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Chemical Reactions And Equilibrium Posted: 02 Feb 2012 05:12 AM PST A chemical reaction involves the interaction of atoms or molecules to form a new compounds. For a chemical reaction to occur, defferent atmos or molecules must move close enough for the-outer-shell electrons to interact. The terms reaction rate describes how quickly a chemical reaction occurs under given circumtanses, such as at a certain temperature or under particular light conditions. Because many reactions occur extremely slowly, scientists sometimes add substances called catalysts to help speed the process. A catalyst reduces the activation energy required to initiate and sustain a sustain a reaction. The catalyst itself is never changed by the reaction. The energy generated by atoms or molecukes moving close together is called kinetic energy. Also The atoms or molecules must collide with sufficient kinetic anaergy to overcome the repelling forces to the electrons surrounding the nucleus of the atoms involved. The total amount of kinetic energy needed to achieve successful collision and reaction is called the activation energy. Various factors can affect the reaction rate: The identity of the reacting substances (reactants) influences the reactions rate, some reactants react more or less quickly than others (for example: nitric oxide reacts very quickly with atmospheric oxygen to form nitrogen dioxide, whereas carbon monoxide reacts very slowly with oxygen to form carbon dioxide). The concentration of the reactants influences the reaction rate (for instance, the greater the number of reactants atoms, the more likely the atoms will cololide and produce a reaction). The surface area of a solid reactants influences the reaction rate (for instance, the greater the surface area available for interaction, the higher the reaction rate). Temperature influences the reaction rate (for instance, atoms and molecules tend to gain more kinetic energy with increased temperatures. Thereby increasing the probability that there will be sufficinet energy to provide necessary actiivation energy for the reaction to occur). Exothermic Reactions Any group of atoms or molecules possesses stored energy (potential energy) that is associated with the attractions and repulsions among the various particles. An exothermic reaction is a chemical reaction that releases energy, in many cases, the released energy appears as heat, which can be measured. In this type of reaction, the products have less potential energy than the reactants do. Once initiated, an exothermic reaction is self-sustaining and continuous. Endhotermic Reactions An endhotermic reaction is a chemical change that requires a continuous input of energy for the reaction to begin and to continue. In this type of reactin, the potential energy of the products is greater than that of the reactants, in other words, energy is absorbed by the reaction. If energy input ceases, the reaction will stop. Oxidation-reduction Reactions Oxidation-reduction reaction is any reaction that involving the transfer of electrons between reactants. Oxidation is the loss of electrons by an atom and reduction is gain of electrons by an atom.  In any oxidation reduction reaction, the number of electrons gained by one reactant must always equal the number of electrons lost by another reactant. An oxidizing agent is any reactant that gains electrons. A reducing agent is any reactant that loses electrons. In the equation :   Ca + Cl2        Ca2+ + 2 Cl- , the calcium (Ca) loses two electrons and the chlorine (Cl) gains two electrons. Ca the element oxidized, is reducing agent and Cl, the element reduced, is the oxidizing agent.

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