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Hydrogengas moleculeHydrogenatomOxygen gasmoleculeOxygenatomWatermolecule (H 2O)FlameEnergyEnergyThe added energy from the flame causes the bondsto break in the oxygen gas and hydrogen gas.When the new bonds form between hydrogen andoxygen to produce water particles, energy is released.Chemical Bonds Energy is stored in the bonds between theatoms in a compound. The stored energy in the chemical bondsis a form of potential energy called chemical energy.The chemical energy stored in oil, gas, and coal is an importantsource of energy that is used every day. The chemical energystored in food provides a source of energy for your body. Themuscles in your body transform some of this chemical energyinto kinetic energy and heat when they move. List some of theother sources of chemical energy you used today.Figure 22 Oxygen and hydrogengas will not react unlessenergy is added.Energy in ReactionsIn every chemical reaction, transformations in energy occur.To break bonds, energy must be added. The reverse is also true.When bonds form, energy is released. Often energy must beadded before the reaction can begin. For example, energy isneeded to start the reaction between hydrogen and oxygen toform water. Look at Figure 22. When a lighted match is placedin a mixture of hydrogen gas and oxygen gas, the mixture willexplode and water will form. The energy to begin the reactioncomes from the heat supplied by the flame. As the reactionoccurs, bonds form between hydrogen and oxygen atoms, andwater molecules form. The energy released as the bonds formresults in the explosion.After the hydrogen and oxygen atoms are bound together toform a water molecule, it is difficult to split them apart.Energy—usually supplied by electricity, heat, or light—isrequired to break the chemical bonds.What is required to break chemical bonds?SECTION 3 Chemical Energy 179
Energy-Absorbing Reactions Some chemical reactionsabsorb energy. A chemical reaction that absorbs heat energyis called an endothermic (en duh THUR mihk) reaction.Endothermic chemical reactions often occur in the preparationof food. Thermal energy is absorbed by the food as it cooks. Forexample, in baking some kinds of cookies, an endothermic reactionproduces a gas that puffs up the cookies.Figure 23 In photosynthesis,plants absorb light energy andmake oxygen and sugar fromwater and carbon dioxide.Photosynthesis Chemical reactionsoccur when sunlight strikes the leaves ofgreen plants. These chemical reactions convert the energy in sunlightinto chemical energy contained in a type of sugar. Oxygenalso is produced by these chemical reactions. This process, shownin Figure 23, is called photosynthesis. When the plant is deprivedof sunlight, the reactions stop. Photosynthesis is probably themost important endothermic process on Earth. Plants provideyou, and almost all other living things, with food and oxygenthrough photosynthesis.Carbon dioxide inthe atmosphereenters the leavesthrough smallopenings.SunlightSunlightSunlightDuring photosynthesis,oxygenis emittedto the atmospherethroughsmall openingsin leaves.Green plants contain thechemical chlorophyll, whichconverts the energy in sunlightinto chemical energy.Plants absorb waterthat travels upthrough their rootsto the leaves.WaterWaterSugar that is made during photosynthesisis stored in the roots orstem of the plant. The plant usesthe sugar for growth.180 CHAPTER 6 Energy
Page 1 and 2: Energysections1 Energy Changes2 Tem
Page 3 and 4: ■ Explain what energy is.■ Desc
Page 5: Food Is ChemicalEnergy You transfor
Page 8 and 9: Increasing Potential Energy When yo
Page 10 and 11: Kineticenergy istransferredto grass
Page 12 and 13: Figure 12 Temperaturedepends on the
Page 14 and 15: The Celsius Scale Another temperatu
Page 16 and 17: Bumping Along Even though conductio
Page 18 and 19: Examples of Convection Eagles and h
Page 22 and 23: Energy-Releasing Reactions Endother
Page 24 and 25: Imagine standing at the top of a mo
Page 26 and 27: 4. Pour 100 mL of room-temperature
Page 28 and 29: Energy Changes1. Energy is the abil
Page 30 and 31: 18. Concept Map Below is a concept
Page 32: Record your answers on the answer s

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