HEAT OF REACTION
The purpose of this experiment is to determine the enthalpy change (∆H) for a reaction that is too
dangerous to perform directly, the partial neutralization of concentrated sulfuric acid with solid sodium
hydroxide:
H2SO4 (l) + NaOH (s)  H2O (l) + NaHSO4 (aq)
Procedure
Construct a calorimeter from a 250 mL beaker in a foam bucket, lid, and temperature probe. Put it on a
magnetic stirrer, and support the probe with a clamp so that it extends into the beaker nearly to the
bottom, but is not struck by the stir bar as it rotates. It is important that the calorimeter and thermometers
be clean and dry for each experiment.
SAFETY PRECAUTIONS
Sulfuric acid (H2SO4) and sodium hydroxide (NaOH) are corrosive. As always, YOU MUST WEAR
GOGGLES AT ALL TIMES IN THE LAB. If you get either on your skin or clothing, rinse the area
immediately with lots of cold water. Assume that all wet spots on the lab bench are harmful. Acid spots
are neutralized with baking soda, then wiped up; base spots are neutralized with vinegar and wiped up.
Leave backpacks and jackets in the desk area; many an expensive item has been ruined by a wet spot that
turned out to be sulfuric acid. Gloves are available if you wish to use them.
1. The calorimeter constant. The examples in your text ignore the heat absorbed by the calorimeter
assembly. To check that assumption and to be as accurate as possible, you will measure the heat
absorbed by the calorimeter assembly and include it in your calculation of the heat of reaction.
Put exactly 50 mL of tap water in the calorimeter. Start stirring, replace the lid and probe, and record
the water temperature. Measure exactly 50 mL of warm water into a beaker. Use a glass thermometer
to measure the temperature of the warm water and record the value. Pour all of the warm water into
the calorimeter. Observe the temperature while mixing and record the highest value reached.
Discard the water and gently dry the calorimeter. You should have three temperature readings: the
initial temperature of the cold water in the calorimeter, the initial temperature of the hot water, and the
final temperature after mixing.
2. Enthalpy of 10 M H2SO4 dissolving in water.
Set up the clean, dry calorimeter on a stirrer as before. Measure 90.0 mL of deionized water with a
graduated cylinder and put it in your calorimeter. Record its initial temperature.
Dispense over 10 mL of 10 M H2SO4 (CAUTION!) into a small dry graduated cylinder, and record
the temperature of the acid using a glass thermometer (NOT a metal probe!). Remove the
thermometer, rinse it immediately, and use a dropper to adjust the volume of 10 M H2SO4 to exactly
10.0 mL.
With the probe and lid in place and the stirrer stirring, add the 10 M H2SO4. Record the maximum
temperature of the mixture. Discard the mixture, rinse the calorimeter thoroughly, and dry it gently.
3. Enthalpy of neutralization of 1M H2SO4 + 1M NaOH.
Set up the clean, dry calorimeter on a stirrer as before. Measure 50.0 mL 1.0 M H2SO4 into the
calorimeter and record its initial temperature. Use another cylinder to measure 50.0 mL 1.0 M NaOH.
Record the temperature of the 1.0 M NaOH in the cylinder, and rinse the thermometer thoroughly.
Add the 1.0 M NaOH to the calorimeter (stirring constantly), and record the maximum temperature of
the mixture. Discard the mixture, rinse the calorimeter, and dry it gently. Don't bother to rinse the
NaOH cylinder (see next experiment).
4. Enthaply of neutralization of 10M H2SO4 + 1 M NaOH.
Set up the clean, dry calorimeter on a stirrer as before and put 50.0 mL of 1 M NaOH into the
calorimeter. Add 45.0 mL deionized water, using the same graduated cylinder. Stir the mixture.
Record the temperature of the mixture, which is its initial temperature.
As before, dispense over 5 mL of 10 M H2SO4 (CAUTION!) into a small dry graduated cylinder, and
sue a glass thermometer to measure the temperature of the acid. Remove the thermometer, rinse it
immediately, and use a dropper to adjust the volume of 10 M H2SO4 to exactly 5.0 mL.
Add the 10 M H2SO4 to the calorimeter (stirring constantly), and record the maximum temperature of
this mixture. Discard the mixture, rinse the calorimeter, and dry it gently. Rinse the graduated
cylinders thoroughly with deionized water.
