Cypress College Determination of The Rate Law Lab Report

Kinetics:
Iodination of Acetone
Introduction
The study and understanding of reaction rates, rate laws and mechanisms are a central part of kinetics. Specifically, kinetics involves determination of the rate of a chemical reaction, and how the rate is affected by other
factors, particularly concentrations and temperature. First and foremost, kinetics is an experimental endeavor.
Only by careful experimentation can one obtain meaningful information about the kinetic properties of a reaction
system. In this experiment, you will determine the reaction rate law and the energy of activation for a reaction.
The rate law is an expression which represents the proportionality between the rate of a reaction and concentrations. In general, for a general reaction such as the one shown in Reaction IA.1,
aA + bB −−−→ cC + dD
(Reaction IA.1)
the rate is defined as the rate of change of any product or reactant concentration versus time (Equation IA.1).
Rate =
Δ[C]
Δ𝑡
(Equation IA.1)
Rates are always positive by convention. The generalized rate law can be written as Equation IA.2
Rate =
Δ[C]
= 𝑘[A]𝑚 [B]𝑛
Δ𝑡
(Equation IA.2)
where 𝑚 and 𝑛 are usually small integers; 𝑚 is the reaction order of reactant A, and 𝑛 is the reaction order of
reactant B. The sum, 𝑚 + 𝑛, is the overall reaction order, and 𝑘 is the rate constant. It is the goal of a kinetics
study to determine experimentally the values of the reaction orders and of 𝑘. In doing so, the general approach
is to first obtain values for the reaction orders by studying the effect that varying the concentration of a single
reactant has on the rate.
Once all the terms in Equation IA.2 are known, it is possible to obtain the value of the activation energy, the
minimum energy required for a reaction to occur. Activation energies are normally determined by studying the
reaction rate as a function of temperature. The relationship between the experimental rate constant, 𝑘, and the
activation energy, 𝐸𝑎 , is given by Equation IA.3
−𝐸𝑎
𝑘 = 𝑍𝑒 R𝑇
(Equation IA.3)
where 𝑒 = 2.7183 (the base of the natural logarithm), 𝑍 =collision factor, R = gas constant, 8.31 J⋅ mol−1 ⋅ K−1 ,
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IA.1
Rates
and 𝑇 =Kelvin temperature.
Mathematical rearrangement of Equation IA.3 gives Equation IA.4:
ln 𝑘 =
−𝐸𝑎 1
⋅ + ln 𝑍
R𝑇 𝑇
Equation IA.4 has the form of a linear relationship where
−𝐸𝑎
R𝑇
(Equation IA.4)
is the slope of the line of ln 𝑘 versus 1/T. One can
calculate 𝐸𝑎 from this slope.
The particular reaction in this experiment is the iodination of acetone, (CH3 )2 CO, as shown in Figure IA.1.
Figure IA.1: Chemical equation for the iodination of acetone.
The reaction is catalyzed by acids (H+ ion). Thus, the general rate law is as shown in Equation IA.5:
rate =
−Δ[I2 ]
= 𝑘[(CH3 )2 CO]𝑚 [I2 ]𝑛 [H+ ]𝑝
Δ𝑡
(Equation IA.5)
The negative sign is needed to make the rate a positive value since I2 is a reactant. Δ[I2 ] equals [I2 ]final – [I2 ]initial .
Note, the catalyst concentration appears in the general rate law. Values for the reaction orders can be readily
obtained by studying Δ[I2 ]/Δt as a function of each reactant concentration, including the catalyst, H+ . Once the
orders 𝑚, 𝑛, and 𝑝 are known, 𝑘 can be calculated. Finally, by studying the reaction at different temperatures, a
value for 𝐸𝑎 can be obtained.
The method, whereby a reaction rate is monitored, is of primary concern. In this experiment, you can take
advantage of the fact that I2 is colored, while I – is colorless. Thus, measuring the period of time required for the
disappearance of color gives the rate (Equation IA.6).
rate =
[I2 ]
𝑡
(Equation IA.6)
Equations such as Equation IA.5 generally are not linear simply due to the fact that the concentrations are constantly changing as the reaction progresses. We can obtain approximately linear behavior if the reaction is studied
over an extremely short interval such that all concentrations, except the one being monitored, remain essentially
constant. In this experiment this is accomplished by using an initial I2 concentration much less than those of the
other reactants. Thus, when I2 is depleted, the other reactants will essentially still have their initial concentrations. This approach, which is commonly used in kinetics, is called the method of initial rates.
