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CHEM150 Labs
Fall 2024
Lab #10: How fast is that chemical reaction?
Chemical Kinetics and the Iodine Clock
Introduction
Whether we want to understand a drug or toxin in our bodies or a pollutant in water, we need to
know how that chemical behaves. Specifically, we need to know how that chemical reacts and
how long that chemical reacts for or remains in the system. Last week in lab, we saw how
thermodynamics tells us about energy in the system and whether reactions release or require
energy to proceed. This week in lab, we will see that kinetics tells us about how fast a reaction
proceeds to reach an energetically stable state. An example of the difference between
thermodynamics and kinetics is: “According to thermodynamics, graphite is the more stable
form of carbon, so all diamonds will eventually transform into graphite. However, after
calculating the kinetics, this reaction will take millions and millions of years to occur at room
temperature – and so our precious gemstones are not in danger of reverting to graphite anytime
soon .” It is important to understand both thermodynamics and kinetics when studying chemical
processes.
Reaction Rates
Chemical reaction rates can be thought of like velocities or speed, for example the speed of a car
in miles per hour. The rate of reaction (R) is defined as the change in concentration of a species
in the reaction per unit time. We measure the rate of the reaction by measuring the increase in the
product concentration or decrease in the reactant concentration.
In the case of the following reaction, A+B→ C
𝑑 𝐴[ ]
𝑑𝑡
𝑑 𝐵[ ]
𝑑𝑡
𝑑 𝐶[ ]
𝑑𝑡
The rate of reaction depends on a balanced chemical equation, accounting for stoichiometric
coefficients.
In the case of 2A+B → C
1
2
𝑑 𝐴[ ]
𝑑𝑡
𝑑 𝐵[ ]
𝑑𝑡
𝑑 𝐶[ ]
𝑑𝑡
Divide by the stoichiometric coefficient (rather than multiply) since the overall rate of reaction is
constant. In other words, substance A goes from 2A to zero in the same time it takes substance B
to go from 1B to zero. Since the overall rate of reaction must be the same regardless of which
species we examine, we must divide the rate of disappearance by the stoichiometric coefficient in
order to maintain the equality.
Rate Law, Order of Reaction and Rate Constants
In many reactions, a rate-limiting step (such as in enzyme kinetics or catalysis) determines the
overall rate, and the addition of more reactants has little to no effect on the overall rate. In other
cases, changing any one of the reactant’s concentration can have dramatic effects on the overall
rate of reaction. The dependence of the reaction rate on reactant concentrations can be seen in the
rate law. The rate law below relates the rate of reaction (R) and the concentration of reactants
Adapted from USF CHEM 114 labs based on an experiment by Jim Klent at Ohlone College,
modified by UC Berkeley Green Chemistry Program and Larry Margerum (version 2020),
(not products). Each reactant concentration is raised to a power that indicates the order of the
reaction. This is determined experimentally.
For the reaction A+B→ C,
with respect to A, 𝑅 = −
𝑑 𝐴[ ]
𝑑𝑡
= − 𝑘 𝐴[ ]
𝑛
for the reaction, the rate law is R= k [A]
n [B]
m
k is the reaction rate constant , if k is small the reaction will proceed slowly while if k is large
the reaction will be rapid. The units of k depend on the order of reaction (1/sec, 1/Molar sec,
1/M
2 sec)
If the order of the reaction with respect to A, n, is 0, then R= k and the rate of reaction does not
depend on the concentration A.
If n=1, then doubling the concentration of A will double the rate of reaction.
If n=2, doubling the concentration of A increases the rate by a factor of 4 (
2 =4)
The overall order of a reaction is the sum of the orders of the individual reactants.
order of the reaction = x + y
In lab, you will study the redox reaction of iodine in acidic solution. The net ionic reaction is:
6I
(aq) → 3I2(aq) + Br
To determine the reaction rate and if the rate depends upon the amount of iodine, bromate, or
acid, we will find the rate law by changing the quantity of one reactant at a time and determine
the effect on the rate:
R = k [I
x [BrO 3
y [H
]
z
To determine the constants x, y, and z, we will use the method of initial rates or instantaneous
rate. You will measure the reaction rate for a short time (because the initial concentrations will
not change much). You will then run the reaction a second time, this time doubling the
concentration of one species (for example, I
- ), while holding the others constant. We will find the
order of the reaction with respect to the changed individual reactant (in this case, x) by diving in
the two rate equations:
𝑟𝑎𝑡𝑒 1
𝑟𝑎𝑡𝑒 2
𝑘[𝐼
− ]
𝑥 [𝐵𝑟𝑂 3
− ]
𝑦 [𝐻
]
𝑧
𝑘[2𝐼
− ]
𝑥 [𝐵𝑟𝑂 3
− ]
𝑦
[𝐻
]
𝑧
Since we have double the concentration of I
- between trials and nothing else changed, the rate is
proportional to 1/time, after simplifying:
𝑟𝑎𝑡𝑒 1
𝑟𝑎𝑡𝑒 2
[𝐼
− ] 𝑡𝑖𝑚𝑒,
[𝐼
− ] 𝑡𝑖𝑚𝑒,
𝑘 𝐼
−
[ ]
𝑥
𝑘 2𝐼
−
[ ]
𝑥
Adapted from USF CHEM 114 labs based on an experiment by Jim Klent at Ohlone College,
modified by UC Berkeley Green Chemistry Program and Larry Margerum (version 2020),
In lab today, you and your partner will study this net ionic reaction and determine the rate law:
6I
(aq) → 3I2(aq) + Br
= k [I
x [BrO 3
y [H
]
z 𝑅 =−
1
6
∆ 𝑆 2
𝑂 3
2− ⎡ ⎢ ⎣
⎤ ⎥ ⎦ ∆𝑡
Goals
● Experimentally determine the rate or reaction, reaction rate constant and order of reaction
for a chemical reaction
Key Concepts
● Kinetics
● Reaction rate constant k
● Reaction rate order
● Rate law
Materials and Equipment
● 4 graduated pipettes
● Two 250 Erlenmeyer flasks
● Graduated cylinder
● White paper
● Chemicals: KI, Na 2 S 2 O3, KBrO 3 , HCl, starch
Pre-lab (to be completed on Canvas by midnight the night before your lab section)
- (1 pt) Why will you add starch to the reactions in this law? What does starch react with?
