Physical Science 8th grade (1).pdf

Endothermic reactions if forming new bonds in the

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Endothermic reactions If forming new bonds in the products releases less energy than it took to break the original bonds in the reactants, the reaction is endothermic . Endothermic reactions absorb energy . These reactions need energy to keep going. An example of an important endothermic reaction is photosynthesis . In photosynthesis, plants use energy in sunlight to make glucose and oxygen from carbon dioxide and water.
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209 10.2 E NERGY AND CHEMICAL REACTIONS C HAPTER 10: C HEMICAL R EACTIONS Activation energy An interesting question Exothermic reactions occur because the atoms arranged as compounds of the products have lower energy then they had when arranged as compounds of the reactants. Since this is true, why don’t all of the elements immediately combine into the molecules that have the lowest possible energy? Activation energy The answer has to do with activation energy . Activation energy is the energy needed to start a reaction and break chemical bonds in the reactants. Without enough activation energy, a reaction will not happen even if it releases energy when it does happen. That is why a flammable material, like gasoline, does not burn without a spark or flame. The spark supplies the activation energy to start the reaction. An example reaction The diagram above shows how the energy flows in the reaction of hydrogen and oxygen. The activation energy must be supplied to break the molecules of hydrogen and oxygen apart. Combining 4 free hydrogen and oxygen atoms into 2 water molecules releases energy. The reaction is exothermic because the energy released by forming water is greater than the activation energy. Once the reaction starts, it supplies its own activation energy and quickly grows (Figure 10.5). Thermal energy A reaction starts by itself when thermal energy is greater than the activation energy. Any reaction which could start by itself, probably already has! The compounds and molecules that we see around us are ones that need more activation energy to change into anything else. Figure 10.5: Because energy released by one reaction supplies activation energy for new reactions, exothermic reactions can grow quickly once activation energy has been supplied. activation energy - energy needed to break chemical bonds in the reactants to start a reaction.
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210 U NIT 4 M ATTER AND C HANGE Figure 10.6: Polymerization is a series of successive addition reactions that combine small molecules into large chain molecules. addition reaction - two or more substances chemically combine to form a new compound. polymerization - a series of addition reactions that join small molecules into large chain molecules. Addition reactions Compounds are made in addition reactions In an addition reaction , two or more substances combine to form a new compound. A good example of an addition reaction is the formation of rust (iron oxide, Fe 2 O 3 ) from pure iron and dissolved oxygen in water. Iron oxide is an ionic crystal because the bonds between iron and oxygen are ionic bonds. Ionic crystals do not have
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