EXPERIMENT 2:
REACTION STOICHIOMETRY BY
THERMOMETRIC TITRATION
INTRODUCTION
The net result of a reaction (a chemical change) is summarized by a chemical equation. In order to write a
chemical equation, a chemist must determine experimentally what the reactants are, in what ratio they react (the
reaction stoichiometry), what the products are, and the amount of each product. This experiment is designed to
determine the relative amounts of reactants (reaction stoichiometry) for a particular reaction.
When two or more substances are mixed, it is possible to determine whether a reaction occurs by noting
whether any property of the mixture changes. Further, by noting how the change in an observed property varies
with the ratios in which reactants are mixed, the stoichiometry of the reaction can be determined.
Consider an example in which solutions of A and B are mixed and a precipitate P forms. If a fixed volume
of solution A is placed in a beaker, and small increments of solution B are gradually added, the amount of
product P will increase. This will continue until the stoichiometric point is reached. The maximum amount of
precipitate is formed when A and B are mixed in the correct stoichiometric amounts, when there is just enough
of each to react, with nothing left over. Any amount of solution B added to the reaction mixture after this point
will not result in the formation of any more product, because no reactant A is left to react. This stoichiometric
point can be determined by plotting the amount of precipitate formed vs. the amount of A used in the
experiments, as in Figure 2-1.
Mass of P, g
stoichiometric point
Volume of B added, mL
Figure 2-1. Plot of the mass of precipitate P formed as a function of the volume of solution
B added to a fixed volume of solution A.
2-1
Experiment 2
If the concentrations of solutions A and B are known, then the moles of each reactant at the stoichiometric
point can be determined.
TECHNIQUE
The formation of a precipitate is just one of many properties that could be used to determine the
stoichiometry of a reaction. In this experiment you will base your decision about the stoichiometric ratio
of reactants on the amount of heat that is evolved during the reaction.
One of the reactants you will use is NaOCl (sodium hypochlorite). NaOCl is a strong oxidizing agent present
in many commercial bleaches, and will react with a large number of reducing agents in an oxidation-reduction
reaction. When NaOCl is reacted, for example, with Na2S2O3 (sodium thiosulfate), the reaction is
x NaOCl + y Na2S2O3 + OH-
products + q
(2-1)
The coefficients in the balanced equation are x and y, and q stands for the heat that is evolved. Since the
objective is to determine the ratio of the coefficients x and y, it is not important that we know the chemical
identity of the products. Also note that hydroxide ion is required as a reactant; it is added in excess to the
solution containing the reducing agent to ensure that the reaction goes to completion.
In this experiment, you will place a measured amount of a reducing agent in a beaker and add an NaOCl
solution of known concentration dropwise to the beaker until the reducing agent has been completely consumed.
By monitoring the temperature of the reaction, you will be able to determine the stoichiometric point--the point
where the reaction no longer occurs and heat is no longer evolved. The technique of adding a solution of known
concentration to another solution dropwise until an endpoint is reached is called a titration. Visualizing the
endpoint by monitoring the reaction temperature makes this a thermometric titration. In this experiment, the
endpoint of the titration occurs at the stoichiometric point, although we will continue adding the NaOCl solution
beyond the stoichiometric point. Once the stoichiometric point is reached, the temperature of the solution will
begin to decrease; not only is heat no longer being generated, but the temperature of the titrant added past the
endpoint is at a lower (room) temperature than the reaction solution.
The LabQuest workstation will be equipped with a drop counter and a temperature probe. By first
calibrating the buret to determine the average number of drops per mL being delivered, a plot of temperature
vs. volume of NaOCl for each reaction can be obtained. You will generate one plot for each of four reactions
involving NaOCl and four different reducing agents. By determining the volume of NaOCl required to reach
the stoichiometric point of the titration, you will be able to calculate the number of moles of each reactant used
at the stoichiometric point and thereby determine the coefficients x and y for the two reactants as seen in Eq. 21.
EQUIPMENT NEEDED
5-mL volumetric pipet
pipet pump
stir plate
buret clamp
temperature probe
drop counter
Experiment 2
beakers
magnetic stir bar
50 mL buret
ring stand
LabQuest
2-2
CHEMICALS NEEDED
reducing agents:
0.20 M Na2SO3 in 0.04 M NaOH; sodium sulfite in sodium hydroxide solution
0.20 M Na2S2O3 in 0.44 M NaOH; sodium thiosulfate in sodium hydroxide solution
0.20 M KNCS in 0.44 M NaOH; potassium thiocyanate in sodium hydroxide solution
0.20 M KI in 0.04 M NaOH; potassium iodide in sodium hydroxide solution
Note: Hydroxide ion is added in excess to each solution to ensure reaction completion
oxidizing agent:
standardized NaOCl solution; sodium hypochlorite (concentration is given on the container)
PROCEDURE
Obtain ~60 mL of the NaOCl solution; be sure to record the labeled concentration. Obtain ~10 mL of
one of the four reducing agents (note: whenever you obtain a solution used by all students, you
obtain an approximate amount. You will measure out 5.00mL of this solution more precisely
with a volumetric pipet once you return to your workstation).
