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Energy-Releasing Reactions Endothermic chemical reactionsare usually important because of the compounds the reactionsproduce. Other reactions are important because theyrelease energy. Exothermic (ek soh THUR mihk) reactions arechemical reactions that release heat energy. A chemical handwarmer releases heat when an exothermic reaction takes placeinside the hand warmer. When a substance burns, atoms in thesubstance combine with oxygen atoms in the air. An exothermicreaction occurs, and energy in the form of heat and light isreleased. The exothermic reaction that occurs when a materialburns by combining with oxygen is called combustion. Burningoil, coal, and gas produces much of the energy needed to heathomes and schools.Topic: Deep Sea VentsVisit red.msscience.com for Weblinks to information about the heatreleased at deep-sea vents.Activity Make a travel brochurefor visiting deep-sea vents. Includewhat is released by the vents,what organisms live near thevents, and the temperatureof the vent fluids.What are chemical reactions that give offheat energy called?Rate of Reaction Chemical reactions can occur at differentrates. They occur very fast when fireworks explode. However, ifyou leave tools or a skateboard outside for a long time, youmight notice the metal parts slowly becoming rusty, as shown inFigure 24. Rusting is a chemical reaction that occurs when ametal combines with oxygen. In a similar way, when silver isexposed to air, it tarnishes. These chemical reactions, however,occur much more slowly than the burning of a candle’s wick ora fireworks explosion.In your body an enormous number of chemical reactions areoccurring every second. The rates of these reactions are carefullycontrolled by your body to enable it to function properly.Figure 24 Rust is a chemicalcompound formed when iron andoxygen combine.This photo shows a wrench beforeit rusts.This photo shows a wrench afterit rusts.SECTION 3 Chemical Energy 181Tony Freeman/PhotoEdit, Inc.