Chapter 3- Chemical Kinetics

Question 1

How do we define the rate of a reaction? (Find the rate of reaction)

Answer 1

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!!

Question 2

How do you find the concentration of a product when given the time intervals and concentration counts of a reactant?

Answer 2

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

Question 3

How do you find the average rate of a reaction?

When given a table of time and concentration

Answer 3

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

Question 4

What is the instantaneous rate of change of a reaction?

Answer 4

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

Question 5

What is the overall rate formula to memorize for this equation:
aA + bB -> cC + dD

Answer 5

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

Question 6

What is the rate law?

Answer 6

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

Question 7

What does it mean if n equals 0? 1? 2?

Answer 7

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

Question 8

How do we find n for a reaction with one reactant when given the concentration and initial rate of that reactant?

Answer 8

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

Question 9

How do we define rate laws for reactions with more than one reactant?
aA + bB -> cC + dD

Answer 9

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

Question 10

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?

Answer 10

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)

Question 11

What is the integrated rate law for a reaction?

Answer 11

The relationship between the concentrations of the reactants and time

Question 12

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)

Answer 12

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

Question 13

What does the graph of first order integrated law look like?

Answer 13

the graph is ln[A]_t over time and the line is linear going down (or up not sure)
Slope= -k

Question 14

What do you do when you have ln[A]=a number?

Answer 14

Switch it to [A] = e^(that number)

Question 15

What does the graph for second order integrated law look like?

Answer 15

It is 1/[A]_t over time and it will be linear as well going up
Slope= k

Question 16

What does the graph for zero order integrated law look like?

Answer 16

Graph of [A]_t over time and is linear as well and going down
Slope= -k

Question 17

What is the half life of a reaction?

Answer 17

Time required for the concentration of a reactant to fall to one half of its initial value

Question 18

What is first order reaction half life formula?

What does the graph look like?

Answer 18

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.

Question 19

What does it mean when a question asks for a concentration at one-eighth it’s initial value?

Answer 19

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

Question 20

What is the equation for second order half life?

Answer 20

t1/2=1/k[A]_0

Question 21

What is the equation for zero order half life?

Answer 21

t1/2= [A]_0/ 2k

Question 22

What is the Arrhenius equation about activation energy and temperature?

Answer 22

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

Question 23

What is the activated complex?

Answer 23

Also known as transition state on the top of the activation energy bump

Question 24

As temperature increases, what happens with molecules that cross the activation energy?
How does this effect exponential factor?

Answer 24

More molecules with sufficient energy overcome the activation energy barrier

Exponential factor increases with increasing temp

Question 25

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?

Answer 25

ln k= -Ea/R (1/T) + ln A
Slope= -Ea/R
Y-intercept= ln A

This is given for tests but good to know

Question 26

What do we use if we have two different rate constants at two different temperatures? And want to find activation energy

Answer 26

Use the formula
ln(k2/k1)= Ea/R (1/T1 - 1/T2)
Given in formula sheet

Question 27

What is the equation for the collision model? (Not on formula sheet but easy to determine)

Answer 27

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)

Question 28

What is a reaction mechanism?
What are elementary steps?
What are reaction intermediates?

Answer 28

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

Question 29

What is molecularity?

What does the rate law have to do with it?

Answer 29

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

Question 30

What is the rate determining step?

Answer 30

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

Question 31

What are the two conditions in which mechanisms can be validated?

Answer 31

1. The elementary steps in the mechanism must sum to the overall reaction (steps 1 and 2 must equal given equation l)
2. The rate law predicted by the mechanism must be consistent with the experimentally observed rate law

Question 32

Where are transition states on a graph?

Answer 32

Top of the curves

Look at graphs on page 165

Question 33

How do you draw the first step with mechanisms with a fast initial step?

Answer 33

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

Question 34

What is (k2k1)/k-1 equal to?

Answer 34

k

Question 35

What is a homogeneous catalyst compared to a heterogeneous catalyst?

Answer 35

Homo- catalyst exists in the same phase as reactants

Hetero- catalyst exists in a different phase than the reactants

Question 36

What’s the difference between non integrated and integrated equations?

Answer 36

Non-integrated related rate and concentration while the integrated relate time and concentration

Question 37

What does the linear graph of zero order look like?

Answer 37

It is concentration over time and it is a negative linear slope

Question 38

What does the linear graph of first order look like?

Answer 38

It is a graph of ln(concentration) over time and it also has a linear negative slope

Question 39

What does the linear graph of second order look like?

Answer 39

It is 1/(concentration) over time and it has a positive linear slope

Question 40

When given a slow and fast reaction mechanism for a n overall reaction what do you use for the rate formula?

Answer 40

You use the reactants of the slow reaction mechanism like so:
rate=k[reactant 1][reactant 2]