Heats of Combustion Lab Report.docx - Grayczyk 1...

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Grayczyk 1 CHEM3860:002 Jonathan J. Grayczyk Experiment 2: Heats of Combustion Technical Report Experiment Run: November 23, 2017 Report Submitted: December 8 th , 2017 TA: Badri Bhattarai Professor: Dr. Findsen
Grayczyk 2 Abstract The heats of combustion and heat capacity for naphthalene and benzoic acid were determined through the use of bomb calorimetry. Using an oxygen bomb calorimeter the heat capacity for benzoic acid was determined to be 9.99 kJ ˚ C . From this value, the heat of combustion for naphthalene was determined to be -5211.8 kJ mol possessing 1.000% error when compared to literature values. With the percent error being so low, the experiment is assumed successful. Introduction In this experiment, the relationship between the heat of combustion and internal energy will be examined using bomb calorimetry. Being one of the oldest known scientific methods used to measure energy transfer, this is often a practical procedure to be employed. In this method, the device used to measure energy is called a calorimeter. In bomb calorimetry, the heat of combustion for a compound can be determined by using a second known compound as a standard. Calorimeters are regularly enclosed by constant temperature water bath which prevent the heat loss from the system to the surroundings. Due to the amount of heat loss being 0, we can consider the calorimeter to be an adiabatic system. Because of this, we can assume that the change in temperature of the water during combustion is equivalent to the total amount of energy released by the combustion process. In order to use this method of calculation, a substance with a known heat of combustion such as benzoic acid must be in order to determine the constant of the calorimeter. The sample substance is placed inside the bomb upon which in comes into contact with an ignition wire in a pressure of around 20 atms. A known volume of water is placed in the bomb and the initial temperature recorded and the bomb ignited which send an electrical current through the wire igniting the substance. This causes the sample to combust and the temperature of the water to subsequently rise. We can them calculate the adiabatic temperature change using equation 1.
Grayczyk 3 ΔT – (T f – T i ) –( dT dt ) I (t d -t i ) –( dT dt ) f (t f -t d ) Equation 1 If we have the mass of the known combustible, we can then determine the heat capacity of it with equation 2. C (S) = ΔE known T 2 ' T ' Equation 2 Using the average heap capacity of the known substance we can then determine the heat capacity of the calorimeter by equation 3: U known = -Cv cal (T f -T i ) Equation 3 Knowing the relationship between the internal energy of a sysem and the heat of combustion we can use equation 4. ∆U = ∆H-∆(pv) Equation 4 If we treat all the products as ideal gases, we can then rewrite equation 4 as: ∆H=∆U + RT∆n gas This then affords us then enthalpy of combustion for a pure compound.

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