Thermochem - Thermochemistry Thermochemistry is the...

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15.1 Thermochemistry Thermochemistry is the application of thermodynamics to chemical systems . This field of study is particularly concerned with the heat effects that accompany chemical reactions, the formation and dilution of solutions, and phase changes. The heat of a reaction is defined as the heat transferred between the system (the reaction) and the surroundings when a known amount of reactants react to form a known amount of products at either a constant temperature and pressure : T, P reactants ------------> products q p = D H or a constant temperature and volume : T, V reactants ------------> products q V = ? Since most reactions are carried out at the constant ambient pressure, we are usually concerned with the enthalpy of reaction and this is what is typically meant when speaking of a heat of reaction.
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15.2 There are between 10 and 20 million known compounds (see the Chemical Abstracts Service daily update of the number of compounds, http://www.cas.org/cgi-bin/cas/regreport.pl ) that can actually or hypothetically react with each other in an astronomical number of ways and it is therefore literally impossible to catalog all the possible heats of reaction. To get around this problem we define for each substance a standard reaction and tabulate its associated heat of reaction. These reactions and their associated heats of reaction can then be used to calculate the heats of other reactions. This standard reaction is known as the standard formation reaction of some substance and is defined as the balanced reaction in which 1 mole of that substance (the only product) is formed from the elements which comprise it (the reactants) in their elemental standard states at 1 bar of pressure and some temperature (usually, but not necessarily 25.0 o C). In older tables the standard pressure was defined as 1 atm, rather than 1 bar. How significant is this change in the definition of standard pressure? For example, the standard formation reaction for the interhalogen, iodine monobromide , is: 298 K, 1 bar 1/2 I 2 (s) + 1/2 Br 2 (l) ------------> 1 IBr (s) The heat associated with the standard formation reaction is known as the standard heat of formation for IBr (s) and is: D H o f, 298 K [IBr (s)] = - 10.5 kJ/mole of IBr (s) The superscript, o , indicates standard state conditions, e.g., that the pressure is 1 bar, while the subscript, f , indicates that this is a formation reaction.
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15.3 This is a page taken from the National Technical and Information Service (NTIS) formerly the National Bureau of Standards (NBS) publication The NBS Tables of Chemical Thermodynamic Properties, Wagman. et. al., eds., Journal of Physical and Chemical Reference Data , Vol 11 (Supp. No. 2), 1982. 0 K
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Thermochem - Thermochemistry Thermochemistry is the...

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