Calorimetry

One method of determining the energy exchange between the reaction system and its environment is to conduct a calorimetric analysis. A calorimeter is a thermally insulated container where a reaction system can be performed and the energy exchange between the system and its environment can be measured. The calorimeter and its contents are considered the environment. The reaction system is a chemical or physical process that occurs within the confines of the calorimeter.

The Qsurr = Qcal + Qcontents

The Qcal can be determined if one knows the Heat Capacity of the calorimeter. This Heat Capacity can be experimentally determined and is expressed in Kilojoules / C degree. In order to determine the Qcal you multiply the Heat Capacity of the calorimeter by the difference between the final and initial temperature. For example if the Heat Capacity of a calorimeter was determined to be 25.4 KJ/Celsius Degree, determine the Qcal if the initial temperature during a calorimetric analysis was 30 C and the final temperature was 50 C.

Qcal = Heat Capacity ( final temp - initial temp) = 25.4 Kj/C ( 50 - 30 C) = 508 Kj

Sometimes the Heat Capacity is determined for the calorimeter and its contents which might be water. Frequently, a coffee cup calorimeter made of Styrofoam is used. Styrofoam has a zero heat capacity so water is usually added to such a calorimeter and the Qsurr = Qwater.

Here is an example of determining the Q for a reaction from calorimetric data:

The combustion of 10 grams of methane, CH4, in a calorimeter changed the temperature insde the calorimeter from 25.4 to 29.4 degrees Celsius. The Heat capacity of the calorimeter and its contents was determined in a separate experimental determination and found to be 26.2 Kj/C. Determine the Heat of combustion per gram of Methane and the heat of combustion per mole Methane.

Here is the process:

  1. Determine the Q calorimeter and its contents which is equal to Qsurr:

    Qcal = Heat Capacity ( final temp - initial temp) = 26.2 Kj/C (29.4 - 25.4 C) = 104.8 Kj = Qsurr

  2. -Qsurr = Qreaction = -104.8 Kj

  3. Determine the Qreaction per gram:

    Q per gram = Qreaction / mass of Methane = -104.8 Kj / 10 grams = -10.48 Kj/g

  4. Determine the Qreaction per mole = -10.48 Kj/g X 16 grams CH4 / 1 mole = -167.68 Kj/mole

Now try this one:

Twenty grams of a salt was dissolved in water inside a calorimeter increasing the temperature of the water from 25.0 to 28.5 C. The Heat Capacity of the calorimeter and its contents was determined to be 21.3 Kj/C. Determine the Heat of solution per mole of salt.

Once you have an answer to the above problem, click here to check your results.

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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

Send Comments to R.H. Logan: Profchm@aol.com 

All contents copyrighted (c) 1996
R.H. Logan, Instructor of Chemistry,DCCCD
All Rights reserved


Revised: 11/23/96


Original Date of Creation: 11/23/96



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