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IA.2
Rates
Procedure
Safety Precautions
Safety/Waste: Nothing goes down the sink today! Put all of your chemicals and hazardous waste in the
recovery/waste bottles in the hood.
Chemical Hazards:
• Acetone – TOXIC! IRRITANT! Acetone is a clear, colorless liquid that is highly flammable. The
potential negative health effects are skin, eye and lung irritation.
• Iodine – TOXIC! IRRITANT! NEVER SWALLOW! Causes severe eye irritation. May cause eye
burns. Vapors cause eye irritation. Harmful if absorbed through the skin. May cause burns to the
digestive tract. Iodine is an easily oxidizable substance.
• Hydrochloric Acid – TOXIC! CORROSIVE! IRRITANT! Will BURN eyes severely. Will IRRITATE
and may BURN the skin. If taken internally there will be severe IRRITATION and DAMAGE.
Chemical Hazards:
• 4M Acetone: FLAMMABLE! IRRITANT! HEALTH HAZARD! Danger: Highly flammable liquid
and vapor. Causes serious eye irritation. May cause drowsiness or dizziness. May cause damage
to organs through prolonged or repeated exposure.
• 1M Hydrochloric Acid: CORROSIVE! IRRITANT! Danger: May be corrosive to metals. Causes
skin burns and eye damage. May cause respiratory irritation.
• 0.0050M Iodine: HEALTH HAZARD! IRRITANT! AQUATIC HAZARD! Danger: Harmful if
swallowed. Harmful in contact with skin or if inhaled. Causes skin irritation. Causes serious
eye irritation. May cause respiratory irritation. Causes damage to organs (Thyroid) through prolonged or repeated exposure if swallowed. Very toxic to aquatic life.
Waste Disposal/Clean-Up: All waste will be disposed in the waste container found in the first fume hood.
After these substances have been disposed into their correct waste container, wash glassware with detergent
and rinse 3 times with tap water and 3 times with DI water. Check-in stockroom items after cleaning them
properly.
This lab will be done with a partner, however everyone will write their own lab
report.
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IA.3
Rates
Part I. Determination of the Rate Law
1.
Obtain two six-inch test tubes. Fill each with distilled water, and view down the tubes against a white background. They should appear identical. Empty and dry one tube. Keep the other as a standard.
2.
For each mixture in Table IA.1 follow the same sequence of mixing reagents. That is, use your graduated
cylinder to measure first the volumes of 4.0 M acetone, then 1.0 M HCl and finally distilled H2 O into a 125
mL Erlenmeyer flask. Measure the volume of 0.0050 M I2 and keep it in a 10 mL graduated cylinder.
Table IA.1: Mixture Reagent Volumes
Reagent Volumes (mL)
MIXTURE
4.0 M
(CH3 )2 CO
1.0 M HCl
0.0050 M I2
Distilled H2 O
I
5
5
5
10
II
10
5
5
5
III
5
10
5
5
IV
5
10
2.5
7.5
low temp
30
10
2.5
7.5
high temp
10
5
10
25
Swirl the flask and then add the quantity of 0.0050 M I2 from the graduated cylinder. Start the timer and
swirl the flask again to mix the reagents thoroughly. Then fill the empty test tube with the mixture. It is not
necessary to transfer all of the reaction mixture since the rate will be the same for all or part of it.
3.
Looking down the test tubes against a white background, watch for the disappearance of the iodine color.
Stop the timer when the solution becomes colorless and record the number of seconds that have elapsed.
Finally record the temperature of the mixture.
4.
Perform Steps 2 and 3 with mixtures II, III and IV.
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IA.4
Rates
Part II. Determination of the Temperature Dependence of the Reaction Rate
You will make additional runs at two different temperatures: one at about 0 °C, the temperature of an ice-water
mixture and one close to 40 °C. The latter is approximately the temperature of the water in the 1.0 L beakers on
the hot plates on the lab benches. Be certain that you use the proper reaction mixture specified in the table.
1.
