Chapter 6
Section 1
Objectives
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Recognize some signs that a chemical reaction may be taking place.
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Explain chemical changes in terms of the structure and motion of atoms and
molecules.
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Describe the differences between endothermic and exothermic reactions.
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Identify situations involving chemical energy.
Chemical Reactions Change Substances
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Chemical reactions occur when substances undergo chemical changes to form
new substances.
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Production of gas and change of color are signs of chemical reactions.
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Chemical reactions rearrange atoms.
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A reactant is a substance or molecule that participates in a chemical
reaction.
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A product is a substance that forms in a
chemical reaction.
Energy and Reactions
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Energy must be added to break bonds.
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Many forms of energy can be used to break bonds:
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heat
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electricity
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sound
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light
Forming bonds releases energy.
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Example: When gasoline burns, energy in the form of heat and light is
released as the products of the isooctane-oxygen reaction and other
gasoline reactions form.
Energy is conserved in chemical reactions.
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Chemical energy is the energy released when a chemical compound
reacts to produce new compounds.
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The total energy that exists before the reaction is equal to the total energy
of the products and their surroundings.
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An exothermic reaction is a chemical reaction in which heat is released to the
surroundings.
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An endothermic reaction is a chemical reaction that absorbs heat.
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The graphs below represent the changes in chemical energy for an exothermic
reaction and an endothermic reaction.
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The graphs below represent the changes in chemical energy for an exothermic
reaction and an endothermic reaction.