7.3 Energy Changes in Reactions
Reading Focus
Key Concepts
What happens to chemical bonds during a chemical reaction?- What happens to energy during a chemical reaction?
Vocabulary
chemical energy
exothermic reaction
endothermic reaction
Reading Strategy
Comparing and Contrasting Copy the Venn diagram. As you read, complete it to show the differences between exothermic and endothermic reactions.
Figure 16 Many portable barbecue grills burn propane gas.
If you've ever had a barbecue, you may have used a gas grill like the one shown in Figure 16. Many types of gas grills use propane, C3H8. You can think of a propane grill as the scene of a chemical reaction—specifically, a combustion reaction. The reactants are propane and oxygen, and the products are carbon dioxide and water. However, the description of this reaction is incomplete unless you consider the heat and light produced. Heat, after all, is the reason for using a propane grill.
Chemical Bonds and Energy
The heat produced by a propane grill is a form of energy. When you write the chemical equation for the combustion of propane, you can include “heat” on the right side of the equation.
This equation states that the heat released in the reaction came from the reactants. Chemical energy is the energy stored in the chemical bonds of a substance. A propane molecule has ten single covalent bonds (eight C—H bonds and two C—C bonds). The chemical energy of a propane molecule is the energy stored in these bonds. Likewise, oxygen, carbon dioxide, and water molecules all have energy stored in their chemical bonds.
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