For the reaction rate at low temperature, use the volumes as indicated in the table. However, before mixing
the two solutions (the acetone-HCl-water mixture and the I2 solution), place them and the test tube in an
ice-water bath (a 600-mL beaker of ice with a little water) and measure the temperature of the mixture in the
flask. When the temperature ceases to change, record it and mix the two solutions. Start the timer and fill
the test tube as before and immediately place it back in the ice-water bath. Make sure the bath is on a piece
of white paper. Watch for the disappearance of the iodine color, stop the timer, and record the number of
seconds that have elapsed. You should begin preparing the second low temperature run as soon as possible
so it will be cooled and ready when the first is completed. Be certain to measure the temperature of the
ice-water bath periodically. If it changes, record the temperatures and use the average in your calculations.
2.
For the reaction rate at high temperature, follow the same procedure as for the low temperature run but use
a 40 °C bath. At high temperatures, acetone and iodine evaporate quite rapidly, so keep them in tightly stoppered containers while in the warm water. The reaction will proceed much faster than at room temperature,
so be alert.
CALCULATIONS
1.
Determining the Reaction Rate Law
a.
Initial Concentration of Reactants
For each run calculate the initial reactant concentrations present in the reaction mixture. The initial
concentration is not the value given in the table but the concentration of the reagent in each mixture
immediately after the reagents are combined. In each case the reagents have been diluted. Present all
the concentrations in tabular form.
b.
Initial Reaction Rates
The rate of your reaction, in units of M⋅𝑠−1 , is defined in Equation IA.6. [I2 ]initial is calculated in step
1a above and 𝑡 is the time in seconds required for the color to fade. Calculate the rate for each run and
present them in a table.
c.
Reaction Orders and Rate Law
As was mentioned, reaction orders are obtained by comparing the rates of two reactions which dif-
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IA.5
Rates
fer only in the concentration of a single reactant. Thus, to obtain the reaction order of acetone, you
should compare two runs at the same temperature wherein the [I2 ] and [H+ ] are the same, but [acetone] varies. Reaction mixtures, I and II, differ only in the concentration of acetone, (CH3 )2 CO. Hence
these two mixtures can be used to determine the value of 𝑚 in Equation IA.5. The relationship is shown
in Equation IA.7.
[(CH3 )2 CO]𝑚
Rate𝐼
𝐼
=
Rate𝐼𝐼
[(CH3 )2 CO]𝑚
𝐼𝐼
(Equation IA.7)
Similarly use your data to calculate the reaction orders of I2 and H+ . Round off the calculated values of
𝑚, 𝑛, and 𝑝 to the nearest integer, including zero. Then rewrite Equation IA.5 using your values for the
reaction orders. This is the experimental reaction rate law.
2.
Determining the Rate Constant Using your rate law, calculate the value of k for all reaction runs, low temperature, room temperature, and high temperature. Calculate the average value of k for room temperature
runs. Present these in a table.
3.
Determination of 𝐸𝑎
Calculate the ln 𝑘 of your average 𝑘 value at room temperature and your 𝑘 values at the low and high temperatures. Calculate 1/𝑇 where 𝑇 is the Kelvin temperature. Prepare a plot of ln 𝑘 versus 1/𝑇 and determine the
value of the slope of the best straight line drawn through your data points. Your slope value equals −𝐸𝑎 /𝑅
and has units of Kelvin. From this value calculate the activation energy for the iodination of acetone. This
activation energy is positive and usually in the range of 60 to 80 kJ.
REPORT
Your report must include your graph, all calculations and the reaction rate law, the values of the rate constants,
the value of the activation energy, and the answers to the questions below.
QUESTIONS
1.
What is the initial rate for the iodination of acetone given the following initial concentrations:
[acetone] = 2.5 M, [H+ ] = 1.5 M, and [I2 ] = 0.005 M?
2.
How does doubling the initial concentration of iodine, while keeping all others constant, affect the
initial rate? Explain your reasoning.
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IA.6
:
MOLECULAR WEIGHT DETERMINATION BY FREEZING-POINT
DEPRESSION
Introduction
The freezing point of the solvent in a true solution will always be lower than that of the pure solvent. This behavior
provides a useful method whereby molecular weights may be determined. The magnitude of the freezing point
depression depends only on the solvent and the proportion of solute to solvent particles, rather than the nature of
the solute. Freezing-point depression is a colligative property. The magnitude of the depression, Δ𝑇f , is directly
proportional to the mole fraction of the solute, 𝑋s (Equation FP.1).
