Unit 6 - Rearranging Bonded Particles - Chemical Potential Energy

Aluminum foil (Al) and bromine (Br2) react vigorously in this test tube to form aluminum bromide (Al2Br3). The reaction is exothermic as evidence by all the heat and light produced. The brown-reddish gas is composed of Br2 molecules vaporized by the intense heat.

click on the picture for a video of this rxn



Chemical reactions involve the rearrangement of atoms in molecules to form new molecules. This rearrangement of atoms results in a change in the chemical potential energy (Ech) of the system. This invariably produces changes in kinetic energy (Ek), and results in energy transfers between system and surroundings.

Mass is conserved because the atoms in the products are the same as those found in the reactants. This is represented symbolically as a balanced chemical equation. Because the grouping of atoms into molecules is changed in a chemical reaction, the total number of molecules (or formula units) in the products need not be the same as that in the reactants.

Substances store varying amounts of chemical potential energy (Ech) due to the arrangement of atoms. It is not possible to measure this amount of energy directly. However, rearrangement of atoms during reaction produces changes in Ek; the resulting energy transfers (as Q) between system and surroundings can be measured. From these one can deduce differences in the Ech of reactants and products. Energy bar graphs are a useful tool for accounting for energy (stored and transferred) during chemical change.


Dr. Michael M. Krop High School







Unit 6 Materials


m r . m o n t e r o . c o m



Methane (CH4), also known as natural gas, reacts with oxygen to form carbon dioxide and water in a typical combustion reaction. The reaction is represented by a balanced chemical equation. Notice how the number of atoms in the reactant side is equal to those in the products side. This shows the law of conservation of mass.

click on the picture for a view of this rxn