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Chemistry

Chemistry 1027A/B

Paul Ragogna

Fall

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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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