17.2. Thermochemical Equations http://www.ck12.org
Calorimetry
Calorimetryis the measurement of the transfer of heat into or out of a system during a chemical reaction or physical
process. Acalorimeteris an insulated container that is used to measure heat changes. The reactions that can most
easily be analyzed in a calorimetry experiment involve only liquids or aqueous solutions. A frequently used and
inexpensive calorimeter is a set of nested foam cups fitted with a lid to limit the heat exchange between the liquid
in the cup and the air in the surroundings (Figure17.5). In a typical calorimetry experiment, specific volumes of
the reactants are dispensed into separate containers, and the temperature of each is measured. They are then mixed
into the calorimeter, which starts the reaction. The reactant mixture is stirred until the reaction is complete, and the
temperature of the reaction is continuously monitored.
FIGURE 17.5
In a simple constant-pressure calorimeter, the temperature of a water-
based reaction is monitored as the reaction takes place. The substances
dissolved in the water are the system, and the water itself is the surround-
ings.The key to all calorimetry experiments is the assumption that there is no heat exchange between the insulated
calorimeter and the room. Consider the case of a reaction taking place between aqueous reactants. The water in
which the solids have been dissolved is the surroundings, while the dissolved substances are the system. The tem-
perature change that is measured is the temperature change that is occurring in the surroundings. If the temperature
of the water increases as the reaction occurs, the reaction is exothermic. Heat was released by the system into the
surrounding water. An endothermic reaction absorbs heat from the surroundings, so the temperature of the water
decreases as heat leaves the surroundings to enter the system.
The temperature change of the water is measured in the experiment, and the specific heat of water can be used to
calculate the heat absorbed by the surroundings (qsurr).
qsurr= m×cp×∆TIn this equation, m is the mass of the water, cpis the specific heat of the water, and∆T is Tf–Ti. The heat absorbed
by the surroundings is equal, but opposite in sign, to the heat released by the system. Because the heat change is
determined at constant pressure, the heat released by the system (qsys) is equal to the enthalpy change (∆H).
qsys=∆H = -qsurr= -(m×cp×∆T)The sign of∆H is positive for an endothermic reaction and negative for an exothermic reaction.
Sample Problem 17.2: Calorimetry and Enthalpy Changes
In an experiment, 25.0 mL of 1.00 M HCl at 25.0°C is added to 25.0 mL of 1.00 M NaOH at 25.0°C in a foam cup
calorimeter. As the reaction occurs, the temperature of the solution rises to 32.0°C. Calculate the enthalpy change