# Shells can be removed but other skeletal elements are

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a shell or as skeletal elements. Shells can be removed, but other skeletal elements are often less easy to separate from the soft tissues. Moreover, depending on the diet of the organism, a substantial amount of calcium carbonate can end up in feces. When we carry out bomb calorimetry on marine organisms or their feces, the following reaction occurs: CaCO 3(s) CaO (s) + CO 2(g) . This reaction does not correspond to any biological oxidation so, if we want to use calorimetry to calculate the amount of energy an organism has stored, the amount of calcium carbonate in the sample must be analyzed, and a correction must be made to the raw calorimetric results corresponding to the heat of this reaction. Incidentally, such corrections are included in the data presented in question 6 from section 1. (a) The initial and final temperatures in a bomb calorimeter are both close to 25 C. Show that this reaction is not thermodynamically allowed at this temperature in the presence of 39 Pa of carbon dioxide (the atmospheric partial pressure of this gas). That being the case, why is this reaction relevant anyway? [9 marks] (b) Calculate the correction to the measured heat per gram of calcium carbonate in the sample. Explain briefly how you would use this correction. The molar mass of calcium carbonate is 100.09 g/mol. [7 marks] 4

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Useful data 0 = 8 . 854 187 817 × 10 - 12 C 2 J - 1 m - 1 k B = 1 . 380 650 3 × 10 - 23 J / K h = 6 . 626 068 8 × 10 - 34 J / Hz R = 8 . 314 472 J K - 1 mol - 1 1 bar = 100 000 Pa To convert degrees Celsius to Kelvin, add 273.15. Debye-H¨uckel formulae: ln γ i = - Az 2 i ( εT ) - 3 / 2 I c or ln γ ± = - A | z + z - | ( εT ) - 3 / 2 I c , with A = 1 . 107 × 10 - 10 . Standard thermodynamic data at 298.15 K Species Δ f H kJ mol - 1 Δ f G kJ mol - 1 C p,m J K - 1 mol - 1 CO 2(g) - 393 . 51 - 394 . 37 37 . 1 CO 2 - 3(aq) - 675 . 23 - 527 . 90 CaCO 3(s) - 1206 . 9 - 1128 . 8 81 . 9 CaO (s) - 634 . 92 - 603 . 30 42 . 8 5
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