Thermochemistry

University:
Eastern Michigan University
Course:
HIST 375 | Modern India
Academic year:

2024
Views:

0

Pages:

1
Author:

Navine Saleh

Thermodynamics/Thermochemistry review AH, Standard enthalpy of formation: the heat involved in the production of one mole of material in its standard state from its elements in their standard states. Units on this and other enthalpies are usually kJ/mol. Most of the data you find in gen chem texts is at 25.0°C. The standard enthalpy of formation of an element in its standard state is zero. So, AH of oxygen gas, O2, is zero. The same is true for l₂(s), and so on. Atomic iodine (1) would have a AH°, since monatomic iodine is not the standard state for the element. Similarly, O or Os would have non-zero AH. Tables of AH° are readily available and can be used in one form of Hess' Law: ΔΗ°χη = Σ ΠΔΗ° (each product) - ΣηΔΗ (each reactant) where "n" corresponds to number of mol of each reactant or product in the balanced chemical equation, i.e. the coefficient of that particular reactant or product. You also saw the relationship for work done under constant pressure and temperature conditions as w = PAV. If you utilize the ideal gas law, also at constant P and T: PV=nRT so PAV = A(nRT) = An(RT) This gives you two ways to calculate work at constant T and P. What is the change in volume in the system and/or what is the change in number of moles of gas in the system. The second law of thermodynamics says that the entropy of the universe must increase for a spontaneous process. Note that "spontaneous" has no speed connotation in thermo! Think of spontaneous as "possible", i.e., the process can happen under the specified conditions. For our purposes, it is reasonable to think of entropy as "disorder." The disorderly/disorganized a system, the higher its entropy. So, the gas phase of a material has a higher entropy than the liquid or solid of the same material. Entropy is represented by an S. There is an equation parallel to Hess's Law for entropy: AS°πη = Σης° (each product) - ΣηS° (each reactant) The third law of thermodynamics specifies that the entropy of a perfect crystal at absolute zero is zero. The units on entropy are J/mol-K. It would often be convenient to know in advance whether or not a reaction or process can occur under a given set of conditions. Can enthalpy or entropy be used individually to determine that? Free Energy, AG or AG°, is used to predict spontaneity (NOT speed!!!) AG is a composite function: it includes both enthalpy, entropy, and temperature terms. AGAH-TAS and AG AH° - TAS° In all the thermodynamic relationships, temperature MUST BE IN KELVIN.

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