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31

منتدى علم الكيمياء / ارجو من الكيميائيين الرائعين الاذكياء في الوطن

« في: نوفمبر 27, 2007, 08:02:30 مساءاً »

Graphite is only a few electron volts more stable than diamond, but the activation barrier for conversion requires almost as much energy as destroying the entire lattice and rebuilding it. Therefore, once diamond is formed, it will not reconvert back to graphite because the barrier is too high

32

منتدى علم الكيمياء / ارجو من الكيميائيين الرائعين الاذكياء في الوطن

« في: نوفمبر 27, 2007, 07:34:19 مساءاً »

spontaneous process
A spontaneous process is a chemical reaction in which a system releases free energy (most often as heat) and moves to a lower, more thermodynamically stable, energy state. The sign convention of changes in free energy follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in free energy, but a positive change for the surroundings.

Thus, for a reaction at constant temperature and pressure where

ΔG = ΔH - TΔS

a negative ΔG would depend on the sign of the changes in enthalpy (ΔH), entropy (ΔS), and the magnitude of the absolute temperature (in kelvins). Changes in the sign of ΔG cannot be changed directly by temperature, because it can never be less than zero.

When ΔS is positive and ΔH is negative, a process is spontaneous

When ΔS is positive and ΔH is positive, a process is spontaneous at high temperatures, where exothermicity plays a small role in the balance.

When ΔS is negative and ΔH is negative, a process is spontaneous at low temperatures, where exothermicity is important.

When ΔS is negative and ΔH is positive, a process is not spontaneous at any temperature, but the reverse process is spontaneous.