THERMOCHEMISTRY II: HESS’S LAW | 83
Thermochemistry II: Hess’s Law
OBJECTIVES:
• Explore quantitative aspects of constant-pressure calorimetry
• Check the validity of Hess’s law
DISCUSSION:
In this part of the experiment, you will use your calorimeter to measure the heat released by three
related reactions. Because two of the reactions sum up to give the third reaction, you can use the
set of measurements to check Hess’s law of heat summation.
In the first reaction, solid sodium hydroxide dissolves in water:
NaOH
(s) → Na+(aq) + OH –(aq) ∆H1 = ?
In the second reaction, aqueous solutions of sodium hydroxide and hydrochloric acid react to
form water and aqueous sodium chloride:
Na+(aq) + OH – (aq) + H+(aq) + Cl –(aq) → H2O (l) + Na+(aq) + Cl –(aq) ∆H2 = ?
In the third reaction, solid sodium hydroxide reacts with aqueous hydrochloric acid to form water
and aqueous sodium chloride:
NaOH
(s) + H+(aq) + Cl –(aq) → H2O (l) + Na+(aq) + Cl –(aq) ∆H3 = ?
As in the first part of the Thermochemistry lab, you will use an EPS cup supported in a beaker as
the calorimeter. Although this assembly is not a perfect insulator, if we assume that the heat
losses are not significantly different from one trial to the next, we can expect valid comparisons.
PROCEDURE:
Reaction 1
1. Insert the temperature probe through a slit rubber stopper. Clamp the stopper to a ring-stand
to suspend the probe in a vertical position.
2. Measure 100.0 mL of DI water into the cup. Place the temperature probe in the water.
3. Weigh the provided vial of NaOH pellets. Because sodium hydroxide readily picks up
moisture from the air, you should weigh it quickly and go on to the next step without delay.
CAUTION: Solid sodium hydroxide and NaOH solutions are quite caustic. Avoid skin
contact, and wash thoroughly in the event of an accidental exposure.
4. As it may take some time for the temperature probe to come into thermal equilibrium with
the solution, wait until you have at least three subsequent readings at the same temperature
before adding the solid NaOH to the water in the calorimeter cup. Stir the pellets in the
solution with a glass stirring-rod, to encourage complete dissolving. Collect data for a full
225 s, or at least until the temperature readings begin to drop after passing through a
maximum value.
5. After collecting the temperature data, weigh the empty vial (with its cap) and record the mass
of NaOH used in your experiment. Dispose of the solution into the proper waste container.
Rinse the probe, the cup, and the stirring rod with DI water, then dry them in preparation for
the next reaction.
6. Read through the data to find the initial solution temperature, T1, and the maximum solution
temperature, T2.. Record T1 and T2 on your data sheet.
