Principles of Chemistry II © Vanden BoutTo d ayReaction CoordinatesActivation EnergyCatalysisPrinciples of Chemistry II © Vanden BoutWe have a balloon with H2 and O2why is not reacting?2H2(g) + O2(g) 2H2O(g)Principles of Chemistry II © Vanden Bout A.! ! this reaction is not spontaneous at room temperature! B.! ! the reaction is very slow at room temperature! C.! ! the reaction is very slow at these concentrations! D.! ! B & C! E.! ! all of the above!We have a balloon with H2 and O2why is not reacting?2H2(g) + O2(g) 2H2O(g)Principles of Chemistry II © Vanden BoutPrinciples of Chemistry II © Vanden BoutWhen the reaction is very very slowthe problem is typically that the rate constant is very smallWhat affects the rate constant?Principles of Chemistry II © Vanden BoutArrhenius PictureReaction CoordinateEnergyPrinciples of Chemistry II © Vanden BoutPrinciples of Chemistry II © Vanden Bout A.! ! all have the same energy! B.! ! have a distribution of energies! C.! ! have one of several fixed energiesAt a given temperature the molecules in a samplePrinciples of Chemistry II © Vanden BoutHow many molecules have enough energy to get over the barrier?Principles of Chemistry II © Vanden BoutArrhenius LawThe rate constant k is a function of temperaturek = A e-Ea/RTArrhenius Lawpre-factorbarriertemperatureThe higher the temperature the more molecules that have enough energy to make it over the barrierPrinciples of Chemistry II © Vanden BoutWhat is A?This is the rate at infinite temperature(not all interactions between the moleculeseven with sufficient energy will lead to products)Principles of Chemistry II © Vanden BoutVery important in organic chemistry“steric effect” “steric hindrance”“steric protection”putting a big unreactive part of the molecule “in the way”to slow (or stop) the reactionPrinciples of Chemistry II © Vanden BoutLet's make a new Equationk = A e-Ea/RT lnk = lnA -Ea/RTlet's look at two temperatureslnk1 = ln A -Ea/RT1lnk2 = lnA -Ea/RT2Principles of Chemistry II © Vanden BoutLet's make a new Equationk = A e-Ea/RT lnk = lnA -Ea/RTlet's look at two temperatureslnk1 = ln A -Ea/RT1lnk2 = lnA -Ea/RT2subtract to get a new equation that doesn't have A ln(k2/k1) = -EaR[]1T2T11-Principles of Chemistry II © Vanden Bout! A.! ! -1.2 x 104 K! B.! ! 1 x 105 J mol-1! C.! ! 1.2 x 104 J mol-1!! D.! ! 1 x 105 K! E.! ! -1 x 102 kJ mol-1!The activation energy for this reaction is?-EaR[]1T2T11-ln(k2/k1) =Principles of Chemistry II © Vanden Bout! A.! ! -1.2 x 104 K! B.! ! 1 x 105 J mol-1! C.! ! 1.2 x 104 J mol-1!! D.! ! 1 x 105 K! E.! ! -1 x 102 kJ mol-1!The activation energy for this reaction is?Slope = -Ea/REa = -R x slopeEa = -8.314 J K-1 mol-1 x (-1.2 x 104 K) = 1 x 105 J mol-1Principles of Chemistry II © Vanden BoutWhy are reactions faster at higher temperatures?More molecules have sufficient energy to getover the barrier. BIG EFFECTMore molecules have collisions (but this is a very small effect) that isignored in Arrhenius viewPrinciples of Chemistry II © Vanden BoutTransition State TheoryPrinciples of Chemistry II © Vanden BoutTransition State TheoryTwo Step Mechanism (Two Barriers)Principles of Chemistry II © Vanden BoutWhat is the rate limiting step?We need to write reaction coordinate vs Free EnergyReaction CoordinatedeltaGPrinciples of Chemistry II © Vanden BoutHow else can affect k?Change the barrier (mechanism)Reaction CoordinateEnergyPrinciples of Chemistry II © Vanden BoutPrinciples of Chemistry II © Vanden BoutCatalystLower the barrier for the reaction (by changing the mechanism)Is not consumed during the course of the reaction(it can be used over and over again)However, it might under go chemistry during the reaction,but the original form is regenerated by reaction.Principles of Chemistry II © Vanden BoutDecomposition of Hydrogen PeroxideH2O2(l) O2(g) + H2O(l)This reaction is very slow at room temperature(thus you can get a bottle of H2O2 at the store)demoPrinciples of Chemistry II © Vanden BoutWhat happens when I add the catalystH2O2 + MnO2 + 2H+ Mn2+ + 2H2O + O2 Mn2+ + 2H2O2 Mn(OH)2 + 2H+ Mn(OH)2 + H2O2 MnO2 + 2H2O Note: During the reaction the catalyst changes. But at the end it is back to the same compound!Principles of Chemistry II © Vanden BoutHow do many catalysts work?Principles of Chemistry II © Vanden BoutCatalyzes three chemical reactions2NOx xO2 + N22CO + O2 2CO22CxHy + (2x+y/2)O2 2xCO2 + yH2OPrinciples of Chemistry II © Vanden BoutHaber Process (Fritz Haber Nobel 1918)Formation of Ammoniaoriginally osmium and uraniumNow iron (keep out the O2)Ertl Nobel Prize 2008Principles of Chemistry II © Vanden BoutEnzymesBiological CatalystsPrinciples of Chemistry II ©
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