Solution Calorimetry
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Heat is the most common form of energy involved in chemical changes, and in
this experiment we will study heat energy in reactions.
Most chemical processes are accompanied by an evolution or absorption of heat.
The heat flow can be measured in a device called a calorimeter. A calorimeter is a
container with insulating walls that minimizes the heat exchange with the surroundings.
Within the calorimeter, chemical changes may occur and heat may pass from one part of
the contents to another, but ideally no heat flows into, or out of, the calorimeter.
The general symbol for heat flow in a process is “q”, and its values are usually
given in units of joules (SI unit) or calories. If the process is carried out at constant
pressure, q is given a special name – enthalpy change, ∆H. Thus, the heat absorbed or
evolved during a process carried out at constant pressure is called the enthalpy change for
that process. We will be working with processes carried out in a calorimeter that is
partially open to the atmosphere; therefore, q = ∆H.
In discussing enthalpy changes, most processes are written in the form of
chemical equations. When studying the enthalpy changes in a reaction, the physical state
of each substance is important. Be sure to include the state of each species in your
balanced chemical reactions. The stoichiometric coefficients in the chemical equations
are understood to represent the number of moles of each substance. Processes that
proceed with the absorption of heat have positive values of ∆H and are said to be
endothermic. Processes that evolve heat have negative values of ∆H and are said to be
exothermic. Considerations of temperature and pressure are important, and unless
otherwise noted, the chemical processes are studied at, or near, 25 °C and one atmosphere
pressure. Thus, equation (1) below means that at 25 °C and one atmosphere of pressure,
one mole of hydrogen gas and 1/2 mole of oxygen gas react to form one mole of liquid
water with the evolution of 286 kJ of heat energy.
H2 (g) + ½ O2 (g) H2O (l) ∆H = - 286 kJ (1)
Temperature Changes
When a “hot” object is placed in contact with a “cold” object (one having a lower
temperature) heat energy will flow from the warmer body to the cooler one until both
objects have the same temperature. It is obvious that the amount of heat energy required
to cause a specific temperature change in a specific object must depend on the mass of
the object. It takes more thought to realize that the amount of heat also depends on the
chemical identity of the object. Thus, we define the specific heat, s, of a substance as the
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