Δ𝑇f = 𝐾f ⋅ 𝑋s
(Equation FP.1)
In Equation FP.1, Δ𝑇f is the freezing point lowering and equals 𝑇f(solvent) − 𝑇f(solution) , 𝐾f is a proportionality constant, and 𝑋s is the solute mole fraction. To get a more useful form of Equation FP.1, we can take advantage of the
fact that generally the moles of solvent are considerably greater than the moles of solute. Using the definition of
mole fraction shown in Equation FP.2,
𝑋s =
𝑛solute
𝑛solute + 𝑛solvent
(Equation FP.2)
and assuming that 𝑛solvent >> 𝑛solute , then Equation FP.3 is true,
𝑋s =
𝑛solute
𝑛solvent
(Equation FP.3)
At this point it is convenient to use the concept of molality as defined in Equation FP.4.
molality = 𝑚 =
𝑛solute
𝑘𝑔solvent
(Equation FP.4)
Starting with Equation FP.1 and using Equation FP.4, an alternative expression (Equation FP.5) can be derived
for very dilute solutions:
Δ𝑇f = 𝐾f ⋅ 𝑚
(Equation FP.5)
𝐾f is the molal freezing point depression constant and is unique for each solvent. Table FP.1 below gives some
examples.
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FP.1
Solutions
Table FP.1: Freezing-Point Depression Constants for Selected Solvents
Solvent
𝐾f (°C ⋅ molal−1 )
acetic acid (HC2 H3 O2 )
3.56
benzene (C6 H6 )
4.90
carbon tetrachloride (CCl4 )
31.80
ethanol (C2 H5 OH)
1.99
water
1.86
Using Equation FP.5, one can determine molecular weights from freezing point depression data. The only requirements for this determination are that the solute be soluble in the solvent of choice, and that the resultant
solution be dilute. Also, one tries to select the solvent with a moderate freezing point and large 𝐾f . The former
merely makes the procedure easier while the latter maximizes accuracy and sensitivity.
The experimental determination of a freezing point is not as straightforward as it may seem at first. Due to
the phenomenon of super-cooling, it is not satisfactory merely to measure the temperature at which a material
solidifies. Instead, one must collect data over a wide temperature range and use it to generate a cooling curve to
obtain the freezing point graphically. As with any experiment, success depends upon collecting data in a careful,
systematic fashion.
In this experiment, you will:
1.
Determine the freezing point of pure 𝑝-dichlorobenzene (solvent) and a solution of 𝑝-dichlorobenzene
with an unknown (solute).
2.
Determine the molecular weight of an unknown solute.
3.
Use your 600 mL (largest) beaker as a hot water bath..
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FP.2
Solutions
Procedure
Safety Precautions
Safety/Waste: Place chemicals in waste containers:
• p-Dichlorobenzene – CANCER SUSPECT AGENT! Toxic! Harmful if swallowed. Avoid breathing vapor. Prevent contact with eyes and skin. Inhalation of vapor may cause drowsiness and
IRRITAION of the nose. Long exposure may result in liver damage.
• Unknown Organic Solids – IRRITANTS! FLAMMABLE SOLIDS! Avoid breathing vapor. Prevent contact with eyes and skin.
• Acetone – FLAMMABLE LIQUID! IRRITANT! Avoid breathing vapor. Prevent contact with the
eyes. IRRITANT! The liquid IRRITATES the eyes and may cause severe damage. Inhalation of the
vapor may cause drowsiness.
Chemical Hazards:
• p-Dichlorobenzene: HEALTH HAZARD! IRRITANT! AQUATIC HAZARD! Danger: Causes serious eye irritation. May cause cancer. Very toxic to aquatic life with long lasting effects.
• Acetone: FLAMMABLE! IRRITANT! HEALTH HAZARD! Danger: Highly flammable liquid and
vapor. Cause serious eye irritation. May cause drowsiness or dizziness. May cause damage to
organs through prolonged or repeated exposure.
• Benzophenone: HEALTH HAZARD! AQUATIC HAZARD! Warning: May form combustible dust
concentrations in air. Suspected of causing cancer. May cause damage to organs through prolonged or repeated exposure. Toxic to aquatic life.
