midterm 2 Flashcards
rates of reaction can be determined using the two formulas
A->B
rate of production of B = Δ[B]/Δt
rate of consumption of A = -Δ[A]/Δt
rate =
change in concentration/change in time
The General Reaction Rate
for a balanced reaction:
aA -> bB + cC
rate = -1/a(Δ[A]/Δt) = 1/b(Δ[B]/Δt) = 1/c*(Δ[C]/Δt)
Initial instantaneous rate vs instantaneous rate
initial instantaneous rate is at t=0.
instantaneous rate can be at any point on the graph
describe the rate law in words
an equation that shows how the reaction rate depends on the concentration of each reactant
Rate Law
aA + bB -> cC + dD
rate = k[A]^m[B]^n
[A], [B] - concentration in M
k - rate constant
m - reaction order in A
n - reaction order in B
what si the overall order of a reaction?
the sum of the individual orders
units of rate
always M/s
units of k
depends on the rate law expression
units of k if:
rate = k
rate = k[A]
rate = k[A][B]
rate = k[A][B]^2
M/s
1/s
1/(Ms)
1/(M^2s)
how are the order of reactions (m, n) and the rate constant (k) determined?
experimentally
give 2 ways of determining order of reaction and rate constant
- method of initial rates:
- vary initial concentration of one reactant at a time
- measure the initial reaction rate for each reaction
- solve a system of rate law equations to determine the order - graphical method/integrated rate law
- monitor the course of a reaction over time
- plot data
- the shape of the curve reveals reaction order
0th order reaction
A –(k)–> product
[A]t = -kt + [A]0
[A]0 is the initial concentration
[A]t is the concentration at time t
- plot the experimental [A] vs time
- if the graph is linear, the reaction is zero-order
- rate constant k = (-slope)
half life
the time required for reactant concentration to reach half of its original value
half life for a 0th order reaction
t(1/2) = [A]0/2k
- dependent on the (initial) concentration
- gets shorter over the course of the reaction (each successive half-life is half as long)
how are 0th order reactions possible?
in reactions whether the kinetics are governed by the availability of a catalyst
1st order reaction
A –(k)–> products
[A]t = [A]0e^-kt
ln[A]t - ln[A]0 = -kt
- plot the experimental ln[A] vs time
- if the graph is linear, the reaction is first-order
- the rate constant k = (-slope)
half life of 1st order reaction
t(1/2) = ln(2)/k
- t(1/2) is independent of the concentration
- every half-life is an equal period of time
2nd order reaction
A –(k)–> products
1/[A]t = kt + 1/[A]0
- plot the experimental 1/[A] vs. time
- if the graph is linear, the reaction is second-order
- the rate constant k = slope
half life of 2nd order reaction
t(1/2) = 1/k[A]0
- t(1/2) is dependent on the (initial) concentration
- t(1/2) gets longer over the course of the reaction, so each successive t(1/2) doubles in length
draw graphs for zeroth order, first order, and second order. also draw straight-line plots used to determine rate constant
how can we find the overall reaction from the elementary steps?
the elementary steps in the true reaction mechanism must sum to give the overall (stoichiometric) reaction
reaction intermediate
a species formed in one step of a reaction mechanism and consumed in a later step
molecularity
a classification of an elementary reaction based on the number of molecules (or atoms) on the reactant side of the chemical equation