5. Enthalpy of solid NaOH dissolving in water.
Set up the clean, dry calorimeter on a stirrer as before. Measure 100.0 mL of deionized water with a
graduated cylinder and put it into the calorimeter. Record its initial temperature
Place a cup on the balance, tare it, and add solid NaOH pellets until you have about 4 grams
(CAUTION! Do not handle these with your fingers! Use forceps to pick them up, and clean up any
dropped pellets immediately). Record the exact mass of the NaOH. Don't worry about its initial
temperature, which you cannot measure accurately.
With the probe and lid in place and the stirrer stirring, add the solid NaOH. Observe the temperature
and record the maximum value reached (make sure it has peaked out, because the solid dissolves more
slowly than the previous reactions). Discard the mixture and rinse the calorimeter thoroughly.
When you have obtained all your data, be sure all glassware is thoroughly rinsed. Carefully rinse the
probe and stir bar, too, and wipe off the surface of the stir plate. Do not leave any acid or NaOH
residue on any of the equipment.
Analysis
1. The calorimeter constant. The highest temperature reached after mixing is Tfinal. Find ∆T and q for
both the warm and the cool water. Assuming that all the heat energy stayed in the calorimeter,
qwarmwater  qcoolwater  qcalorimeter  0
Calculate qcalorimeter, then calculate the heat capacity of the calorimeter (B) in J/°C:
B=
qcalorimeter
T calorimeter

2. Enthalpy of 10 M H2SO4 dissolving in water.
Determine ∆T for the acid and ∆T for the water in the calorimeter. If energy is conserved,
qreaction  qwater  qacid  qcalorimeter  0
For qwater and qacid, use q = mc∆T, assuming the density of water is 1.00 g/mL and the density and
specific heat of the acid are the same as for water. For qcalorimeter, use the relationship q = B∆T, using
∆Tcalorimeter = ∆Twater. Solve the equation for qreaction.
Calculate the moles of H2SO4 used and determine ∆Hreaction in kilojoules per mole H2SO4 (that is, the
ratio of energy released to moles reacted). Be attentive to significant figures.
3. Enthalpy of neutralization of 1M H2SO4 + 1M NaOH.
Determine ∆T for the acid and ∆T for the base. Use an equation and assumptions similar to the one
above to solve for qreaction:
qreaction  qbase  qacid  qcalorimeter  0
As before, ∆Tcalorimeter is the same as ∆T for whatever was in the calorimeter initially. Calculate the
moles of H2SO4 used and determine ∆Hreaction in kJ per mole H2SO4.
4. Enthaply of neutralization of 10M H2SO4 + 1 M NaOH.
Determine ∆T for the acid and ∆T for the base + water. Use an equation and assumptions similar to
the one above to solve for qreaction (note that the amount of base + water in the calorimeter is 95 mL):
qreaction  qbase water  qacid  qcalorimeter  0
Calculate the moles of H2SO4 used and determine ∆Hreaction in kJ per mole H2SO4.
5. Enthalpy of solid NaOH dissolving in water.
Determine ∆T for the liquid in the calorimeter. Use the equation
qreaction  qsolution  qcalorimeter  0
to solve for qreaction. For the mass of the solution, combine the mass of water plus the mass of NaOH.
Calculate the moles of NaOH used and determine ∆Hreaction in kJ per mole NaOH.
Discussion
1. Verify that your ∆Hreaction values are additive by setting up this combination:
H2SO4 (10M)  H2SO4 (1M)
H2SO4 (1M) + NaOH (1M)  H2O (l) + NaHSO4 (aq)
H2SO4 (10M) + NaOH (1M)  H2O (l) + NaHSO4 (aq)
∆H2 = (observed value)
∆H3 = (observed value)
∆H4 = (∆H2 + ∆H3)
∆H4 = (observed value)
You can see that reactions 2 and 3 add together to give reaction 4. Compare the value you calculated
for reaction 4 (∆H2 + ∆H3) to the value you actually observed for reaction 4. Calculate the %
difference between the two values, using the observed value as the correct one.
% difference = calculated ∆H4 – observed ∆H4 x 100
observed ∆H4
This will give you a way to judge the reliability of the value you report for the reaction we are
studying.
2. Now combine these reactions to get the one we are looking for but could not perform directly:
H2SO4 (10M)  H2SO4 (1M)
NaOH (s)  NaOH (1M)
H2SO4 (1M) + NaOH (1M)  H2O (l) + NaHSO4 (aq)
H2SO4 (10M) + NaOH (s)  H2O (l) + NaHSO4 (aq)
∆H2 = (observed)
∆H5 = (observed)
∆H3 = (observed)
∆H = (∆H2 + ∆H5 + ∆H3)
Discuss error and the likely reliability (based on the reliability of your calculated value for reaction 4,
above) of this final ∆Hreaction, which we could not safely measure.