For example, consider the burning of graphitic carbon in oxygen, the change in enthalpy is:
Ho
Cgraph + O2 → CO2 -94.0 kcal/mole
In the combustion of 1 mole (12 g) of pure carbon as graphite (a reasonable approximation of coal) 94 kcal are released. This is enough to raise the temperature of 1.0 L of water 94 °C.
The superscript ° stands for standard conditions (25 oC and 1.00 atm). Because we started with elements in their standard state H° = Hf
o the standard heat of formation for CO2. There is a table of Hf
The heat capacity depends on degrees of freedom in the molecule enjoys.
Translation = 1/2 R each (every gas has 3 translational degrees of freedom)Rotation = 1/2 RVibration = RFor a gas with N atoms you see 3N total degrees of
freedom and 3N - 3 internal (rot + vib) degrees of freedom.
Equipartition principle: As a gas on warming takes up energy in all its available degrees of freedom.
The table shows 5.0; what's wrong? Not all energy levels are populated at 300 K. Not all the degrees of freedom are active (vibration). O2 vibration occurs only with high energy; vacuum uv radiation.At 2000K Cv (O2) approx 7.0 cal/mole K
Students: show that on the primordial Earth the dry adiabatic lapse rate was about 12.6 K/km.
G is the Gibbs free energy that stands as the criterion of feasibility. G tends toward the lowest values, and if G for a reaction is positive, the reaction cannot proceed!
The Gibbs free energy of a reaction is the sum of the Gibbs free energy of formation of the products minus the sum of the Gibbs free energy of formation of the reactants.
3. BOND ENERGIES See Appendix III of Pitts for a table of bond energies. The quantity is actually heat not energy. Definitions:Bond Dissociation Energy - The amount of energy required to break a specific bond in a specific molecule.Bond Energy - The average value for the amount of energy required to break a certain type of bond in a number of species. EXAMPLE: O-H in waterWe want H2O → 2H + O +221 kcal/mole We add together the two steps: H2O → OH + H +120 OH → O + H +101 --------------------------------- NET +221 Bond energy (enthalpy) for the O-H bond is 110.5 kcal/mole, but this is not the b.d.e. for either O-H bond.
The bond energy for C-H in methane is +396/4 = +99 kcal/mole.Bond energies are useful for "new" compounds and substances for which b.d.e. can't be directly measured such as radical.
We have a problem, neither internal energy (E or U) nor enthalpy (H) is the "criterion of feasibility". Chemical systems generally tend toward the minimum in E and H, but not always.
Everyday experience tells us that water evaporates at room temperature, but this is uphill in terms of the total energy.
This holds only for reactants that start (state 1) at standard conditions and products that finish (state 2) at standard conditions (1.00 atm). Standard conditions are 25 °C and 1.00 atm pressure. Watch out for units – Gibbs Free Energy of formation is tabulated for these standard conditions.
You can show that the mole fraction of NO at equilibrium is nearly independent of pressure. Try repeating this calculation for 2500 K; you should obtain Keq = 3.4E-3 and [NO] = 2.3%.
In high temperature combustion, such as a car engine or power plant, NO arises from similar conditions.
Allen, D. J., and K. E. Pickering, Evaluation of lightning flash rate parmaterization For use in global chemical transport models, J. Geophys. Res., 107(23), Art. No. 4711, 2002.
Chameides W. L., et al., NOx production in lightning, J. Atmos. Sci., 34, 143-149, 1977.
Rakov V., and M. A. Uman, Lightning: Physics and Effects, University Press, Cambridge, 2003.
