During Class Invention # Introduction to Kinetics Name(s) with Lab section in Group ______________________ __________________________________________ ________ 1. List four factors which affect the rate of a chemical reaction. For each provide a brief statement describing how it affects the speed of a chemical reaction. 2a. Define the term reaction rate. b) For the following chemical reaction 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) write the rate equation in terms of i) the change in concentration of N 2 O 5 with time; ii) the change in concentration of NO 2 with time; iii) the change in concentration of O 2 with time; iv) write a statement that relates the rate of appearance of NO 2 to the rate of appearance of O 2 ;
Transcript
During Class Invention #
Introduction to Kinetics
Name(s) with Lab section in Group ________________________________________________________________________
1. List four factors which affect the rate of a chemical reaction. For each provide a brief statement describing how it affects the speed of a chemical reaction.
2a. Define the term reaction rate.
b) For the following chemical reaction
2N2O5(g) 4NO2(g) + O2(g)
write the rate equation in terms of i) the change in concentration of N2O5 with time;
ii) the change in concentration of NO2 with time;
iii) the change in concentration of O2 with time;
iv) write a statement that relates the rate of appearance of NO2 to the rate of appearance of O2;
v) write a mathematical statement that relates the rates of the reactants and products in the reaction to each other.
3. In the plot below, three lines, labeled A, B and C are shown. Identify (use the letter) which line best represents the average rate, instantaneous rate and initial rate for the chemical reaction.
During Class Invention #
Rates of Reaction
Name(s) with Lab section in Group ________________________________________________________________________
Plot the data for Exp. #1 and determine the average rate of the reaction between 8 and 24 min., the instantaneous rate of the reaction at 8 minutes and the initial rate of the reaction.
b. Plot the data for Exp. #2 and determine the average rate of the reaction between 8 and 24 minutes, the instantaneous rate of the reaction at 8 minutes and the initial rate of the reaction.
c. By what factor did the initial concentration change in going from Exp #1 to Exp #2?
d. By what factor did the initial rate change in going from Exp #1 to Exp #2?
e. Write an equation which describes how the initial rate of the reaction depends on the initial concentration.
During Class Invention #
Method of Initial Rates
Name(s) with Lab section in Group ________________________________________________________________________
1. Define the terms; rate equation and rate law for a chemical reaction.
2. Write the general rate law for the following reaction;
2NO(g) + Cl2(g) 2NOCl(g)
Identify the rate constant in the rate law. What are the exponents in the rate law called?
3. What experimental data is needed to determine the order of a chemical reaction?
4. Consider the reaction2NO(g) + 2H2(g) N2(g) + 2H2O(g)
iii) Write the specific rate law for the reaction.
iv) Determine the rate constant for the reaction (include units).
b. The following initial rate data were collected for the reaction
2NO2(g) + F2(g) 2NO2F(g)
at 100 ˚C.
Exp. [NO2] [F2] initial rate (M/sec)1 0.0482 M 0.0318 M 1.90 x 10–3
2 0.0120 M 0.0315 M 4.69 x 10–4
3 0.0480 M 0.127 M 7.57 x 10–3
i) Determine the reaction order for NO2 and F2.
ii) Determine the overall order of the reaction.
iii) Write the specific rate law for the reaction.
During Class Invention #
Integrate Rate Law Part I
Name(s) with Lab section in Group ________________________________________________________________________
1. Write the stepwise derivation of the general mathematical equation which relates concentration to time for a simple first order reaction.
2. The decomposition of H2O2 to H2O and O2 follows first order kinetics with a rate constant of 0.0410 min–1 at a particular temperature.
H2O2(l) 2H2O(l) + O2(g)
Calculate the [H2O2] after 10 mins if [H2O2]0 is 0.200 M.
3. The decomposition of N2O5 to O2 and NO2 follows first order kinetics. If a sample at 25 ˚C with the initial concentration of N2O5 of 1.25 x 10–3 M falls to 1.02 x 10–3 M in 100. minutes, calculate the rate constant for the reaction.
4. Show how a plot of ln[concentration] versus time can provide the rate constant for a reaction which follows simple first order kinetics.
5. Using the following data, establish that the decomposition N2O5 according to the reaction,
2N2O5(g) 2NO2(g) + O2(g)
follows first order kinetics. Determine the rate constant for the reaction.Time (sec) [N2O5] (M)
0 1.50 x 10–3
2000 1.40 x 10–3 5000 1.27 x 10–3
7000 1.18 x 10–3
11000 1.03 x 10–3
15000 9.00 x 10–4
During Class Invention #
Half Life
Name(s) with Lab section in Group ________________________________________________________________________
1. Derive a mathematical equation for the half-life for a reaction that follows simple first order kinetics.
2. The decomposition of H2O2 to H2O and O2 follows first order kinetics with a rate constant of 0.0410 min–1 at a particular temperature.
H2O2(l) 2H2O(l) + O2(g)
How long would it take for half of the H2O2 to decompose?
During Class Invention #
Integrate Rate Law Part II
Name(s) with Lab section in Group ________________________________________________________________________
1. Write the stepwise derivation of the general mathematical equation which relates concentration to time for a simple second order reaction.
2a. The decomposition of NOCl(g)
2NOCl(g) 2NO(g) + Cl2(g)
is a second order reaction with a rate constant of 0.0480 M–1·sec–1 at 200 C. In an experiment at 200 C, the initial concentration of NOCl was 0.400 M. What is the concentration of NOCl after 15.0 min have elapsed?
b. How many minutes will it take for the concentration of NOCl(g) to drop to 0.150 M?