Setting up the workstation and calibrating drops/mL of the buret
Note: If the workstation is displaying a plot of data from a previous user’s experiment, select New
from the File menu and touch Discard.
1. The temperature probe should already be plugged into a side port on the workstation, and a
temperature (the temperature of the lab room) displayed on the screen. If the drop counter has not
yet been plugged in, do so using the DIG1 port at the back of the station.
2. Attach the drop counter to a ring stand. Position the drop counter above a 50 mL beaker so that the
beaker is directly below the notch in the drop counter (this is where the drop detector is located).
3. Clamp a buret to the ringstand with a buret clamp and position the tip of the buret directly above
the notch in the drop counter so that the drops will be collected in the beaker. Fill the buret close
to the 0.0mL mark with tap water and allow some water to drain out the tip so that tip is filled with
water. Record the initial volume to the nearest 0.01 mL.
4. Touch the Volume box on the workstation and select Calibrate. Ignore instructions 1 and 2 on
the workstation screen; we will be using the volume readings from the buret rather than a graduated
cylinder to ensure greater precision.
5. Touch the Calibrate Now box. Carefully open the buret stopcock so that the drip rate is ~1 drop
per second. Make sure the drops are being detected by the drop counter (the red LED light on the
drop counter will flash when a drop is detected) and that the drops are being counted in the upper
right corner of the workstation.
6. When approximately 10 mL of water has been drained from the buret, close the stopcock. Record
the final volume and determine the precise volume of water drained from the buret. Touch the box
on the workstation after Enter the precise volume and enter this volume. Touch Equation to see
the calculated drops/mL. Record this value.
7. Touch Calibrate and then Redo Calibration, and repeat the calibration steps. Continue repeating
the calibration until you have two drops/mL values that agree within 0.5 drops/mL. Touch OK.
2-3
Experiment 2
Setting up the Apparatus
8. Touch the Graph icon on the workstation screen; from the Graph menu select Graph Options.
Change the value in the Right box (maximum volume, x-axis) to 20, and change the values in the
Top and Bottom boxes (max and min temperatures, y-axis) to 40 and 20, respectively. Touch
OK—you should see the screen ready to plot data with the temperature and volume ranges you
just set up.
9. Remove the buret from the buret clamp. Drain the water from the buret, rinse the inside walls of
the buret with a ~5 mL of the NaOCl solution, then open the stopcock and allow the solution to
drain into a 400 mL waste beaker. Fill the buret close to the 0 mL mark with the NaOCl solution.
Reclamp the buret, and drain a small amount of the NaOCl solution into the waste beaker until the
tip of the buret is completely filled with solution.
10. Position the drop counter over a stir plate, leaving enough room to place a 50 mL beaker
underneath it. Insert the temperature probe into the small hole in the drop counter.
11. Position the buret so that the tip is slightly above and centered over the notched portion of the drop
counter.
Measuring Temperature Change
12. Using a volumetric pipet, deliver 5.0 mL of the reducing agent to a 50 mL beaker. Place a
magnetic stir bar in the beaker, and place the beaker on the stir plate underneath the drop counter.
13. Lower the temperature probe into the beaker so that it is just above the bottom of the beaker, but
not touching the beaker or the stir bar. Tighten the set screw to hold the temperature probe in place.
Make sure that the buret tip is positioned so that it is not only over the notch on the drop counter,
but also over the beaker.
14. When everything is arranged over the beaker, begin the titration by touching Collect (green arrow)
on the workstation. Turn on the stir plate to the lowest setting that will allow the stir bar to spin.
Carefully turn the stopcock on the buret to begin adding the NaOCl solution a rate of approximately
1 drop per second. The red LED on the drop counter will flash each time a drop is counted; if the
light does not flash, stop the titration and adjust the position of the buret tip so that it is centered
over the notch. While the titration is running, monitor the drip rate and make small adjustments of
the stopcock if needed to keep the drip rate fairly constant.
15. Continue adding the NaOCl solution dropwise until you observe the slope of the temperature curve
on the workstation screen go from positive to negative. Allow the titration to continue for about
10 seconds after this point, then close the stopcock and touch Stop (red square).