• Benzil: IRRITANT! Warning: Causes skin irritation. Causes serious eye irritation. May cause
respiratory irritation.
• Naphthalene: FLAMMABLE! IRRITANT! HEALTH HAZARD! AQUATIC HAZARD! Danger:
Flammable solid. Harmful if swallowed. May cause cancer. Very toxic to aquatic life with long
lasting effects.
• Biphenyl: IRRITANT! AQUATIC HAZARD! Warning: Causes skin irritation. Causes serious eye
irritation. May cause respiratory irritation. Very toxic to aquatic life with long lasting effects.
Waste Disposal/Clean-Up: Any test tubes with p-Dichlorobenzene shall be treated by putting it into the
hot water bath in the second fume hood. Add a small amount of acetone into the test tube to clean out the
𝑝-dichlorobenzene. Dispose the 𝑝-dichlorobenzene and acetone into the beaker labeled “PDB Waste.” Any
other extra organic solids shall be disposed into the “Organics Waste” found in the first fume hood. After
these substances have been disposed into their correct waste container, wash glassware with detergent, rinse
3 times with tap water and 3 times with DI water. Check-in stockroom items after cleaning them properly.
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FP.3
Solutions
Part I. Determination of the Freezing Point of p-Dichlorobenzene (solvent)
1.
Obtain the necessary apparatus and assemble it by copying the set-up on display.
2.
Record the mass of the test tube to the nearest 0.0001 g . Add about 15 g of pure 𝑝-dichlorobenzene to the
test tube and record the new mass of the test tube + 𝑝-dichlorobenzene to the nearest 0.0001 g.
3.
Set up a hot water bath by adding about 500 mL of distilled water to your 600 mL beaker and heating it to
about 75 °C.
Do not touch the surface of hot plate! Do not hold the hot beaker by hand!
Place the test tube containing the 𝑝-dichlorobenzene into the beaker and heat it until the compound is completely melted.
Stirrer but must be kept under the liquid!
Carefully fit the stirrer and thermometer into the test tube so that they are well immersed in the liquid.
Continue to heat the beaker and gently move the wire stirrer up and down to mix the liquid.
4.
When the temperature of the liquid is about 70 °C, carefully remove the apparatus from the hot water bath
and turn off the burner. Dry the outside of the test tube and allow it to cool while you occasionally stir the
liquid.
5.
Record temperature and time readings every 30 seconds, beginning at 65 °C. Note and record the temperature of the first appearance of solid. After the solid appears, continue to mix the contents and take
temperature-time data until the sample solidifies and it is no longer possible to stir it. Save your test tube
and 𝑝-dichlorobenzene since it will be used in Part II.
Reminder, you should have weighed the solvent container full and empty.
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FP.4
Solutions
Part II. Determination of the Molecular Weight of an Unknown (solute)
1.
Record the number of the unknown provided.
2.
Determine the mass of about half of the unknown to the nearest 0.0001 g and record it (it should be about
1.5 g).
3.
Add it to the test tube containing the 𝑝-dichlorobenzene from Part I.
4.
Melt the solids and collect the temperature-time data in the same manner as in Part I.
5.
When you have completed the first run, determine the mass of the remaining half of unknown (again, about
1.5 g) to the nearest 0.0001 g and record the mass.
6.
Add it to the mixture already in the test tube. Once again collect the temperature-time data as before.
When you have finished, discard the contents of the test tube into the waste container.
7.
Clean up: Rinse the test tube, thermometer, and stirrer with small portions of acetone over the waste container
in the fume hood. Wash these items accordingly as stated in the “waste disposal/clean-up” section and return
them to the stockroom.
Reminder, you should have weighed the unknown solute container full, after ½
removed, and empty.
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FP.5
Solutions
CALCULATIONS
1.
Temperature versus time plots by computer graphical method
Use your data from Part 1 and millimeter graph paper to make a plot of temperature versus time. You
should notice two distinct linear regions in your data points, draw two straight lines, one through each
linear region of the data points. If you do not see clear linear regions, use the first 3-4 data points to draw
one straight line and the final 3-4 data points to draw the second straight line.
2.
Freezing point of 𝑝- dichlorobenzene
The f reezing point of a l iquid i s t he t emperature a t t he point of i ntersection of t he t wo l ines
in t he graph a bove. Determine t he f reezing point of 𝑝- dichlorobenzene a nd e ach mixture f rom
the intersection of t he s traight-line portions of t he g raphs. Clearly i ndicate t he f reezing point on
each plot.