3. Derive a mathematical equation for the half-life for a reaction which follows simple second order kinetics.
4. The initial concentration of NOCl, described in 2a above, is 0.400 M. Calculate the half-life for the decomposition reaction.
5. Show how a plot of ln [concentration] versus time can provide the rate constant for a reaction which follows simple second order kinetics.
6. Using the following data establish that the decomposition NO2 according to the reaction,
2NO2(g) 2NO(g) + O2(g)
following second order kinetics. Determine the rate constant for the reaction.Time (sec) [NO2] (M)
Name(s) with Lab section in Group ________________________________________________________________________
1. The following rate data was obtained at different temperatures for the reaction
O3(g) + NO(g) O2(g) + NO2(g)
Temperature (K) k (M-1·sec-1)600 0.28650 0.22700 1.30750 6.00800 23.0
Sketch the plot of ln k (y-axis) versus
2. Write the Arrhenius equation and define each term.
3. Define the term activation energy.
4. At 300 ˚C the rate constant for the reaction
H2C=CH-CH3
is 2.41 x 10-10 sec-1. At 400 ˚C the rate constant is 1.16 x 10-6 sec-1. Calculate the activation energy for the reaction.
b. Estimate the rate of the rearrangement reaction at 800 ˚C.
c. If the activation energy for the decomposition of N2O5 is 1.0 x 102 , calculate the temperature change necessary to double the rate at room temperature.
5. Sketch the energy profile diagram for the exothermic reaction
NO(g) + O3(g) NO2(g) + O2(g)
and label the important features, including reactants, products, activated complex, the energy of activation and the enthalpy of the reaction.
During Class Invention #
Reaction Mechanisms
Name(s) with Lab section in Group ________________________________________________________________________
1. Given the chemical equation
R + BG RG + B
For the reaction to occur above an R atom must collide with a BG molecule. The collision must have the correct orientation, and the particles in the collision must have enough energy to break the B-G bond.
2. Write the general differential form of the rate law for the reaction in Q1 above.
Rate = k[R]1 [BG]1
3. Define the terms reaction mechanism and rate determining step.
A reaction mechanism attempts to describe the stepwise sequence of elementary reactions that take reactants to products. The mechanism describes in detail the bonds that are broken and formed as the reaction proceeds.
Every mechanism consists of a series of stepwise reactions. Each reaction in the mechanism has a rate associated with it. The overall speed of the reaction depends upon the slowest step of the mechanism. The rate law of this step is identical to the experimental rate law. The slow step of the mechanism is also called the rate determining step of the mechanism. The sum of all the steps of the mechanism must equal the overall balanced chemical equation.
4. List several characteristics of a reasonable mechanism for a chemical reaction.
Each elementary step of a mechanism typically involves one, two or three reactants combining to form products. The steps of a mechanism must add together to yield the overall balanced chemical equation. The coefficients of the reactants in the rate determining step of the mechanism must correspond to the exponents, or order of
the reactants in the experimental rate law.
5. Look at the simulation (http://introchem.chem.okstate.edu/DCICLA/K2GBM.htm) for the reaction,
2G + B G2BThe rate law is known to be,
Rate = k[G]2
Suggest a possible mechanism for the reaction.
1) G + G G22) G2 + B G2B
6. The rate law for following reaction 2NO2(g) + F2(g) 2NO2F(g)
was experimentally determined to berate = k[NO2]1[F2]1
which of the following mechanisms is the most reasonable? Explain your reasoning for making the choice you did.
a) 2NO2(g) + F2(g) 2NO2F(g)
The experimental rate law indicates the slow step of the mechanism involves the collision of a single NO2 molecule and a single F2 molecule. This mechanism has two molecules of NO2 molecule colliding with a single F2 molecule. This is not in agreement with the experimental rate equation.
This reaction is a rearrangement of the CH 3, methyl, group. In the reactants, the methyl group is bonded to the carbon atom and, in the product, the methyl group is bonded to the nitrogen atom. Since the reaction is first order, the mechanism consists of a single step. The methyl group must move from the carbon atom to the nitrogen atom through an intermediate where the methyl group interacts with both atoms.A diagram of the intermediate is shown below.
8. Suggest a mechanism for the reaction
OH- + CH3Br CH3OH + Br-
if the experimental rate law is rate = k[CH3Br]1[OH-]1.
The mechanism for this reaction is single step. When a hydroxide ion collides with a CH 3Br molecule in the correct orientation, the hydroxide ion displaces the bromide ion forming methanol and bromide ion. The hydroxide ion must collide with the CH3Br on the side opposite of the bromine atom.
9. Suggest a mechanism for the reaction
NO2(g) + CO(g) NO(g) + CO2(g)
if the experimental rate law is rate = k[NO2]2.
In this example, the experimental rate law suggests the slow step of the mechanism involves the collision of two NO 2 molecules. A possible mechanism is shown below,
The elementary steps of the mechanism sum to yield the overall balanced chemical equation. Notice that NO3 is an intermediate in this mechanism.
10. Consider the following set of equations
Step I: Cl(g) + O3(g) ClO(g) + O2(g)
Step II: ClO(g) Cl(g) + O(g)
O3(g) O(g) + O2(g)
Describe the process illustrated by the above set of equations and the role each of the species plays in the process. Use words like mechanism, elementary steps, overall reaction, reactants, products, intermediate, and catalyst in your description.
The mechanism for the decomposition of ozone, O3, is catalyzed by the presence of chlorine radicals, Cl. The first step in the decomposition is the reaction between Cl(g) and O3(g) to form an intermediate, ClO, and oxygen, O2. The intermediate decomposes to reform the chlorine radical and an oxygen atom.