16. Touch the plot as close as you can to the maximum temperature. Use the cursor controls (red circles
at bottom of screen) to position the cursor line at the highest temperature. The volume reading (in
box at lower right of screen) represents the volume of NaOCl required to reach the stoichiometric
point. Record this value. It is not required, but not a bad idea, to email the screenshot of the
temperature curve to your email account as a backup of the data. Be sure to include the name of
the reducing agent in the Subject box. It is also recommended that you check with your TA to
make sure the plot looks OK before moving on to the next titration.
Experiment 2
2-4
17. Unscrew the set screw and lift the temperature probe to remove the beaker from the stir plate. Pour
the reaction solution into the waste beaker, rinse the 50 mL beaker with distilled water and dry the
beaker.
18 Touch the File Cabinet icon next to the Run 1 box to start a new run--the box should now read
Run 2. Obtain ~10 mL of another of the reducing agents and repeat steps 12-17 above (remember
that you will be pipetting 5.00 of this solution into the reaction beaker). Follow the same procedure
with the other reducing agents, refilling the buret with NaOCl solution only when the level of
liquid in the buret falls below the 40 mL mark. Make a note of any color changes you observe
for any of the reactions.
19. Note: if you run out of NaOCl solution during the experiment, use the solution in the Leftover
NaOCl container in hood.
Clean Up and Waste Disposal
20. At the end of the experiment, drain the buret into the beaker you used to obtain the NaOCl solution
at the beginning of the experiment, and pour the solution into the Leftover NaOCl container in
the hood. Combine all other leftover reagents in the waste beaker, and pour this waste down the
sink.
21. Rinse the temperature probe with tap water.
22. Use a sponge from the sink area to wipe down your bench top and dry with paper towels.
23. Open the File menu on the workstation and select New. Touch Discard to clear out the workstation
for the next team of students.
Results
Consult the postlab PowerPoint on canvas for instructions on how to perform the necessary
calculations and complete the lab report.
2-5
Experiment 2
EXPERIMENT 2
REPORT SHEET
Name: _______________________________________ Date:__________
Data
Concentration of NaOCl solution
Reducing agent
Concentration of Volume of
Reducing Agent
Reducing Agent
Volume of NaOCl at
Stoichiometric Point
KI
Na2S2O3
Na2SO3
KNCS
x NaOCl + y KI
products + q
Moles of NaOCl reacted
__________________
Moles of KI reacted
__________________
Mole ratio, NaOCl /KI
__________________
Simplest whole number mole ratio, NaOCl /KI
__________________
x NaOCl + y Na2S2O3
products + q
Moles of NaOCl reacted
__________________
Moles of Na2S2O3 reacted
__________________
Mole ratio, NaOCl / Na2S2O3
__________________
Simplest whole number mole ratio, NaOCl / Na2S2O3
__________________
Experiment 2
2-6
x NaOCl + y Na2SO3
products + q
Moles of NaOCl reacted
__________________
Moles of Na2SO3 reacted
__________________
Mole ratio, NaOCl / Na2SO3
__________________
Simplest whole number mole ratio, NaOCl / Na2SO3
__________________
x NaOCl + y KNCS
products + q
Moles of NaOCl reacted
__________________
Moles of KNCS reacted
__________________
Mole ratio, NaOCl / KNCS
__________________
Simplest whole number mole ratio, NaOCl / KNCS
__________________
2-7
Experiment 2
Notes for Experiment 2
NaOCl is a strong oxidant—it is the main component of household bleach. Wear protective
equipment (goggles, gloves, lab coat) to avoid bleaching your clothing, skin or eyeballs.
Each of the reducing agent solutions contains a ~0.4M concentration of NaOH, which is a caustic
material. Wear gloves and keep your goggles over your eyes at all times in the lab, and clean up any
small spills of reducing agent solution with a wet paper towel.
Be sure to write down concentration of NaOCl solution!
𝑚𝑜𝑙𝑒𝑠 𝑠𝑜𝑙𝑢𝑡𝑒
Molarity (M) = 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 , so the moles of each reactant at the stoichiometric point can be
calculated by
Mole reactant = M x volume of solution at stoichiometric point (in L)
Make sure not to confuse Na2SO3 (sodium sulfite) and Na2S2O3 (sodium thiosulfate)!
When using a common reagent, you should never place anything in the bottle (such as a disposable
pipet, unless there is a dedicated pipet attached to the bottle), and you should never pour any leftover
solution back into the bottle!
Be on the lookout for a color change in one of the reactions—record your observations!
Experiment 2
2-8