3.
Unknown Molecular Weight
For e ach r un use Equation FP.5 a nd a v alue of 7 .10 ° C⋅ molal−1 f or 𝐾f f or 𝑝- dichlorobenzene t o
calculate t he molality of t he mixture. Use Equation FP.4 t o c alculate t he moles of s olute. Then f rom
the definition of moles a nd t he weight of s olute, c alculate t he molecular weight of t he unknown
substance. Determine t he a verage v alue of t he molecular weight f or t he t wo determinations.
REPORT
In addition to the standard sections, your report must contain all temperature versus time plots, all calculations,
and the answers to the questions below.
QUESTIONS
1. What is supercooling?
2. Give two experimental errors, which would result in too high a value for the molecular weight. Explain
your reasoning.
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FP.6
Guidelines for writing a procedure in the style of a scientific paper
The Experimental (Materials and Methods) section should describe all experimental procedures in
enough detail so that someone else could repeat the experiment. Some guidelines to follow:
• Explain the general type of scientific procedure you used to study the problem.
• Describe what materials, subjects, and equipment you used (Materials).
• Explain the steps you took in your experiment and how did you proceed (Methods).
• Mathematical equations and statistical tests should be described.
This section explains how and, where relevant, when the experiment was done. The researcher
describes the experimental design, the apparatus, methods of gathering data and type of control. If any
work was done in a natural habitat, the worker describes the study area, states its location and explains
when the work was done. If specimens were collected for study, where and when that material was
collected are stated. The general rule to remember is that the Materials and Methods section should be
detailed and clear enough so that any reader knowledgeable in basic scientific techniques could
duplicate the study if she/he wished to do so.
DO NOT write this section as though it were directions in a laboratory exercise book. Instead of writing:
First pour agar into six petri plates. Then inoculate the plates with the bacteria. Then put the
plates into the incubator . . .
Simply describe how the experiment was done:
Six petri plates were prepared with agar and inoculated with the bacteria. The plates were
incubated for ten hours.
Also, DO NOT LIST the equipment used in the experiment. The materials that were used in the research
are simply mentioned in the narrative as the experimental procedure is described in detail. If wellknown methods were used without changes, simply name the methods (e.g., standard microscopic
techniques; standard spectrophotometric techniques). If modified standard techniques were used,
describe the changes.
How much detail? Be precise in providing details but stay relevant. Ask yourself, “Would it make any
difference if this piece were a different size or made from a different material?” If not, you probably
don’t need to get too specific. If so, you should give as many details as necessary to prevent this
experiment from going awry if someone else tries to carry it out. Probably the most crucial detail is
measurement; you should always quantify anything you can, such as time elapsed, temperature, mass,
volume, etc.
Structure and Style
Organization is especially important in the Methods section of a lab report because readers must
understand your experimental procedure completely. Many writers are surprised by the difficulty of
conveying what they did during the experiment, since after all they’re only reporting an event, but it’s
often tricky to present this information in a coherent way. There’s a fairly standard structure you can
use to guide you, and following the conventions for style can help clarify your points.
Subsections: Occasionally, researchers use subsections to report their procedure when the following
circumstances apply: 1) if they’ve used a great many materials; 2) if the procedure is unusually
complicated; 3) if they’ve developed a procedure that won’t be familiar to many of their readers.
Because these conditions rarely apply to the experiments you’ll perform in class, most undergraduate
lab reports won’t require you to use subsections. In fact, many guides to writing lab reports suggest that
you try to limit your Methods section to a single paragraph.
Narrative structure: Think of this section as telling a story about a group of people and the experiment
they performed. Describe what you did in the order in which you did it. We’re used to reading about
events chronologically, and so your readers will generally understand what you did if you present that
information in the same way. Also, since the Methods section does generally appear as a narrative
(story), you want to avoid the “recipe” approach: “First, take a clean, dry 100 ml test tube from the rack.
Next, add 50 ml of distilled water.” You should be reporting what did happen, not telling the reader how
to perform the experiment: “50 ml of distilled water was poured into a clean, dry 100 ml test tube.”
HINT: most of the time, the recipe approach comes from copying down the steps of the procedure from
your lab manual, so you may want to draft the Methods section initially without consulting your manual.
