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Calorimetry

Calorimetry is the science of measuring the heat of chemical reactions or physical changes and usally involves the use of a calorimeter.

A very crude calorimeter could be assembled from an steel can filled with a known volume of water and heated by a candle.

Allowing the candle to burn for a fixed period and then weighing the candle before and after would allow us to estimate the heat of combustion for candle wax. We would proceed to measure the end temperature of the can/water system and use it to calculate the total heat absorbed by the system. Dividing the heat absorbed in Joules by the weight of the candle wax consumed in grams, would provide us with an estimate of the heat produced per gram of wax burned.

Example Calculation

Suppose we were to measure the weight of the can to be 150 grams and the water to be 100 grams and we then allow the water and the can to be heated by the candle for ten minutes. By weighing the candle we determine that we burned 1 gram of wax and by measuring the before and after temperatures of the water we determined that the water/can system was heated up by 40°C.

The amount of heat absorbed by the can and the water can be calculated using the specific heats, or heat capacity, of iron and water, found in the heat capacity section of this topic area.

H = msteel cpsteel (Tfinal-Tinitial) + mwater cpwater (Tfinal-Tinitial)
= (150)(0.45)(40) + (100)(4.18)(40)
= 19.4 kJ

According to tables burning 1 g of paraffin wax should generates 41.5 kJ of energy while our crude calorimeter collected only about half that amount.

The explanation of course is that most of the heat of combustion went astray because the heat from the candle had many other places to escape to, in order to avoid being absorbed by our can/water system. However this does lead us to understand the starting point for calorimetry design and better appreciate the various devices and strategies that you will see used to avoid the loss of stray heat.

Note that the majority of chemical reaction do not result in an open flame, but do release or absorb heat. The principles discussed here still work, however the reaction vessel containing the chemical reactants and products will need to be immersed in the water, or other liquid, bath in order to facilitate heat transfer to the liquid.

A more detailed description of calorimeter and of calorimetry can be found by following these links to Wikipeda.


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