How do we measure heat capacity?

Updated 1/5/2019

What is heat capacity? What is calorimetry? Good questions. Calorimetry is the study of heat in its various forms. This is of particular concern to chemists and mechanical engineers. Let’s look at some common terms.

Thermal Conductivity is the property of a material to conduct heat. The unit of measurement is watts per meter kelvin or BTU per hour foot Fahrenheit. This is typically used to determine the heat transfer through materials in series in order to calculate how large of a heat sink will be required for a thermal system. The major influencing variable when measuring thermal conductivity is the average material temperature. Also, thermal conductivity will usually change when the measured material undergoes a phase transition. Some crystalline structures will only act as thermal transmitters in certain directions due to the crystalline lattice structure. Another consideration are air pockets or void pockets in polystyrene-like materials.

Heat Capacity is the amount of heat that is required to change a substances temperature by a certain amount. The SI unit of measurement denotes Joules per kelvin. Specific heat capacity is the same thing except expressed as Joules per degree kelvin * unit mass. Specific heat capacity is useful when trying to determine the heat sink properties of different sizes of material. Heat capacity may be affected by the volume of the substance, density due to thermal expansion, impurities in the material, and type of atomic bond the material uses.

One of the major accuracy constraints in calorimetric determinations is the sampling probe. Inaccuracies in the probe will contribute to the total error of the experiment. This can be minimized by frequent calibration. Another inaccuracy is uneven heating of the substance being tested. Another common problem is using an irregularly shaped material rather than a thin film. The irregular shape can cause unexpected conduction problems. Poor sampling preparation can also be a problem. If a material is placed in a pan that is to be heated, ensure that all of the samples use the same type and dimension of container.

Pictured above is a “bomb” calorimeter.

This device works by measuring the temperature of a material that is typically in water suspended above a combustion chamber. The temperature of the material is noted before and after the combustion chamber has been operated. This will allow for the specific heat capacity of a material to be determined based on how much heat was absorbed and also by how much heat has been radiated away.

As a trivia note: If you use a calorimeter in college, you’ll probably make it out of two Styrofoam cups.

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