Calorimetry

Heat of Solution of Ammonium Nitrate

Concepts

Calorimeters are designed to be well-insulated, so no heat is gained from or lost to the

surroundings. If no heating element is used to introduce heat in the system, the total heat (q) for

the entire calorimeter system must equal zero. The total heat can be split into heats for each

component in the system.

Imagine a reaction in which solid ammonium nitrate (a component in some fertilizers and an

explosive) is dissolved in water to produce an aqueous ammonium nitrate solution.

NH4NO3 (s) → NH4+ (aq) + NO3

- (aq)

The heat flow (qrxn) for this reaction is called the heat of solution for ammonium nitrate. When

the reaction is finished, the system contains two substances, the calorimeter itself and the

aqueous solution, and there is a heat associated with each component. The heat balance for this

experiment is thus

0 = q = qrxn + qcal + qsoln

The basic strategy in calorimetry is to use a temperature change and a heat capacity to determine

a heat flow. In this experiment all substances have the same initial and final temperatures.

qcal = Ccal ΔT = Ccal ( Tf - Ti )

qsoln = Csoln ΔT = msoln ssoln ( Tf - Ti )

One typically determines the heat capacity of the aqueous solution (Csoln) from the mass of the

solution (msoln) and the specific heat capacity of the solution (ssoln). The mass of the solution is

the sum of the masses of the water and ammonium nitrate originally placed in the calorimeter.

The specific heat capacity of the aqueous solution is usually close to that of pure water (4.184 J oC

-1 g

-1).

The objective of this experiment is to determine the heat of reaction (in this case a heat of

solution). The above equations can be combined and rearranged to yield a working equation:

qrxn = - qcal - qsoln = - ( Ccal + msoln ssoln ) ( Tf - Ti )

Just as the heat capacity of a substance is an extensive property, so the heat of solution is an

extensive property. It is generally more convenient to report intensive properties, thus the heat

capacity of a substance is usually reported as a specific heat capacity, that is, the heat capacity

per gram of substance. Similarly one can report a specific heat of solution, which is the heat a

solution per gram of solute. More commonly, though, the molar heat of solution is reported. The

molar heat of solution (ΔHsoln) is the heat of solution (qrxn) per mole of solute (n).

ΔHsoln =

qrxn

n