 # Calorimetry Lab- Enthalpy of Combustion of Alcohols

Hello, I want to know whether I can be able to calculate the enthalpy of combustion of a couple of alcohols in the calorimetry lab but apparently in order to do that I have to find the specific heat capacity of the bomb calorimeter. Is there any clarification or help I can get on how to do that?

Hi Nour_Ibrahim,

Yes, there is some information on this! There are some brief directions on how to do it in the “Help” menu (which you can find by clicking on the stockroom bell or “Help” on the General Chemistry landing page). The directions can be found by navigating to “Calorimetry” > “The Simulation.” I will also post what it says for you:

The calculation of the heat produced or consumed by a process, including a combustion reaction in the bomb calorimeter, is based on the equation C=Q/ΔT where Q is the heat produced or consumed, ΔT is the temperature changed measured by the thermometer, and C is the heat capacity of the entire system. If C is known, then the heat can be calculated by rearranging the equation to Q=CΔT . Measuring ΔT is rather straightforward, however, the trick is knowing the heat capacity of the system. The heat capacity of the system includes the sample being measured (for example, the reaction solution, the water plus the metal, the water and the salt, the water and the ice, the organic compound) plus the calorimeter (the dewar; coffee cup; or the bomb, water, and bucket). To a first order approximation for the dewar and coffee cup, the heat capacity of the system can be taken as the heat capacity of the solution neglecting the contribution of the dewar or coffee cup. For the coffee cup, this is not necessarily a poor assumption since the coffee cup has so little mass, but the dewar makes a non-trivial contribution. A further approximation can be made that the heat capacity of any solution in the calorimeter can be approximated by the heat capacity of water. These assumptions cannot be applied to the bomb calorimeter since the combined mass of the bomb and bucket is several kilograms.

The heat capacity of the system (sample plus calorimeter) can be determined using an electrical calibration. For the dewar and coffee cup, an electrical heater has been provided where the heater can be turned on and off, the current going through the heater and the voltage across the heater can be measured, and the time the heater is on can be measured with a stopwatch. From this, the heat, Q , dissipated by the heater can be calculated using Q = power x time = V · i x t where V is the voltage across the heater while the heater is on, i is the current going through the heater (in amps), and t is the length of time the heater was on. With Q and the resulting temperature rise, ΔT , caused by the electrical heating, the heat capacity, C , can be calculated using the equation C=Q/ΔT .

Unfortunately for the bomb calorimeter, the heat capacity of the entire calorimetric system cannot be determined with an electrical heater. Instead, the calorimeter is calibrated by measuring the heat of combustion of a standard (such as benzoic acid), which produces a known heat per mole of sample. From this, the heat capacity of the calorimetric system can be calculated using the same method as described above. For calibration purposes, use ΔH° = 3226.9 kJ/mol or ΔH = 3228 ± 2 kJ/mole for the heat of combustion for benzoic acid where ΔH° is the standard state value and ΔH is the value under the actual conditions in the calorimeter or, in other words, the value that would be measured in the experiment. The 2 kJ/mole uncertainty in ΔH is caused by changes in the correction from the standard state for differing amounts of sample and O2 in the bomb during the combustion.

Kelsey