Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: orgo814 on August 04, 2013, 11:12:15 AM
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"Calculate ΔHr and ΔUr at 298 K and ΔHr at 348 K for the hydrogenation of ethyne (acetylene) to ethene (ethylene) from the enthalpy of combustion and heat capacity data in Tables 2.6 and 2.8. Assume the heat capacities to be constant over the temperature range involved"
I understand the general procedure for this problem but my question is this (focusing just on 298 K which is the temp the data is given). Wouldn't I use enthalpy of formation for each compound/element in the reaction to calculate the enthalpy of reaction? I'm confused as to why they asked me to use enthalpy of combustion data. Did they want me to use the enthalpy of combustion and figure out the overall enthalpy using Hess's Law instead of just using the enthalpy of formations multiplied by coefficient way (ΔH = ΣHf products - ΣHf reactants)? Any insight appreciated
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Did they want me to use the enthalpy of combustion and figure out the overall enthalpy using Hess's Law instead of just using the enthalpy of formations multiplied by coefficient way (ΔH = ΣHf products - ΣHf reactants)?
Yes. The reason you do it using heats of combustion rather than heats of formation is because that's what the problem asked you to do. In the end it's effectively the same thing. Heats of formation are essentially determined from combustion measures in calorimeters. Here you are effectively cutting out the middle man.