Bomb calorimeter


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Bomb calorimeter

  1. 1. Bomb calorimeter The bomb calorimeter is a device for carrying out combustion reactions in a constant-volume stainless steel container under a high pressure of O2 while measuring the amount of heat evolved. The bomb is immersed in a known volume of water inside an enclosure which is either insulated to reduce heat loss or held at a temperature which matches that of the water (adiabatic bomb). In either case, while the heat capacity of the water could be calculated, that of the bomb, bucket and its container cannot. For this reason, results are calibrated by the temperature increase when ~1 g of benzoic acid is burned. The project this semester will be to burn a series of CHO compounds and determine ∆U and ∆H of combustion. We will compile results and, by graphing ∆Hc per mol C vs. average oxidation number of C, we will attempt to decide whether the oxidation number is a reliable index of energy available by combustion. In a constant volume system, the heat of combustion, qc equals the internal energy change: (1) qc = ∆uc , and (2) ∆Hc = ∆Uc + ∆(pV). ∆uc refers to the energy change of the sample; ∆Uc refers to the energy change per mole of substance. The balanced combustion reaction for your compound [products are CO2(g) and H2O(l)] is needed since ∆(pV) = ∆(ngasRT) ≈ ∆ngasRTinitial.1 Important: heat of combustion is sometimes given as a positive quantity in the literature. However, in this report, use standard sign conventions: energy losses by the reaction system are given a negative sign. Procedure Detailed instructions will be given in lab. Synopsis: a pellet of sample of known mass is placed in the bomb, in contact with an ignition wire (“fuse”). The bomb is sealed, pressurized with O2 and immersed in 2L of water. The sample is ignited electrically and the temperature of the water is recorded until it reaches a maximum. 1. Standard Combust a ~1g pellet of benzoic acid and measure the temperature change, ∆t. Based on the exact mass, calculate heat liberated. For benzoic acid, ∆Uc = 3226 kJ/mol.2 Some of the fuse wire will oxidize and add a small amount of heat to the total (9.6 J per cm burned). The total heat is given by (3) q = qsample + qwire . Calculate the heat capacity of the calorimeter, C: (4) t q ∆ =C , 1 Neglecting the fact that both ng and T change will lead to some error, but this is much smaller than the likely experimental uncertainty. 2 P.Atkins, Physical Chemistry, 5th edition, Freeman, San Francisco (1999)
  2. 2. in which ∆t is the observed temperature change. Find the average over 3 trials and 95% confidence limits. Our calorimetric thermometers read in 0 F, so C will have units of J/0 F. There is no need to convert to 0 C or K. 2. Compound Combust a ~1 g sample of your compound. Use C to find q; subtract off any heat contributed by the fuse wire. The energy of combustion per gram of compound is (5) mass qq u wire c − =∆ . Run 3 valid trials. (If you find unburned sample or soot in the sample cup after the combustion, that trial is invalid.) Find average value of ∆uc and report with 95% confidence limits or propagated uncertainty, whichever is larger. Some compounds, e.g. sugars, are difficult to ignite. In this case, press a thin disk of benzoic acid (~ 0.2g but accurately weighed), place it on top of the sample pellet and place the fuse wire on the benzoic acid. Calculate and subtract off the heat of combustion of the benzoic acid. 3. The report should contain the following, with 95% confidence limits where applicable: Intro. 1. Balanced combustion reaction along with the discussion of ∆U vs. ∆H. 2. Average oxidation number of C in your compound Table 1: 3. Raw data: mass of benzoic acid, ∆T and C for all trials 4. Average value of C obtained Table 2: 5. Raw data: mass of sample, ∆T and ∆uc for all trials 6. Average value of ∆uc obtained Table 3: 7. ∆Uc kJ/mol 8. ∆Ηc kJ/mol Compare to literature value 9. ∆Hf kJ/mol Calculate using ∆H0 f for CO2(g) = -393.51kJ/mol, H2O(l) = -285.83kJ/mol.Compare to literature value of ∆H0 f and discuss. 10. ∆Hc kJ/mol of C (for our study of oxidation numbers) 11. ∆hc kJ/gram compound (of interest if the substance is to be used as fuel)