2NH3(g )
The above chemical reaction is so endothermic that it can produce a drop in temperature from room temperature ( 22oC) down to -30oC!  The graph in diagram "D" below, shows that the total kinetic energy of the product is much more than the energy of the reactants. Obviously this reaction has gained energy from its surroundings. Endothermic chemical changes take in heat from their surroundings, that's why they feel cold. Endothermic chemical changes move heat from their surroundings into the reaction. If the reaction is taking place inside a beaker, and you hold the beaker in your hand, the reaction will remove heat from your hand!
 
 
Diagram D
Diagram E
                                                                               
The graph in Diagram "E" above, illustrates the energy changes associated with an exothermic chemical reaction, such as the burning of Methane gas in a Bunsen burner. The reaction looks like this: CH4 + 2O2 --> CO2 + 2H2O. Since the product has less energy than the reactants did, the energy must have been released to the surroundings. In other words, exothermic reactions, like this one, release energy, this is why you can feel heat being radiated from burning Methane. Here is another way to look at these two reactions:
Diagram F
 
Diagram G
    Chemical equations can show you the elements involved in a chemical reaction as well as the ratio with which the atoms combine to form new substances. All chemical equations must support the law of conservation of matter that states, " In a chemical change matter is not created nor destroyed, atoms are simply rearranged to produce new substances with new physical and chemical properties. Look at this chemical equation for the reaction between Hydrogen and Oxygen that produces water.
H2 + O2 --> H2O
     The above equation is not balanced. The number of Oxygen atoms on the reactant side and the number of Oxygen atoms on the product side of the equation are not the same. We can adjust the number of atoms of each element on the reactant and product sides by using numbers called coefficients. A coefficient is an integer number placed in front of a molecule or formula unit to adjust the number of atoms of a particular element. Here is how we adjust the number of Oxygen and Hydrogen atoms on each side of this reaction to balance this equation.
2H2 + O2 --> 2H2O
     Here is a link to Dr. Hue-Ling Wong's chemical equation balancing tutor. You can use it to learn how to balance chemical equations.