Principles of Chemistry II © Vanden BoutTodayKineticsHow fast are reactions?What are the rates?Principles of Chemistry II © Vanden BoutThermodynamics vs. KineticsDiamond Graphite∆RG° = -3 kJ mol-1Graphite is lower in free energy than DiamondReaction of Diamond to Graphite is spontaneousTHE REACTION IS JUST VERY VERY SLOWPrinciples of Chemistry II © Vanden BoutThermodynamicsCompares Free energy of reactants and productsThis is the ideal case assuming everything can find its lowest energy state (time is irrelevant)KineticsWhat is actually happeningHow long does it take convert reactants to productsDiamonds are unstable Diamonds are "kinetically trapped" in the unstable statePrinciples of Chemistry II © Vanden BoutWhat prevents reactions form going"downhill" in energy?2H2(g) + O2(g)2H2O(g)EnergyThermodynamics deals with the initial and final statesKinetics deals withthe path between themPrinciples of Chemistry II © Vanden BoutWhy is there a "barrier"?You have to break the "old" bonds before you can form the "new" onesHow do you speed up a reaction?Raise the temperature (more molecules over the barrier)Add a catalyst(lower the barrier)Principles of Chemistry II © Vanden BoutHow do we know how fast a reaction is?We look at the rateRate is change per timeReaction rate is change in concentration per timePrinciples of Chemistry II © Vanden BoutFor this reactionA. the rate for all the species is constantB. the rate if largest at the start of the reactionC. the rate is largest at equilibriumD. the rate is randomly fluctuatingPrinciples of Chemistry II © Vanden BoutRate is change in concentration per unit timeRate is the slope of the graph of concentration vs timeSteepest slow at the startat equilibrium rate = 0 (reaction has stopped)Principles of Chemistry II © Vanden BoutCO(g) + H2O(g) CO2(g) + H2(g)If you know the rate of one reactant or product you know them allPrinciples of Chemistry II © Vanden Bout2H2(g) + O2(g) 2H2O(g)Rate of consumption of H2=∆[H2]∆tChange-reactant decreaseRate of formation of H2O∆[H2O]∆t+products increase=Principles of Chemistry II © Vanden Bout2H2(g) + O2(g) 2H2O(g)Rate of consumption of H2Rate of formation of H2O2 x the Rate of consumption of O2= =H2 and H2O has rates that are faster since 2 moles reaction for each 1 mole of O2Principles of Chemistry II © Vanden BoutFor this reactionsN2(g) + 3H2(g) 2NH3(g)the rate of production of NH3 is! A.! ! 2 times the rates of consumption of H2! B.! ! 1.5 times the rate of consumption of H2! C.! ! 2/3 times the rate of consumption of H2!2x N2 2/3x H2Principles of Chemistry II © Vanden BoutCharacterizing ratesWe want the sloped[C]dt∆[C]∆t~NoteRate is changing with concentrationPrinciples of Chemistry II © Vanden BoutPrinciples of Chemistry II © Vanden BoutRate LawsHow does the rate depend on the concentrations?Rate is some function of the concentrationof the reactant moleculesWhat is the function?Principles of Chemistry II © Vanden Bout2NO2(g) NO(g) + O2(g)Rate = k[NO2]nunknown constantk is the "rate constant"unknown exponentn is the "reaction order" with respect to
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