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Final Exam Notes

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Department
Chemistry
Course
Chemistry 1027A/B
Professor
Paul Ragogna
Semester
Fall

Description
Chapter 3: Chemical Kinetics 3.1 Reaction Rates and Rate Laws - chemical kinetics is study of how quickly a reaction will proceed - speed at which a reaction takes place depends on several factors - nature of reactants and their concentrations - temperature - presence of a catalyst - rate of chemical reaction is a positive quantity defined by comparing chainge in product or reactant concentration over time rate = -1/a([A]/t) = -1/b([B]/t) = -1/c([C]/t) = -1/d([D]/t) x y - rate of reaction (rate law) is equal to k[A] [B] - k is specific rate constant for a reaction at a given temperature - exponents are experimentally measured and DO NOT correlate with the coefficients in the reaction equation - if A products is a first order reaction, rate = k[A] = -([A]/t) - as the reaction proceeds, [A] decreases - rate can be integrated into integrated rate law: [A]t= [A] 0 -kt - [A]tis the concentration after time has elapsed - [A]0is the initial concentration ln[A]t= -kt + ln[A] 0 - if reaction is first-order, plot of ln[A]tas a function of time gives you a straight line with a slope of k and a y-intercept of ln[A] 0 - half life (1/2 is the amount of time it takes to use up half of the reactant t1/2= 0.693/k - half life of first-order reaction is exponential decay - half life is constant length of time and only depends on the rate constant, k fraction remaining = (0.5) n # of elapsed half lives (n) = time elapsed/length of half life 0 - if A products is zero-order, rate = -([A]/t) = k[A] = k [A]t= -kt + [A] 0 - plot of [A] as a function of time gives a straight line with slope of k and y-intercept [A] 0 2 - if A products is second-order, rate = -([A]/t) = k[A] 1/[A] t 1/[A] 0 kt - plot of 1/[A] as a function of time gives a straight line with positive slope - to determine if reaction is zero, first or second-order, plot [A], ln[A] or 1[A] as a function of time and see which one gives a straight line - half life for second-order reaction: t1/2= 1/k[A] 0 Order Rate Integrat Straigh Slope Units Half- ed Rate t Line Law of Plot of k Life Law Plot 0 Rate = [A]t= -kt [A] vs Negativ Mol/Ls [A] 02k k + [A]0 time (t) e ln[A]t= 1 Rate = -kt + ln[A] vs Negativ 1/s 0.693/k k[A] time (t) e ln[A]0 2 Rate = 1/[A]t 1/ 1/[A] vs positive L/mols 1/k[A} 0 k[A]2 [A]0= kt time (t) 3.2 Reaction Mechanisms and the Arrhenius Equation - thermodyndamis: A B, results in net energy difference (E) - kinetics: speed of A B conversion, depends on size of barrier - thermodynamics and kinetics are distinct - reaction coordinate illustrates the energy changes that occur on route from products to reactants - Ea= activation energy - collision theory explains the various factors that influence reaction rates - molecules must overcome activation barrier (average kinetic energy is relative to temperature) - energy required to overcome the activation barrier is called activation energy - energy needed to overcome the activation barrier comes from heat - heat has a direct impact on kinetic energy - temperature is a measure of average kinetic energy - there is a distribution of kinetic energies at any given temperature - for a chemical reaction to occur: - reactants must collide with sufficient energy to overcome the activation barrier - must collide in proper orientation - rate of reaction is affected by three factors: - reactant concentration - probabilities of colliding in particular geometry and continuing to products at transition state - Eaand temperature - rate = number of collisions x probability (steric) factor x fraction of collisions with enough energy to overcome E a - Arrhenius equation: k = Ae -Ea/RT - A = Arrhenius probability factor for specific reaction - Ea= activation energy for specific reaction - R = gas constant (8.314 J/molK) - T = temperature (Kelvin) - E a is a constant that can be determined without knowing the probability factor by performing two experiments at different temperatures while maintaining the same reactant concentrations rateT2rate T1 = k 2k1= Ae -Ea/R/Ae-Ea/RT1 ln(rateT2rate T1= ln(k /2 )1= E /Ra1/T 1/T ) 2 - activation energies are usually expressed in kJ/mol lnk = -E aR(1/T) + lnA - Eacan be determined experimentally by measuring a reaction rate at different temperatures - then plot lnk versus 1/T, making a straight line with a slope of Ea/R and y-intercept of lnA - catalyst is a species that increases the rate of reaction but is not consumed in the reaction - provides an alternate pathway with a lower E , incraasing k - it has no effect on the net enthalpy change - it does not affect the equilibrium constant but allows a system to attain equilibrium faster - rate enhancement factor is ratio of k values for the catalyzed and uncatalyzed reaction - to determine the magnitude of the E reducaion (E ): a ln(ratecatate uncat= ln(k cat uncat = Ea(uncat)Ea(cat)T = E /aT - reactions occur in multiple steps - reaction mechanisms describe the sequence of steps that occur - each step is an elementary step, which cannot be broken down further - molecularity refers to how many species react together in an elementary step - if there is only one reactant species, it is a unimolecular process A products; rate = k[A] - for two reactant species, it is a bimolecular process 2 A + A products; rate = k[A] A + B products; rate = k[A][B] - for two or more elementary steps, the steps can have different molecularity - ONLY in elementary steps are the coefficients of the reactants become the exponents in the rate law - overall rate of reaction is determined by rate of slowest or rate- determining step (RDS) - reaction of alkyl halides is a nucleophilic substitution - electron-bearing nucleophile (Cl) replaces bromide on electrophilic (electron-deficient) carbon atom - if reaction occurs in one step, the overall reaction equation is the only elementary step - reaction is bimolecular - rate law is overall second-order - - rate = k[R-Br][Cl ] - if reaction occurs in two steps with the first being slow, reaction coordinate diagram will show an intermediate and two transition states - the slow or rate determining step is the step with the highest E a - if a reaction occurs in two steps with second being slow, overall rate law is based on slow step (step 2) - rate = k2[R ][Cl ] - [R ] = k [R Br]/k [Br ]- 1F 1R - 5 guidelines for deriving a rate law: - look for slow/rate determining step (RDS) - write rate law in terms of reactant concentrations - maximum of two species should appear in rate law - if there are intermediates, express their concentration in terms of stable reactants - may be done by writing equilibrium constant expression - substitute concentrations of stable reactants for concentrations of intermediates - fast steps following RDS in mechanistic sequence may be ignored - many reaction are multi-step reactions, including hydration of an alkene - first step: H adds to alkene in slow reaction - second step: water adds to carboation to form oxonium ion in fast reaction - third step: oxonium undergoes fast deprotonation to regenerate catalyst, H + + - since step 1 is slow step, rate = k[alkene][H ] Chapter 4: Transition Metals 4.1 Electronic Configurations, Properties and Complexes - 30 d-block elements are transition metals with special properties associated with their partially-filled d orbitals - first row elements: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn - for neutral atoms, 4s orbital is filled before the 3d orbitals, with two exceptions - in chromium, both orbitals are half filled, maximizing parallel spins - half hilled d orbital has lower energy than partially filled d orbital and more stable - all six electrons are unpaired - arrangement obeys Hunds rule and minimizes inter-electron repulsions - in copper, energy of 3d orbital is below that of 4s due to increased nuclear charge - trends in electronic configuration are not obvious
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