Chapter 15 Laboratory: Thermochemistry and Calorimetry 271LABORATORY 15 .1:
dETERmINE HEAT of SoLUTIoN
Heat of solution, also called enthalpy change
of solution, is heat that is absorbed or released
when a solute is dissolved in a solvent.
Dissolution is a complex process that involves
both absorption and release of energy. Energy
is absorbed (endothermic energy) to break the
attractions between the solute molecules and
the attractions between the solvent molecules.
Conversely, energy is released (exothermic
energy) as new attractions form between
solute molecules and solvent molecules. The
net difference between the energy absorbed
and the energy released per mole of solute is
defined as the heat of solution for that solute.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance and weighing papers£ calorimeter£ thermometer£ graduated cylinder, 100 mL£ ammonium nitrate (40.0 g = 0.5 mol)£ sodium chloride (29.2 g = 0.5 mol)£ sodium hydroxide (20.0 g = 0.5 mol)£ water (at room temperature)SBSTITUTIU oNS ANd modIfICATIoNS- At the risk of some loss of accuracy, you can run this
lab on a semi-micro scale by reducing the quantities
of solutes and water by a factor of 10 and substituting
a test tube for the calorimeter. (This is not ideal,
because the glass of the test tube has much higher
thermal mass than a foam cup and is a poor insulator.) - You may substitute pure ammonium nitrate fertilizer
for laboratory- or reagent-grade ammonium nitrate. - You may substitute table salt for sodium chloride.
- You may substitute crystal drain opener that lists its
contents as 100% sodium hydroxide for laboratory- or
reagent-grade sodium hydroxide. - If you are using a calorimeter in which the solutions
will come into contact with aluminum or glass,
substitute calcium chloride for the sodium hydroxide.
In areas that have snowy winters, relatively pure
calcium chloride is sometimes available as ice-melt
crystals, although many different formulations are
used, so it’s necessary to check the label.
For a particular solvent, some solutes have a positive heat of
solution, because the energy that must be absorbed to break the
solute-solute bonds and the solvent-solvent bonds is greater than
the energy that is released by the formation of solute-solvent
bonds. Dissolving such a solute reduces the temperature of
the solution relative to the original temperature of the solvent.
Other solutes have a negative heat of solution, because less
energy is needed to break the solute-solute and solvent-solvent
attractions than is released by the forming of solute-solvent
bonds. (Remember, heat of solution is defined as endothermic
energy minus exothermic energy, so dissolving a compound with
negative heat of solution causes the temperature of the solution
to increase.)
Heat of solution (represented as ΔHsolution) is properly quantified
using SI units of kJ/mol (kiloJoules/mole), but many older
sources (and many older chemists.. .) continue to use the old-
style C/mol (kilocalories/mole), where one kJoule equals 2.390 x
10 –1 kilocalories (0.2390 C) and one kilocalorie (kcal or C) equals
4.1841 kJ. For any particular solute/solvent combination, a value
for heat of solution can be determined experimentally from data
derived by dissolving a known mass of the solute in a known mass
of the solvent and measuring the temperature change. That’s
what we do in this lab.