Teaching Chapter 6: rates of reactions.
Day 1: “POGIL” lesson on collision theory
Day 2: Phet Colorado simulation: rates of reactions- students investigate how temperature, concentration and activation energy affect the rate of a reaction.
Day 3: Students investigate how different factors affect the rate of a reaction by performing the iodine clock lab.
Day 4: Students are lectured about the rate law and the mechanism of a reaction.
Evaluation: Formal lab report
Learning expectations:
D2: investigate and analyse rates of reaction and solve related problems:
D2.8 plan and conduct an inquiry to determine how various factors (e.g., change in temperature, addition of a catalyst, increase in surface area of a solid reactant) affect the rate of a chemical reaction [IP, PR, AI]
D3: demonstrate an understanding of rates of reaction.
D3.5 explain, using collision theory and potential energy diagrams, how factors such as temperature, the surface area of the reactants, the nature of the reactants, the addition of catalysts, and the concentration of the solution control the rate of a chemical reaction
D3.6 describe simple potential energy diagrams of chemical reactions (e.g., the relationships between the relative energies of reactants and products and the activation energy of the reaction)
D3.7 explain, with reference to a simple chemical reaction (e.g., combustion), how the rate of a reaction is determined by the series of elementary steps that make up the overall reaction mechanism
1) Lesson plan for Learning about the collision theory.
Use lesson developed by ©POGIL 2005, 2006 Authored by: Bryan Horan; Revised by: Kelly Levy and Kenneth Levy Edited by Linda Padwa and David Hanson, Stony Brook University
· Using baseball to explain the collision theory
· Learning Objectives: Identify the requirements needed for a successful reaction to occur between reactant particles.
· Success Criteria: Explain the meaning of an effective collision and explain the requirements needed for a reaction to occur between reactant particles.
· This activity can be followed up by using the simulation reaction rate from PHET as outlined below:
Reactions & Rates
Single Collision
Pull back the knob. What happened? (Discuss with your partner(s))
Click the “Reload Launcher” button and expand the two windows on the right side of the program by clicking the “+” button for the Separation View and the Energy View. Now release the knob from various distances.
Indicate on the potential energy diagram when the reaction proceeds forward
Now click the Angled Shot option in the top right corner.
Try launching from a different angle or two.
Did the reaction proceed as before?
Why do you think this was the outcome despite having enough energy for the reaction to proceed?
Set the Choose a reaction option to the last preset chemical reaction that isn’t Design your own.
Now Change the Launcher Options back to Straight Shot and release the knob.
What happens to the translational speed of the molecules as the reaction goes forwards and backwards? (Hint: the effect will be easiest to observe at a low energy)
Please explain why this occurs.
Rate Experiments
Click on the “Rate Experiments” tab found at the top middle of the program window.
Set Options (bottom right corner) to Strip.
Increase the Initial Temperature (right side of window) until the Average Energy (green bar) is equal to the potential energy of the product
Set each reactant to 1. How much time does it take to react?
(Stop if it has been 2 minutes or roughly 3000 seconds on the programs graph)
If no reaction occurred, stop the reaction after 2 minutes and increase reactant by 1. Repeat until the reaction occurs.
How many reactants had to be used?
Explain how this reaction took place below the activation energy?
2) Lesson Plan for Reaction Rate Simulation
Approximate Length: 55 min
Learning Goal:
Students will be able to demonstrate their knowledge of reaction rates through illustrations and descriptions. These illustrations and descriptions should include:
· How varying the temperature changes the rate of a reaction.
· How varying the concentration changes the rate of a reaction.
· How activation energy changes the rate of a reaction.
Background
The previous day, the students were introduced to the “collision theory” through a POGIL activity, which is also linked to the Phet webpage. The activity begins by describing an effective collision in terms of baseball. Effective collisions will occur for the hits that results in a homerun. This analogy was used to discuss the aspects of the collision theory:
· Particles must have enough kinetic energy to produce an effective collision
· Particles must collide in the correct orientation for an effective collision to occur
Lesson Plan with Simulation:
Students will begin this lesson by discussing what “rate” means and provide examples. In small groups, students will draw the number of reactants and products present in a reaction over time. Reactants are represented by squares and products are represented by circles. Students will draw what the reaction looks like every 15 seconds. The illustrations will be discussed and shared as a large group.
