# Gibbs free energy question

1. Mar 20, 2014

### sero2000

I calculated some gibbs free energy values for a chemical reaction at 298K and 1773K both giving me negative values . the amount for the reaction at 298K is -162443J and the one at 1773K is -81147.85J.

Both shows that the reaction are spontaneous and so thermodynamically the process at 298K is favorable right?

If so, why would someone still want the reaction to occur at 1773K?

the process is an exothermic reaction.

Last edited: Mar 20, 2014
2. Mar 20, 2014

### Staff: Mentor

Maybe the rate of the reaction is too slow at 298.

Chet

3. Mar 20, 2014

### sero2000

Hi there!

I did some researching again and found this
http://chemistry.illinoisstate.edu/standard/che360/handouts/360gibbshelmholtz.pdf

of which the last example has the same results as what I have but however, I cant seem to understand it.

From the top of the file, it states that dG = -Sdt + VdP what does this actually mean? Does it have any reference to the normal eqn of Delta G = Delta H - Delta TS.

I went to check out the rate of reaction as well but it's something im not taught yet.

4. Mar 20, 2014

### abitslow

Thermodynamics is NOT kinetics. The rate of a reaction is generally an increasing function of temperature.
You want us to speculate on why "someone" "want" something? Not sure why you posted a freshman chemistry question in Advanced Physics Homework... actually your question should be posted in Psychology.
Since there are a plethora of possible benefits for increasing temperature, and since there is no specific reaction given, I won't waste any more time speculating about which reasons might apply.
-=-=-
At 25°C, you'll get more energy out of reaction. Your second post makes no sense at all to me. In elementary thermodynamics, kinetics is almost always ignored. The pdf file seems to be about elementary thermo. Your question had NOTHING to do with entropy ...why do you bring it up??? In thermo, the typical situations are constant temperature, constant pressure, constant volume, constant energy, or some combination of these. Consider rusting of iron in water. Do you think -Sdt+VdP is useful? (A: not really). Meaning: equation is for a specific set of materials (ideal gases?) in a specific context.
-=-
Rates of reactions are almost, almost, impossible to predict (with more than ballpark accuracy...meaning plus or minus a couple of powers of 10 of the correct rate). (predict from first principles, without knowledge of similar reactions, or without knowledge of activation energy, etc.).

Last edited: Mar 20, 2014
5. Mar 20, 2014

### Staff: Mentor

ΔG0(T) represents the change in Gibbs free energy in going from the pure reactants to the pure products at temperature T and and reference pressure (usually 1 atm). If you only know the free energies of the pure reactants and products at 298 reference temperature, you need to determine the free energies of the reactants and products at temperature T to find ΔG0(T). The reference you cites shows how to do this, based on knowing their enthalpies, or knowing the change in enthalpy for the reaction in the reference state (neglecting its variation with temperature).

Chet