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Chapter 14 Chemical Kinetics.docx

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David Brock

Chapter 14 Chemical KineticsBernard HoJanuary 13 2011Rate of a Chemical ReactionThe rate of reaction describes how fast the concentration of a reactant or product changes with timeRate of formationchemicalotRate of disappearancechemicalotGiven the general equation aAbBgGhHABGH1111oThe rate of reaction is atbtgthtoIn this expression we take the negative value of Xt when X refers to a reactant to ensure that the rate of reaction is a positive quantityEx A2GAG1oAverage rate t2tRates are always positiveMeasuring Reaction RatesFollowing a chemical reactionoEx HOaqHOl12Og2222oWe can follow the progress of the reaction by focusing either on the formation of Og or on the disappearance of HO222Measure the volumes of Og produced at different times and 2relate these volumes to decreases in concentration of HO22Remove small samples of reaction mixture from time to time and analyze these samples for their HO content22One way to do this is by titration with KMnO in acidic 4solutionRate of reaction expressed as concentration change over timeoWhen the rate of reaction is expressed as HOt the result is an 22average value for the time interval tRate of reaction expressed as the slope of a tangent lineoGraph time vs HO and determine the slope of a line tangent to the 22graphoThe rate of reaction determined from the slope of a tangent line to a concentrationtime curve is the instantaneous rate of reaction at the point where the tangent line touches the curveInitial rate of reactionoThe initial rate of reaction is the rate of reaction when the reactants are first brought together rate when t0Given by v0oThis rate can be obtained from the tangent line to the concentrationtime curve at t0oAn alternative way is to measure the concentration of the chosen reactant as soon as possible after mixing in this way obtaining reactant for a very short time interval at essentially t0G1If average rategtG11dGlimoInstantaneous rategtgdtt0Experimental rates are always average ratesTo approach instantaneous rates experimentally use short measurement times and measure rates near t0Effects of Concentration on Reaction Rates The Rate Law
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