Section 3.3 Chemical Reactions Involve Energy Changes.
Chemical reactions release and absorb energy.
Chemical reactions break the chemical bonds in reactants and make new bonds in the products.
Breaking bonds releases energy.
The energy in chemical bonds is called bond energy.
More energy is released when the bonds in products form than is used to break the bonds in reactants.
Energy is released, often as heat.
Exothermic reactions (cont.)
Reactants have lower bond energies than products.
All common combustion reactions are exothermic.
Jellyfish glow because of exothermic chemical reactions.
More energy is needed to break the bonds in reactants than is released when the bonds in products form.
The reactants have higher bond energies than the products.
Energy is absorbed.
Photosynthesis is an important endothermic reaction. 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2
Exothermic and endothermic reactions work together to supply energy.
Endothermic and exothermic reactions can form a cycle.
Example: The energy stored by the series of endothermic reactions in photosynthesis can be released by exothermic reactions such as combustion.
A plant absorbs energy from the sun during photosynthesis. 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 Endothermic Equation If the plant burns, the energy is released. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Exothermic Reaction Combustion Page 90
Section 3.4 Life and Industry Depend on Chemical Reactions
Living things require chemical reactions.
Photosynthesis is an endothermic process.
Plants absorb energy from the sun.
The energy is stored in the chemical bonds of sugars.
The sugars break down during exothermic reactions of respiration, the process that produces energy for living organisms.
Photosynthesis: 6CO 2 + 6H 2 O + energy C 6 H 12 O 6 + 6O 2 Respiration: C 6 H 12 O 6 6CO 2 + 6H 2 O = energy
Most reactions that take place in living organisms use enzymes, catalysts that cause reactions to take place at the relative temperatures of living tissue.
Chemical reactions are used in technology.
Combustion engines use gasoline in a chemical reaction that releases energy.
Catalytic converters are technological devices that remove unwanted pollutants from the burning of gasoline in automobile engines.
Use metals as catalysts: platinum, palladium, rhodium
Catalysts allow reactions between exhaust gases to occur.
Convert pollutants into gases that are typical parts of Earth’s atmosphere: oxygen, nitrogen, water vapor, carbon dioxide.
Industry uses chemical reactions to make useful products.
The electronics industry produces silicon for microchips by refining SiO2 (quartz) into pure silicon.
Silicon treated with photoresist and light produces miniature electronic circuits.