Finding Heat Capacity of a Bomb Calorimeter
Introduction
Bomb calorimeters are high-precision instruments used in various scientific disciplines to determine the heat of combustion or the caloric content of a sample. Understanding the heat capacity of the bomb calorimeter itself is crucial for accurately analyzing the sample and getting precise results. In this article, we will outline the steps to find the heat capacity of a bomb calorimeter.
The Role of Heat Capacity in Calorimetry
Calorimeters measure heat absorption or evolution during chemical reactions. In bomb calorimeters, this involves dropping the sample (typically an organic compound) into a calorimeter and allowing it to react with oxygen or nitric acid, leading to the production of carbon dioxide and water. During this reaction, heat is liberated, and the temperature increase is measured.
To perform accurate calorimetric experiments, the heat capacity (C) of the bomb calorimeter needs to be known. Heat capacity, measured in units of energy per temperature change per mass (e.g., Joules per degree per kilogram, J kg¯¹°C¯¹), represents the amount of energy required to change the temperature of a given substance.
Experimental Method
Several methods exist for measuring heat capacity. We will describe the common technique: Calorimetric self-comparison, where an energy equivalent Standard Fuel Oil (SFO) sample is combusted simultaneously with the test sample (to be measured) to equalize temperatures. Then, the bomb calorimeter contains only air or is maintained at its original (baseline) temperature. Next:
Initialize the calorimeter’s measuring system by filling with oxygen, or by pressurizing it with gases suitable for calorimetry.
Stir the mixture carefully but thoroughly.
- Record the baseline temperatures of the system, such as the internal thermistors, as a measure. This Baseline measurement will serve as the standard 0 oC mark in the entire procedure.
- Inject standard fuel oil (SFO) as quickly and with minimal delay into a hole in the **"tare cap" ( a removable piece on top the calorimeter vessel).
- Calculate (at least twice a day, during this session) a new ΔT(SFO). Measure the corresponding change, if any.
Now fill the test sample(s).
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Importance and Control of Conditions
Ensuring precise experimental conditions, is crucial when determining heat capacity. Variables such as atmosphere, surface cleanliness, electrical connections, and heat absorption by experimental components and ambient conditions influence results accuracy.
Critical considerations when dealing with precision heat capacitimeters:
To establish a strong energy conservation;
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