Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Energy Changes Energy changes associated with chemical reactions. Notes (1) All chemical reactions involve energy. If you have ever sat near a campfire, you have heat experienced this energy as _____and _______. light In addition to producing energy, chemical use energy. reactions also _____ photosynthesis For example, plants perform ____________ which is a chemical reaction that uses energy from sunlight. Notes (2) Energy is involved in chemical reactions in two ways: (1) At the start of a chemical reaction, energy is break used to ___________ bonds between atoms in the reactants so that the atoms are available to form new bonds; and (2) Energy is ___________ released when new bonds form as the atoms recombine into the new compounds of the products. We classify chemical reactions based on how the energy used in (1) compares to the energy released in (2). Notes (3) If forming new bonds releases more energy than it takes to break the old bonds, the reaction is exothermic Once started, exothermic ____________. reactions tend to keep going because each releases enough energy to start the reaction _______ reaction in neighboring molecules. Notes (4) 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. In fact, endothermic reactions need more energy to keep going. An example of an important endothermic reaction is photosynthesis __________. An interesting question Exothermic reactions occur because the atoms ___________ products arranged as compounds of the ____________ have lower energy and are more stable than they are when arranged as compounds of the reactants ___________. Chemical reactions ________________—like other systems—move toward more stable circumstances. If this is true, why don’t all the elements combine into the molecules that have the lowest possible energy? Notes (5) activation energy a reaction Without enough ________________, will not happen even if it is __________. exothermic 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. Notes (6) The activation energy must be supplied to break the molecules of hydrogen and oxygen apart. Energy is then released when the four free hydrogen and two free oxygen atoms combine to form two water molecules. 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. Enthalpy The symbol ______, H is used to represent internal energy of a enthalpy, which is the _______________ substance. It is this change in enthalpy, or ΔH that determines which way the energy ______, flows. negative (-) If the energy flows out, the sign is __________, exothermic and the reaction is __________. positive (+) If the energy flows in, the sign is __________, endothermic and the reaction is ____________. Notes (7) You learned that photosynthesis is an endothermic reaction. Through photosynthesis, ______________ plants capture energy from the Sun. The way that humans and other life forms obtain this energy is by respiration a chemical reaction called _______________ which is the reverse of photosynthesis. Glucose is metabolically “burned” in the presence of oxygen and carbon dioxide and water is released. What kind of reaction is respiration—exothermic or endothermic? How do you know? Fossil Fuels Energy is obtained from fossil fuels (like natural gas, kerosene, and coal) by using _______________________, which are Combustion Reactions exothermic ____________. In addition to producing energy, combustion reactions yield ______________ carbon dioxide and _______ water as products. Notes (8) Certainly, it’s useful for chemical reactions to __________ produce more energy than they use. But, how so we benefit from reactions that use more energy than they produce? It turns out that most of the reactions used in industry to produce useful materials _________ require more energy than they produce. This is one of the reasons sources of energy are so important to industry. In other words, exothermic reactions are needed to cause endothermic reactions to run! Notes (9) ICE PACKS - An ice pack gets very cold because it ionic bonds in takes energy to dissolve the ____________ the ammonium nitrate. Besides being endothermic this reaction is also a ____________, dissolution reaction __________________.