Later, of course, you can go back and fill in any part of the procedure you inadvertently overlooked.
Past tense: Remember that you’re describing what happened, so you should use past tense to refer to
everything you did during the experiment. Writers are often tempted to use the imperative (“Add 5 g of
the solid to the solution”) because that’s how their lab manuals are worded; less frequently, they use
present tense (“5 g of the solid are added to the solution”). Instead, remember that you’re talking about
an event which happened at a particular time in the past, and which has already ended by the time you
start writing, so simple past tense will be appropriate in this section (“5 g of the solid were added to the
solution”.
Passive voice vs. first person: Scientific journals encourage their writers to avoid using the first person
(“I” or “we”), because the researchers themselves aren’t personally important to the procedure in the
experiment. Remember that other researchers should ideally be able to reproduce experiments exactly,
based on the lab report; using first person indicates (to some readers) that the experiment cannot be
duplicated without the original researchers present. To help keep personal references out of lab reports,
scientific conventions also dictated that researchers should use passive voice, in which the subject of a
sentence or clause doesn’t perform the action described by the verb. These examples might explain the
distinction between active and passive voice:
• Active: We heated the solution to 80°C. (The subject, “we,” performs the action, heating.)
• Passive: The solution was heated to 80°C. (The subject, “solution,” doesn’t do the heating–it is
acted upon, not acting.)
Materials and Methods Examples
Sample 1: In preparing the catecholase extract, a potato was skinned, washed, and diced. 30.0 g of the
diced potato and 150 ml of distilled water were added to a kitchen blender and blended for
approximately two minutes. The resulting solution was filtered through four layers of cheese cloth. The
extract was stored in a clean, capped container.
Four individually labeled spectrophotometer tubes were prepared using different amounts (as
represented in Table 1) of the following reagents: a buffer of pH 7, a 0.1% catechol substrate, and
distilled water. The wavelength of the Spectronic 20 spectrophotometer was set at 540 nm. To
calibrate** the specrophotometer at zero absorbance, a blank control tube prepared with no catechol
substrate and labeled “tube 1” was inverted and inserted into the spectrophotometer.
It is important to note that the extract to be tested was added to each tube immediately before placing
the tube into the spectrophotometer. 1.0 ml of catecholase extract was pipetted into tube 2. Tube 2 was
immediately inverted and placed in the spectrophotometer. The absorbance was read and recorded for
time zero (t0), the ten minute mark (t10), and each minute in between. Tube 2 was removed from the
spectrophotometer and the same measurements were taken for tube 3 and tube 4 using the same
protocol.
* In this example the writer gives enough detail about the procedure so that is can be understood, but
not so much that there is an excess of unnecessary detail.
** Calibration is a small but important detail to include in this section so that the experiment would be
able to be repeated by anyone reading the report. Keep this in mind while deciding what to include in
this section.
Sample 2: A potato and a knife were obtained for this experiment. Also, distilled water, a blender,
cheese cloth, a clean container with a cover, and eight spectrophotometer tubes were used. A
Spectronic 20 spectrophotometer was used for this experiment, as were buffers of pHs 4, 6, 7, and 8.
Catechol substrate, Parafilm coverings, KimWipes, a black pen, and pipettes were also obtained for this
experiment. Finally, a pencil and pad were obtained for recording results.
* This example has a list of materials at the beginning which are not necessary in the materials and
methods section. The body of the section should mention the materials and equipment used during the
experiment so that it is not necessary to list them in order to know what was used for the procedure.
Sample 3: In preparing the catecholase extract, a potato was skinned, washed, and diced.
A balance*was used to obtain 30.0 g of the diced potato. 150 ml of distilled water was poured into a
beaker. The water was added to the diced potato. The cover of the kitchen blender* was removed. The
potato and water were added to the blender. The solution smelled like potato. The cover was placed on
the blender and the power button was depressed. The clock was observed until the second hand circled
twice. The power button* was pushed again to stop the blender. The resulting solution was filtered
through four layers of cheese cloth. The extract was stored in a clean, capped container.
* This is extraneous detail that is not needed to explain the procedure. The reader would know how to
turn the blender on and off without being told that a button was pushed, and knowing that the solution
smelled like potato is completely unrelated to knowing how to perform the experiment.