The following graph is then shown to the students. In a small group, they are asked to label the terms “reactants”, “products”, and “activation energy” on the graph. This graph is then used to discuss how activation energy affects the number of effective collisions and the formation of the products in a reaction.
Students will then receive the handout and data tables used with the Reaction Rate Simulation. See Student Handout
After the Lesson:
The next day in class, students will be testing how variables such as temperature and concentration change the rate of a reaction by performing the Iodine Clock Reaction.
3) Performing the experiment
4) Lecturing rate Law and Mechanism of reactions
Use the same simulation go to reaction rate and set the average kinetic energy equal to the potential energy of the products.
Start out with 5 of each reactant.
Record how long it takes for 1 reaction to take place (The program counter will suffice).
Repeat the experiment for a total of 5 times.
Then throw out the fastest and slowest time and average the remaining 3 experiments.
Repeat this process for 10 reactants each and 15 reactants each.
Lecturing:
Rate Law
To find the rate law of a reaction, a minimum of two experiments will have to be performed where only one reactant’s initial concentration is changed. This will allow you to determine the order of the reaction for that particular reactant.
Finding Order of the Reaction for a Reactant and Rate Constant
Ex
Run 1 / 0.85 / 0.39Run 2 / 1.10 / 0.65
Run 3 / 1.60 / 1.38
Using Run 1 & 3:
Find Rate Constant
Using Run 3:
Find Rate Law
Problem
Find the Rate Law given the following information.
Run 1 / 1.00 / 1.00 / 1.30 x 10-3Run 2 / 1.50 / 1.00 / 2.93 x 10-3
Run 3 / 1.50 / 3.00 / 8.78 x 10-3
Chemistry 12
Worksheet 1-3 - Reaction Mechanisms
1. It is known that compounds called chlorofluorocarbons (C.F.C.s) (eg. CFCl3) will
break up in the presence of ultraviolet radiation, such as found in the upper atmosphere, forming single chlorine atoms:
CFCl3 à CFCl2 + Cl
The Cl atoms then react with Ozone (O3) as outlined in the following mechanism.
Step 1: Cl + O3 à ClO + O2
Step 2: ClO + O à Cl + O2 (single "O" atoms occur naturally in the atmosphere.)
a) Write the equation for the overall reaction. (Using steps 1 and 2)
______
b) What is the catalyst in this reaction? ______
c) Identify an intermediate in this reaction ______
d) Explain how a small amount of chlorofluorocarbons can destroy a large amount of ozone.
______
______
e) What breaks the bond in the CFCl3 and releases the free Cl atom?
______
2. Given the following mechanism, answer the questions below:
Step 1: O3 + NO à NO2 + O2 (slow)
Step 2: NO2 + O à NO + O2 (fast)
a) Give the equation for the overall reaction.
______
b) What could the catalyst be in this mechanism? ______
c) What is an intermediate in this mechanism? ______
d) Given that the uncatalyzed overall reaction is a slow exothermic reaction, draw a potential energy graph which shows the possible shape of the curve for the
uncatalyzed reaction. On the same graph, show a possible curve for the catalyzed
reaction.
Progress of Reaction
3. Consider the following mechanism:
Step 1: H2O2 + I- à H2O + IO- (slow)
Step 2: H2O2 + IO- à H2O + O2 + I- (fast)
a) Give the equation for the overall reaction.
______
b) What acts as a catalyst in this mechanism? ______
c) What acts as an intermediate in this mechanism? ______
4. What is meant by the rate determining step in a reaction mechanism? ______
______
5. What is meant by a reaction mechanism? ______
______
6. How are reaction mechanisms determined? ______
______
7. Given the following Potential Energy Diagram for a 3 step reaction, answer the questions below it:
a) Which arrow indicates the activation energy for the first step of the reverse reaction?
