CHEMISTRY 457/557
PHYSICAL CHEMISTRY LABORATORY I
Lab Notes: Solution Calorimetry
Modifications:
Significant! We use a commercial solution calorimeter to determine
the enthalpy of formation of MgO(s) rather than the enthalpy of solution that Sime
describes. Additionally, we use 0.5 M HCl, in lieu of the 0.1 M concentration specified in
the Parr manual, because the magnesium (50 mesh) does not react quickly enough in the
more dilute acid.
Instrumentation:
Parr solution calorimeter, personal computer.
The availability of a commercial calorimeter with a self-contained bridge circuit
simplifies the experimental procedure, and we will use the procedures in the Parr solution
calorimeter manual. Photocopies of this manual have been placed in folders in 202 Oelman;
they are to remain in the lab for everyone to use. Students should review the manual before
beginning the experiment. Since we will use a computer to acquire data, most references to
a chart recorder do not apply.
The enthalpy of formation of MgO(s) is to be determined from the reactions of Mg (s)
and MgO(s) with HCl(aq) (Table 4-2, system 1). This reaction is described in Sime's reference
5; however, the author of that article describes a much different calorimeter, and only his
reference to ∆H values is of interest.
+
Mg(s) + 2 H
(aq)
+
MgO(s) + 2 H
→ Mg2
(aq)
+
(aq)
→ Mg2
+
∆H1
+ H2(g)
(aq)
∆H2
+ H2O(l)
+
Note that because ∆Hf (H+(aq)) = 0, ∆H1 is equivalent to ∆Hf (Mg2 (aq)), and may be
compared to a literature value. Hess' law tells us that reversing the second reaction and
adding it to the first results in the following reaction and enthalpy change.
Mg(s) + H2O(l) → MgO(s) + H2(g)
∆H1 - ∆H2
You should be able to show that the enthalpy of formation of MgO(s) is given by:
∆Hf (MgO(s)) = ∆H1 - ∆H2+∆Hf (H2O(l))
Details of the apparatus are given in the Parr Instrument Company literature, which
should be reviewed in the laboratory prior to the experiment. A thermistor (thermally
sensitive resistor) and bridge circuit is used to measure the temperature of the water in the
calorimeter. The bridge circuit output is a voltage that will change with temperature. This
voltage will be measured with a analog-to-digital (A/D) converter and transmitted as a digital
reading to the PC.
Solution Calorimetry - page 2
A data capture program running on the PC will place the data points into a data file
which will be read by Excel or SigmaPlot. You will analyze the data to obtain ∆T values by
obtaining linear fits to the pre-reaction and post-reaction data. Compare your ∆T value from
the graphical treatment with that from the analysis with SigmaPlot.
The current configuration of the calorimeter is set to take data at a rate of one point
per second for ten to twelve minutes, using the first three minutes to collect pre-reaction
"temperature drift" data before beginning the reaction. Determination of the heat capacity of
the calorimeter will be performed by the standardization method in the Parr manual (use 0.5
M HCl). There is no need to determine the energy equivalent of the empty calorimeter
described in step 8 on page 17.
We recommend using ≈1 mmole of Mg and MgO reactants as a starting point. Since
50 mmole HCl is available in 100 mL of 0.5 M HCl, there is a 25-fold excess of HCl and the
reaction should be sufficiently rapid for our needs. It would seem that using a 100 mL
volumetric pipette (±0.08 mL tolerance) provides a reasonable accuracy and is simpler than
weighing 100 grams of the HCl solution. Consult the TA or instructor if unreacted solid is
noticed after the reaction. Carefully consider any modifications of these initial recipes before
making them.
Compare your results with that from a reference source. Use J or kJ (not calories) for
the energy units in your report.
Show sample calculations of energy input (Q E) and energy equivalent of the
calorimeter and its contents (e) from one of the standardization reactions. Also show sample
calculations for the energy evolved (Q) and the enthalpy change (∆H) for one of the Mg
reactions.
Finally,
show
a
sample
calculation
of
∆Hf
(MgO(s)).
Solution Calorimetry
Simple instructions for use of the Parr 1755 calorimeter.
On arrival to the laboratory, turn on the calorimeter and computer next to it. (They are
both connected to the same power strip so they should turn on together, make sure they
are both on)
1. Start the data capture program on the computer by double clicking on the
Calorimeter icon on the desktop.
2. Start the stirring motor by entering *101 on the calorimeter keypad, then 1.
3. Start the calorimeter data logging by entering *190 on the calorimeter keypad, then
the key marked “ON”.
4. Check to see that the computer is receiving data.
5. Start the data capture by selecting the Transfer menu and clicking on Capture Text.
A dialog box will open. Click on the browse button, select the C:\PchemData
subdirectory and name the capture file.
6. After approximately 3 min, depress the plunger to start the reaction.
7. After approximately 7 additional minutes turn off the data logging by entering *190 on
the calorimeter keypad, then the key marked “OFF”.
8. Stop the stirring motor by entering *101 on the calorimeter keypad, then 2.
9. Clean up calorimeter and prepare another run.
10. Goto step 2.
Calculations
The data that will be captured from the calorimeter consists of five comma separated
values for each data point. The data file is readable by Excel. Excel may parse the data
into five columns, or you may have to tell Excel to parse the data. Once parsed, the first
three columns and the fifth column may be deleted. (Note: To completely remove the
columns, select the column, then select Delete on the Edit menu.) Now select the
remaining column and insert a column to it’s left. Enter the value “0” in the A1 cell and “1”
in the A2 cell. Select the A1 and A2 cells, then autofill the remainder of the column. The
first column now contains the elapsed time in seconds and the second column contains
the corresponding temperature.
Solution Calorimetry - page 2
Create a x:y chart of the time and temperature data.
Using the SLOPE and INTERCEPT functions in Excel, determine the slope and intercept
on the linear lines defined by the first 50 and last 50 data points. The lines that these
data points define can be called the pre- and post-reaction temperature functions.
In columns 3 and 4 calculate new temperature data points using the determined slopes
and intercepts. In column 5 calculate the difference between the calculated pre- and
post-reaction temperature values contained in columns 3 and 4. In column 6 calculate the
difference between observed temperature (column 2) and pre-reaction calculated
temperature (column 3) divided by the difference between calculated pre- and postreaction temperatures (column 5).
Examine column 6 for a value closest to 0.63. The value in column 5 at the same time is
the ∆T for the reaction.
This value for ∆T way be used in the equation ∆H = -Cp∆T to determine the heat of
reaction.
17.85
17.8
y = 3.43492 x 10-4x + 17.59699334
17.75
Temperature
17.7
T(0.63∆T) = 17.63447 ºC
t = 398 s
∆Τ = 0.275821 ºC
17.65
17.6
17.55
∆T
17.5
y = 2.12771 x 10-4 x + 17.37319972
17.45
17.4
17.35
0
100
200
300
400
Seconds
500
600
700
800