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25 Cards in this Set
- Front
- Back
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4 factors that affect reaction rates
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1. physical state of reactions (increased if surface area of solids is increased)
2. concentrations of the reactants (increases as conc. increases b/c frequency of collisions increase) 3. temperature at which reaction occurs (increases the kinetic energies of molecules, causing them to move more rapidly) 4. presence of a catalyst (affect the mechanism-kinds of collisions) |
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Define reaction rate
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Change in the concentration of reactants or products over time (usually molarity per second)
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For the reaction A --> B, the average rate can be expressed by disappearance of a reactant or appearance of a product
flip for expression in equation form |
avg rate of appearance of product B = delta B / delta t
avg rate of disappearance of reactant A = - delta A / delta t (minus sign is used b/c rxn rates are always expressed positive) avg rate of A = avg rate of B |
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How does the reaction rate change as time passes?
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Rates typically decrease as rxns proceeds b/c reactant concentrations decrease
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How do you determine the instantaneous rate?
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Determined by slope or tangent of curve at point of interest
instantaneous rate = - delta[A] / delta t initial rate is the instantaneous rate at t = 0 |
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If stoichiometric relationships are not one-to-one, what is the reaction rate?
Reaction: aA + bB --> cC + dD |
Rate = - 1/a delta[A]/delta t
= -1/b delta [B]/delta t = 1/c delta [C]/delta t = 1/d delta [D]/delta t |
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Rate law for aA + bB --> cC + dD
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Rate = k[A]^m [B]^n
k = rate constant (changes with temperature, not with conc.) |
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Reaction orders in rate = k[A]^m [B]^n
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exponents m, n = reaction orders, which indicate how rate is affected by the conc. of reaction
e.g. if A is raised to 2, doubling the conc. quadruples rxn rate values of exponents must be determined experimentally overall reaction order is sum of all the orders in rate law |
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units of rate constant k
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units of rate = units of k * (units of conc)^reaction order overall
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How do you determine the rate law from initial rate data?
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divide rate law of trial 1 by rate law of trial 2
trial 1 and 2 must have the same initial conc. except for one reactant solve for the exponent, round to a whole or half integer repeat for the other exponents |
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First-order reaction
Give differential (on conc.) and integrated (on time) rate laws |
rate depends on the conc. of a single reactant to the first power
diff. rate = -delta[A]/delta t = k[A] inte. rate ln[A]_t = -kt + ln[A]_0 can be graphed as a straight line y = mx + b to verify that it is a first-order rxn |
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Second-order reaction
Give differential and integrate rate laws |
depends on the reactant conc. raised to the second power, or on the conc. of 2 reactants
diff. rate = -delta[A]/delta t = k[A]^2 inte. rate 1/[A]_t = kt + 1/[A]_0 This will also graph a linear plot |
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How can you distinguish first and second order rxns from graphs?
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Plot ln[A] and 1/[A] vs. time.
ln[A] vs. t will be linear for 1st order 1/[A] vs. t will be linear for 2nd order |
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Define half-life.
How can you calculate half-life for a 1st order rxn? for a 2nd order rxn? |
Time required for conc. of a reactant to reach 1/2 initial value, [A]_t1/2 = 1/2[A]_0
1st order ln((1/2[A])/[A]) = -kt_1/2 t_1/2 = -ln(1/2) / k = 0.693/k -half life remains constant in a rxn, so conc. keeps decreasing by 1/2 at regular time intervals 2nd order t_1/2 = 1/k[A]_0 -half life depends on conc., so it changes as rxn progresses (the lower the initial conc., the greater the half life) |
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rate constant increases with temp. why?
-collision model -orientation factor -activation energy |
-based on kinetic molecular theory (10.7)
molecules must collide to react, the greater # of collisions per sec, the greater the rxn rate increasing temp increases molecular speeds, so molecules move faster, collide more frequently, and more forcefully with energy -molecules have to collide in a certain orientation to react -Svante Arrhenius proposed that molecules must have a minimum amt of E to react, called activation energy, Ea |
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Reactants get sufficient E to overcome E barrier through collisions w/ other molecules.
Fraction of molecules that have E equal to or greater than Ea is: |
f = e ^(-Ea/RT)
R = 8.314 J/mol-K T = abs temp |
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Arrhenius found that the increase in rate with increasing temp is nonlinear, based on what three factors?
State Arrhenius equation and derivative based on data from two temps |
1. fraction of molecules possessing Ea or greater
2. # of collisions occurring per sec 3. fraction of collisions w/ correct orientation k = A*e^(-Ea/RT) A = frequency factor (pretty constant, even if temp changes) lnk = -Ea/RT + lnA ln(k1/k2) = Ea/R ((1/T2)-(1/T1)) |
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Define elementary rxns (elementary processes)
What is molecularity? |
Mechanisms with one collision
molecularity = # of molecules that participate as reactants in a elementary reaction unimolecular (single molecule) bi- (two molecules colliding) ter- (three, very rare) |
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Multi step mechanisms
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-Based on sequence of elementary reactions
-must add to give the eq. of overall process |
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intermediate vs. transition state
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intermediate-formed in a multi step mechanism (not a reactant or product of overall rxn)
transition state-arrangement of molecules at the top of rxn pathway (at Ea hill) |
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Rate laws have to be determined experimentally or from its mechanism
Rate law of elementary rxn is based directly on its _____ |
molecularity.
unimolecular-1st order bi-2nd order ter-3rd order A --> B, rate = k[A] A + A --> B, rate = k[A]^2 A + B --> C, rate = k[A][B] |
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Rate-determining step (rate-limiting)
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Slowest elementary step of mechanism
-determines rate law for whole mechanism |
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determine rate law for mech. with slow initial step
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rate of overall rxn is rate law of step 1
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determine rate law for mech. with fast initial step
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slow, rate-determining step has rate law that includes unknown conc. of intermediate
fast step 1 is in equilibrium, w/ forward rate = reverse rate solve for intermediate conc, and plug into rate law experimental rate constant = k2 * k1/k-1 |
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Define catalyst, homogeneous and heterogeneous
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catalyst-substance that changes speed of rxn w/o undergoing permanent change itself
-lowers the Ea for rxn, usually by changing mech. for rxn homogeneous-in same phase as reactants heterogeneous-in diff phase from reactants, usually metals or metal oxides, where rxns occur on its surface (of a solid catalyst) -initial step of catalysis is adsorption of reactants(binding of reactants to surface) |
