Unformatted text preview: temperature increases, reaction rate increases. root mean square speed increases
average kinetic energy increases
(J) Ea CHM1311 Chemical Kinetics 52 Effect of Temperature on Rate Constants reaction rates, and thus rate constants, increase when the temperature is increased there are very few exceptions to this rule! for example, an enzyme- catalyzed reaction can begin to slow down as the temperature is decreased, since the enzyme can become denatured at high T CHM1311 Chemical Kinetics 53 Effect of Temperature on Reaction Rates Arrhenius demonstrated that many rate constants vary with temperature according to the equation: Rate constant is inversely proportional to the Ea. k = Ae-Ea/RT Rate constant is directly proportional to temperature Ea = activation energy T= Temperature of reac. A= pre-exponential factor taking the ln of both sides: " !E %" 1 % ln k = $ a '$ ' + ln A # R &# T & CHM1311 Chemical Kinetics 54 Temperature Dependence of k For graphs of ln k versus 1/T: " !E %" 1 % ln k = $ a '$ ' + ln A # R &# T & y = mx + b -Ea slope = R y-intercept = ln A CHM1311 Chemical Kinetics 55 Using an Arrhenius Plot N2O5 N2O4 + O2 -E a R = -1.2 x 104 K
Ea = 1.0 x 102 kJ mol-1 CHM1311 Chemical Kinetics 56 The Arrhenius Equation it is possible to determine the activation energy from two rate constants, k1 and k2, measured at two temperatures, T1 and T2 Ea ln k1 = ln A ! RT1 ln k 2 = ln A ! Ea RT2 subtracting the first equation from the second gives: FOR 2 POINTS ONLY Ea " 1 1 % k2 ln = ! $ ! ' k1 R $ T2 T1 ' # & CHM1311 Chemical Kinetics 57 Reaction Mechanisms MECHANISM: A step-by-step description of a chemical reaction. Each step is called an ELEMENTARY PROCESS (sub steps) Any molecular event that significantly alters a molecules energy of geometry or produces a new molecule. Stoichiometry for the overall reaction The experimentally determined rate law how we convert reac ->prod Reaction mechanism must be consistent with: CHM1311 Chemical Kinetics 58 Elementary Processes 1. Can be unimolecular or bimolecular (A-> products) why not 3 species? Probability is exceedingly small (A+B--> Products) or (A+A->products 2. Exponents for concentration terms are the same as the stoichiometric factors for the elementary process 3. Elementary processes are reversible 4. Intermediates are produced in one elementary process and consumed in another 5. One elementary step is usually slower than all the others and is known as the RATE DETERMINING STEP, RDS Slowest step determines the overall rate (bottle neck) Rate overall = Rate RDS CHM1311 Chemical Kinetics 59 MECHANISMS The O3 + NO reaction examined previously occurs in a single ELEMENTARY step HOWEVER: Many reactions involve a sequence of elementary steps The sum of the elementary steps gives the NET REACTION CHM1311 Chemical Kinetics 60 Example: Decomposition of Ozone Overall reaction: 2 O3 3 O2 UV CHM1311 Chemical Kinetics 61 Example: Postulating a mechanism Postulate a plausibl...
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