Chapter 3- Chemical Kinetics Flashcards
How do we define the rate of a reaction? (Find the rate of reaction)
Pick an element we have data on, then use the negative(if using reactant) or positive(if using product) change in concentration of the element over the change in time
The rate is the same for every element in the reaction!!
How do you find the concentration of a product when given the time intervals and concentration counts of a reactant?
Take the concentration of the reactant and subtract it from 1.000 M then multiply by any stoichiometric ratios between the product and the reactant
SEE PAGE 124 example
How do you find the average rate of a reaction?
When given a table of time and concentration
You find the time interval the question would like
Then if it’s a reactant you use the rate formula with a negative
-Δ[element] / Δt (M/s)
If it’s a product use the rate formula with a positive
Page 125
What is the instantaneous rate of change of a reaction?
The rate at any one point in time represented by a tangent line at the point desired with 20s on either side of it and make a triangle with change in concentration and change in time
Then use the rate formula to solve
What is the overall rate formula to memorize for this equation:
aA + bB -> cC + dD
Rate= -1/a (Δ[Α]/Δt) = -1/b (Δ[B]/Δt) = +1/c (Δ[C]/Δt) = +1/d (Δ[D]/Δt)
Ex: H2 + I2 -> 2HI
+1/2 Δ[HI] / Δt
Look over example on page 127
What is the rate law?
The relationship between the rate of the reaction and the concentration of the reactant
Rate= k[A]^n
k is a constant of proportionality (rate constant)
n is the reaction order
A is the concentration of the reactant
What does it mean if n equals 0? 1? 2?
n=0 means zero order, the rate is independent of concentration A, rate= k[A]^0= k
n=1 means first order, the rate is directly proportional to the concentration of A
Rate= k[A]^1
n=2 means second order, the rate is proportional to the square of concentration A
Rate= k[A]^2
How do we find n for a reaction with one reactant when given the concentration and initial rate of that reactant?
Use the rate law equation and divide the bigger rate by the smaller rate equation (page 133)
Rate 2/rate 1= k[A]v1^n / k[A]v2^n
Then solve and take log of both sides to find n
How do we define rate laws for reactions with more than one reactant?
aA + bB -> cC + dD
Rate= k[A]^m[B]^n
M and n are the orders for each reactant
The overall order is the sum of m and n
How do you find the rate law and rate constant for a reaction when given a table of the two reactants concentrations and the initial rates?
Example 3.2 and 3.3 on page 135-6!!
Take this equation: Rate= k[A]^m[B]^n
Find m for A by doing concentration of experiment 2 over concentration of experiment 1 to the power of m= rate of experiment 2/rate of experiment 1
Then do same for B
Then pick an experiment and plug the rate and he m and n into the formula up top and solve for k (rate constant)
What is the integrated rate law for a reaction?
The relationship between the concentrations of the reactants and time
What is the integrated rate law for first order, second order, and zero order?
What are the units for k for each?
(Don’t need to memorize but must be able to tell what’s what on test)
First- ln[A]_t = -kt + ln[A]_0 k=s^-1 Second- 1/[A]_t = kt + 1/[A]_0 k= M^-1•s^-1 Zero- [A]_t = -kt + [A]_0 k=M•s^-1
[A]_t is the concentration of A at the time (t)
[A]_0 is the concentration of A at time 0
Y=mx+b
REMEMBER THIS FOR GRAPHS OF EACH
What does the graph of first order integrated law look like?
the graph is ln[A]_t over time and the line is linear going down (or up not sure)
Slope= -k
What do you do when you have ln[A]=a number?
Switch it to [A] = e^(that number)
What does the graph for second order integrated law look like?
It is 1/[A]_t over time and it will be linear as well going up
Slope= k
What does the graph for zero order integrated law look like?
Graph of [A]_t over time and is linear as well and going down
Slope= -k
What is the half life of a reaction?
