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Heats of Combustion

Heats of Combustion. Bomb Calorimetry. Principles of Calorimetry. Heat capacity can be related to either the enthalpy or internal energy The conditions of the experiment determine which thermodynamic quantity is determined

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Heats of Combustion

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  1. Heats of Combustion Bomb Calorimetry

  2. Principles of Calorimetry • Heat capacity can be related to either the enthalpy or internal energy • The conditions of the experiment determine which thermodynamic quantity is determined • There are two heat capacities, Cp and Cv. The first is determined at constant pressure and the second at constant volume.

  3. Where the general thermodynamic expressions for the heat capacities are: With the integrated quantity at constant volume, say: In bomb calorimetry, the measurement is carried out in a closed vessel so the volume is a constant. Thus the heat evolved in the process being measured is the internal energy. The heat capacities are not strongly temperature dependent and are regarded as constant over limited temperature ranges.

  4. For a combustion measurement, a known quantity of a pure organic material is placed in a closed volume and burned in pure oxygen to achieve complete combustion. This is an adiabatic process that is associated with a temperature rise: CaHbOc(s)[To] + (a +b/4 –c/2)O2(g)[To]  aCO2(g)[T1] + (b/2)H2O(l)[T1] However, because the conditions are isothermal, the products have to be brought back to the initial temperature. aCO2(g)[T1] + (b/2)H2O(l)[T1]  aCO2(g)[To] + (b/2)H2O(l)[To] For these two processes: ΔE = ΔEa + ΔEc But ΔEa = 0, since q = 0 and w = 0. Thus, ΔE = ΔEc.

  5. Using the equation relating Cv to ΔE gives: • ΔEc = [a Cv(CO2) + (b/2)Cv(H2O) + Cv(S)](Tm – To) • Srefers to the system, i.e. the calorimeter and all of its parts. • (Tm – To) is the instantaneous rise in temperature for the adiabatic step. • Determination of ΔE consists of two parts: • A known mass of test material is burned in a bomb and the temperature rise is determined. • The sum of the heat capacities must be obtained.

  6. Step 1: The temperature rise is a function of time. The initial slow rise is due to the stirrer. At to the bomb is ignited and the rise in temperature due to combustion is monitored. The first portion of the graph is extrapolated forward and the second portion back to a vertical line, tc, such that equal shaded areas are created. The vertical distance between the extrapolated lines at tc is taken as the instantaneous temperature change.

  7. Step 2: The sum of the heat capacities must be obtained. This is best done by combustion of a standard sample (usually benzoic acid) of known ΔE in the calorimeter. The burning must be under the same conditions as the determination of the unknown. Thus, after obtaining the temperature rise from a known mass of the benzoic acid, the summed heat capacity of the calorimeter is obtained: Where: C = [a Cv(CO2) + (b/2)Cv(H2O) + Cv(S)]

  8. Experimental • Reaction occurs in a stainless steel bomb at constant temperature • Sample is ignited in an oxygen atmosphere through contact with a hot wire • Heat generated by combustion causes a temperature rise in a known amount of water in the bucket in which the bomb is immersed.

  9. Procedure • Form and accurately weigh a pellet of 0.5 to 0.8 g sample and place in the cup • Attach 10 cm of iron ignition wire tightly to electrodes. Let the loop of wire just touch the sample or fuse into the pellet (better). • Install pellet and wire in bomb. • Assemble bomb and screw top on tightly (use no wrenches – only manual force) • Fill the stainless steel can with 2L distilled water at 25oC.

  10. Fill bomb with oxygen to a pressure of about 25 atm. • Attach the electrode wire to the slotted contact in the bomb head • Ignite the charge by pressing the firing button for no more than 5 sec. • Take time-temperature readings every 30 sec. Estimate the temperature to thousandths of a degree if possible. • Do not interrupt the readings until the run is complete!! • After the readings are complete, remove bomb and open the release or outlet valve to vent excess gas. • Open bomb and remove and weigh the unburned wire. • Clean the bomb, pan, empty water and return to original condition.

  11. Important Points • Be sure there are no unburned carbon particles on the pan or in the bomb before starting. Clean with steel wool. • If there are unburned particles after combustion, repeat with a smaller sample. • In closing the bomb, tighten cap only by hand. • Multiply the length of the wire consumed by 2.3 cal/cm. This is part of the heat input. • Be sure that the wire is connected tightly to the binding posts or there may be no ignition.

  12. Filling of Bomb with O2 • Close the outlet valve. • Attach the union from the oxygen tank to the inlet valve, turning it down tightly by hand. • Close the filling valve between the bomb and the gauge (turn clockwise). • Open tank valve one-quarter turn, open filling valve slowly, and allow the pressure in the line to rise slowly to desired pressure.

  13. Close the filling valve and the tank valve and open the relief valve under the gauge. • The excess pressure in the bomb will close the inlet valve. • Unscrew the union. • Remove atmospheric nitrogen before combustion by putting 10 – 15 atm of oxygen into the bomb and then opening the release valve. Formation of nitrogen oxides will corrupt the measurement.

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