Experiment 13: Determination of Enthalpy
Changes Associated with a Reaction
and Hess’s Law
Background:Thermochemistry is the study of heat changes and transfers associated with
chemical reactions. In this thermochemical laboratory study, you will determine the enthalpy
change that occurs when a strong base, sodium hydroxide, reacts with a strong acid, hydrochlo-
ric acid. Other mixtures studied will include ammonium chloride mixed with sodium hydroxide
and ammonia mixed with hydrochloric acid. These three reactions are represented as:
NaOH()aq++HCl()aq"NaCl()aq H O 2 ()l
NH Cl 432 ()aq+++NaOH()aq"NH()aq NaCl()aq H O()l
NH 34 ()aq+HCl()aq"NH Cl()aqSo that there is no gain nor loss of heat, thermochemical reactions that are called “adiabatic” are
carried out in well-insulated containers called calorimeters. The reactants and products in the
calorimeter are called the chemical system. Everything else is called the surroundings, and in-
cludes the air above the calorimeter, the water in the calorimeter, the thermometer and stirrer, etc.
Hess’s law, or the law of constant heat summation, states that at constant pressure, the enthalpy
change for a process is not dependent on the reaction pathway, but is dependent only upon the
initial and final states of the system. The enthalpy changes of individual steps in a reaction can
be added or subtracted to obtain the net enthalpy change for the overall reaction.
An exothermic reaction occurs when the enthalpy of the reactants is greater than the enthalpy
of the products, −∆H (∆H = Σ∆Hf products−Σ∆Hf reactants). In endothermic reactions, the enthalpy
of the reactants is lower than that of the products, +∆H.
Some of the heat transferred to the surroundings during an exothermic reaction are absorbed by
the calorimeter and its parts. In order to account for this heat, a calorimeter constant or heat ca-
pacity of the calorimeter is required and usually expressed in J ⋅°C−^1.
Scenario:A student constructed a ‘coffee cup calorimeter’ (see Figure 1). To determine the
heat capacity of the calorimeter, the student placed 50.0 mL of room temperature distilled
water in the calorimeter. A calibrated temperature probe recorded the temperature as 23.0°C.
The student then added 50.0 mL of warm distilled water (61.0°C) to the calorimeter and
recorded the temperature every 30 seconds for the next three minutes. The calorimeter was
then emptied and dried. Next, the student measured the temperature change when 50.0 mL of
2.00 M HCl was added to 50.0 mL of 2.00 M NaOH. Again, temperature change was recorded
every 30 seconds for three minutes. The same procedure was followed for 2.00 M NH 4 Cl with
2.00 M NaOH and finally, 2.00 M NH 3 with 2.00 M HCl.
Part III: AP Chemistry Laboratory Experiments