______
b) Which arrow indicates the activation energy for the first step of the forward reaction?
______
c) Which arrow indicates the activation energy for the second step of the forward reaction?
______
d) Which arrow indicates the enthalpy change (DH) or "enthalpy change" for the overall
forward reaction? ______
e) Which arrow indicates the enthalpy change (DH) or "enthalpy change" for the overall
reverse reaction? ______
f) Which arrow indicates the activation energy for the overall forward reaction?
______
g) Which step would be the rate determining step in the forward reaction? ______
h) In a dashed line or another colour sketch a possible curve that would represent the route
for the uncatalyzed overall reaction. Label this on the graph.
8. Given the reaction:
4HBr + O2 à 2H2O + 2Br2
a) Would you expect this reaction to take place in a single step? ______
Why or why not? ______
b) This reaction is thought to take place by means of the following mechanism:
Step 1: HBr + O2 à HOOBr (slow)
Step 2: HBr + HOOBr à 2HOBr (fast)
Step 3: 2HBr + 2HOBr à 2H2O + 2Br2 (fast)
c) Identify the two intermediates ______
d) A catalyst is discovered which increases the rate of Step 3. How will this affect the rate
of the overall reaction? ______
Explain your answer. ______
______
e) A catalyst is discovered which increases the rate of Step 1. How will this affect the rate
of the overall reaction? ______
Explain your answer. ______
______
f) Which step has the greatest activation energy? ______
g) How many "bumps" will the potential energy diagram for the reaction mechanism have?
______
h) Which step is called the rate determining step in this mechanism? ______
i) In order to have successful collisions, the colliding particles must have both the proper
amount of energy and the proper ______
j) On the set of axes below, draw the shape of the curve you might expect for the reaction
in this question. The overall reaction is exothermic! Make sure you get the "bumps" the
correct relative sizes.
9. The equation for an overall reaction is:
I- + OCl- à IO- + Cl-
a) The following is a proposed mechanism for this reaction. One of the species has been left out. Determine what that species is and write it in the box. Make sure the charge is correct if it has one!
Step 1: OCl- + H2O à HOCl + OH- ( fast )
Step 2: I- + HOCl à IOH + Cl- ( slow )
Step 3: IOH + OH- à + H2O ( fast )
b) Which species in the mechanism above acts as a catalyst? ______
c) Which three species in the mechanism above are intermediates? ______
d) Step ______is the rate determining step.
e) On the set of axes below, draw the shape of the curve you might expect for the
reaction in this question. The overall reaction is endothermic! Make sure you get the
"bumps" the correct relative sizes.
10. Given the following steps for a mechanism:
Step 1: Br2 à 2Br (fast)
Step 2: Br + OCl2 à BrOCl + Cl (slow)
Step 3: Br + Cl à BrCl (fast)
a) Write the equation for the overall reaction.
______
b) A substance is added that decreases the activation energy for step 1. Will this speed
up, slow down, or have no effect on the rate of the overall reaction? ______
Give a reason for your answer. ______
c) Is there a catalyst in this mechanism? ______. If so, what is it? ______
d) Is there an intermediate in this mechanism?______. If so, what is it? ______
e) Which step is the rate determining step? ______
11. The following potential energy diagram refers to a very slow one-step reaction of ozone (O3) and oxygen atoms in the upper atmosphere.
On the axis below, draw a potential energy diagram which could represent the catalyzed mechanism for the reaction:
Step 1: O3 + NO à NO2 + O2 (slow)
Step 2: NO2 + O à NO + O2 (fast)
12. A certain chemical can provide a reaction with an alternate mechanism having a greater activation energy. What will happen to the rate of the reaction when this chemical is added?
______
Explain your answer. ______
______
13. The following overall reaction is fast at room temperature:
H+ + I- + H2O2 à H2O + HOI
A student proposes the following two-step mechanism for the above reaction:
Step 1 : H+ + H+ + H2O2 à H4O22+
Step 2 : H4O22+ + I - à H2O + HOI + H+
Would you agree or disagree with this proposed mechanism? ______