Principles of Chemistry II © Vanden BoutTo d ayKineticsHow fast are reactions?What are the rates?Principles of Chemistry II © Vanden BoutDiamond Graphite∆RG° = -3 kJ mol-1Interconversion of two forms of carbonIf you own diamond jewelry, why should you not worry about this reaction?! A.! ! we know diamond is more stable from ∆RG°! B.! ! the reaction is extremely slow! C.! ! the reaction will only happen if we add energy!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 BoutKineticsTo understand how fast a reaction proceeds we need to think about what is actually happening during the reactionZn(s) + 2H+(aq) Zn2+(aq) + H2(g)What actually happens?Presumably somehow the reactants have tophysically interact before there can be any chemistry(assuming we are not doing this in a electrochemical cell)Principles of Chemistry II © Vanden BoutLet’s look at some reactionsZn(s) + 2H+(aq) Zn2+(aq) + H2(g)Cu(s) + 2H+(aq) Cu2+(aq) + H2(g)! A.! ! Zn! B.! ! Cu! C.! ! no way to since, since kinetics doesn’t have anything to do with thermodynamics!which will be faster to react?∆RG° = +65 kJ mol-1∆RG° = -147 kJ mol-1Principles of Chemistry II © Vanden BoutLet’s look at some reactionsZn(s) + 2H+(aq) Zn2+(aq) + H2(g)! A.! ! Zn dust! B.! ! Zn chunks! C.! ! they will react at the same rate (they’re both Zn)!which will be faster to react?Principles of Chemistry II © Vanden BoutLet’s look at some reactionsZn(s) + 2H+(aq) Zn2+(aq) + H2(g)! A.! ! higher concentration HCl! B.! ! lower concentration HCl! C.! ! they will react at the same rate (they’re both strong acids)!which will be faster to react?Principles of Chemistry II © Vanden BoutWhat can affect the rate of reactions?1. Nature of the reactants2. Concentration of the reactants3. Temperature4. Presence of a CatalystPrinciples of Chemistry II © Vanden BoutFirst you have to get the reactants togetherOften we have two species that need to interact (physical contact)before there can be a chemcial reaction (bond breaking/forming)Principles of Chemistry II © Vanden BoutFirst you have to get the reactants togetherHow do you speed up a reaction?More interactions“Nature” of the reactanct (more surface area for solids)Higher concentrations (more collisions for species in solution)Often we have two species that need to interact (physical contact)before there can be a chemcial reaction (bond breaking/forming)Principles 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 statesPrinciples 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" onesPrinciples 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 slope 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== d[H2]dt- d[H20]dt+ =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 have 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
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