- (2 pt) What color is the color change and what does it indicate?
- (2 pt) How many moles of iodine react with how many moles of thiosulfate?
Watch Crash Course Chemistry #32: Demolition Derby
- (2 pt) What branch of chemistry studies how collisions between molecules affect the rate
of reactions?
- (2 pt) What is a reaction rate? Is it determined theoretically or experimentally?
- (1 pt) What are the units for the rate of a chemical reaction?
- What questions do you have about this lab? What did you find interesting or confusing?
Adapted from USF CHEM 114 labs based on an experiment by Jim Klent at Ohlone College,
modified by UC Berkeley Green Chemistry Program and Larry Margerum (version 2020),
Procedure
- Label two 250 mL Erlenmeyer flasks with tape, one labeled “A” and the other labeled “B”
- Label 4 pipettes with tape: “KI” “Na 2 S 2 O 3 ” “KBrO 3 ” “HCl”
- Set up the reaction mixtures in Table 1 one at a time.
- Pour the contents of flask B into flask A and immediately start the timer. 5. Swirl the mixture on the bench for about 10 seconds and then let it sit on a piece of white
paper so you can see the color change. (Swirl like you did during the titrations)
6. Stop the timer the second blue color appears. Watch closely for the slightest color change.
- Pour the mixture in the waste bottle in the hood and rinse the flasks with DI water in between
trials.
- Do three trials for each reaction mixture.
- Record all your raw data.
Table 1. Reaction mixtures for kinetic trials.
* Note: before adding the starch, gently shake it because starch can settle out of solution
Reaction
Mixture
Mixture A Mixture B
0.01000M KI
mL added
H 2 O mL
added
0.001000 M
Na 2 S 2 O 3
mL added
0.04000 M
KBrO 3
mL added
0.1000M
HCl
mL added
Starch
(drops)
4 10.00 0.00 10.00 10.00 20.00 5]
Question #1: What is the final volume of each reaction mixture?
Table 2. What should the title be?
Reaction
Mixture
What variable
changed in the
reaction mixture?
Reaction Times Average
Time
Trial 1 Trial 2 Trial 3
1 Compared to rxn 1
Question #2: Describe any trend you see in the time data.
Calculate the concentration of each reactant in the reaction mixture.
Table title: What should go here?
Reaction
Mixture
[I
- ] [BrO 3 - ] [H
+ ] [S 2 O 3
2- ]
Provide one sample calculation. Be sure to include all units.
Adapted from USF CHEM 114 labs based on an experiment by Jim Klent at Ohlone College,
modified by UC Berkeley Green Chemistry Program and Larry Margerum (version 2020),
Post-lab Questions
Please complete all post-lab questions in a word document (graphs made in excel can be cut and
paste into the word document) and upload a pdf to Canvas. If you use external sources to answer
the following questions, include all citations.
- (3 pt) In your own words and at the level that a non-chemist could understand, describe
the difference between thermodynamics and kinetics. Use examples from the
biodiesel/fuels lab last week and the lab this week. \
- (2 pt) Describe what is happening in the reaction mixture before you observed a color
change to blue?
- (2 pt) Why did the change in time before you saw a color change differ between the
reaction mixtures?
- (2 pt) What is the reaction rate for each reaction mixture? Be sure to include units. Are
any of the rates twice as much or four times as much as the rate for reaction mixture 1?
What does this tell you about the order of reaction and whether the rate is dependent on
the concentration of the reactants?
- (2 pt) What is the rate constant, k , you calculated based on your experiments? Be sure to
include units. Is the rate constant similar between all the reaction mixtures? Why or why
not?
6. (2 pt) What is the order of the reaction? What does this tell you about the reaction? Does
the rate of reaction depend on the concentration of I
? How? Describe in
words.
- (4 pt) Describe an environmental or engineering example of when knowing the kinetics
of a chemical reaction would be important.
- (1 pt) What are the potential sources of error in this lab? What are the potential
weaknesses or limitations in your data collection process? How would these sources of
error affect your results? How would you minimize them in future calorimetry
experiments?
- (2 pt) In 2-3 sentence(s), what are your main takeaways?
Adapted from USF CHEM 114 labs based on an experiment by Jim Klent at Ohlone College,
modified by UC Berkeley Green Chemistry Program and Larry Margerum (version 2020),