Time required for the concentration of a reactant to fall to one half of its initial value
What is first order reaction half life formula?
What does the graph look like?
T1/2= 0.693/k
Graph is concentration over time and the line drops down half concentration over a time after one half life then half that over another half life, etc.
What does it mean when a question asks for a concentration at one-eighth it’s initial value?
1 half life= 1/2 conc 2 half life= 1/4 3 half life= 1/8 Use 3 half life’s So 3t1/2
Example 3.6 on page 146
What is the equation for second order half life?
t1/2=1/k[A]_0
What is the equation for zero order half life?
t1/2= [A]_0/ 2k
What is the Arrhenius equation about activation energy and temperature?
k=Ae^(-Ea/RT)
A- frequency factor (times the reactants approach the activation barrier per unit time)
Ea- activation energy
R= gas constant (8.314 J/mol•K
What is the activated complex?
Also known as transition state on the top of the activation energy bump
As temperature increases, what happens with molecules that cross the activation energy?
How does this effect exponential factor?
More molecules with sufficient energy overcome the activation energy barrier
Exponential factor increases with increasing temp
What equation do you get when you take the ln of both sides of the Arrhenius equation?
[k=Ae^(-Ea/RT)]
What is slope? Y intercept?
ln k= -Ea/R (1/T) + ln A
Slope= -Ea/R
Y-intercept= ln A
This is given for tests but good to know
What do we use if we have two different rate constants at two different temperatures? And want to find activation energy
Use the formula
ln(k2/k1)= Ea/R (1/T1 - 1/T2)
Given in formula sheet
What is the equation for the collision model? (Not on formula sheet but easy to determine)
from k=Ae^(-Ea/RT)
k=pze^(-Ea/RT)
p=orientation factor (ratio of effective collisions to non effective)
Z=collision frequency (# of collisions per unit time)
What is a reaction mechanism?
What are elementary steps?
What are reaction intermediates?
A series of chemical steps by which an overall chemical reaction occurs
Elementary steps are the series of chemical steps that cannot be broken down into simpler ones
RI’s are elements that are consumed (crossed out cause they cancel) in the simpler steps of a reaction (they are the lows in the graph) U
What is molecularity?
What does the rate law have to do with it?
Number of reactant particulars involved in the step Unimolecular- A-> products Rate=k[A] Biomolecular- A+B-> products Rate= k[A][B] Termolecular- A+B+C-> products (rare) Rate= k[A][B][C]
A+A rate= k[A]^2
What is the rate determining step?
The slowest step that has the highest activation energy that the rate relies on
Ex: the slowest member of a track team
NOT NECESSARILY THE FIRST STEP
What are the two conditions in which mechanisms can be validated?
- The elementary steps in the mechanism must sum to the overall reaction (steps 1 and 2 must equal given equation l)
- The rate law predicted by the mechanism must be consistent with the experimentally observed rate law
Where are transition states on a graph?
Top of the curves
Look at graphs on page 165
How do you draw the first step with mechanisms with a fast initial step?
First step is reversible so k1 going right is equal to k-1 going left
Rate(forward)=rate(reverse)
See example on pages 167-169
What is (k2k1)/k-1 equal to?
k
What is a homogeneous catalyst compared to a heterogeneous catalyst?
Homo- catalyst exists in the same phase as reactants
Hetero- catalyst exists in a different phase than the reactants
What’s the difference between non integrated and integrated equations?
Non-integrated related rate and concentration while the integrated relate time and concentration
What does the linear graph of zero order look like?
It is concentration over time and it is a negative linear slope
What does the linear graph of first order look like?
It is a graph of ln(concentration) over time and it also has a linear negative slope
What does the linear graph of second order look like?
It is 1/(concentration) over time and it has a positive linear slope
When given a slow and fast reaction mechanism for a n overall reaction what do you use for the rate formula?
You use the reactants of the slow reaction mechanism like so:
rate=k[reactant 1][reactant 2]
