# Explaining Gibbs Free Energy

1. Jul 4, 2007

### PiRsq

1. The problem statement, all variables and given/known data
A. Explain the need for Gibbs Free Energy equation.
B. Explain what it means when:
$$\Delta G <0$$ $$\Delta G >0$$ $$\Delta G =0$$

2. Relevant equations

$$\Delta G= \Delta H-T \Delta S$$

3. The attempt at a solution

A. Gibbs eqn comes from the need to calculate the entropy of the universe, which is the sum of entropy of system + entropy of surroundings. The need for the Gibbs eqn is because of the problem arising with calculating the entropy of surroundings as there are uncountable entropy changes that take place when heat is released into the surroundings. Gibbs eqn deals with this by eliminating the entropy of surroundings and replacing with an eqn that only has entropy of system on the right side of the eqn.

B. I am confused over here:

If $$\Delta G<0$$ then the process is spontaneous. This means$$\Delta H<0$$ and $$\Delta S>0$$ But how can the system release heat and also have an increase its entropy?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 5, 2007

### chemisttree

Heat and entropy are not linked. Think of some chemical process for which entropy is clearly increasing. Heat is often given off as well. Burning a hydrocarbon, for example.

3. Jul 5, 2007

### PiRsq

How about burning of propane. C3H8 + 5O2 ---> 3CO2 + 4H2O + heat

The entropy does increase in this rxn, due to 7 moles of gas in products vs. 6 in the reactants. However, C3H8 is relatively more complex relative to all the other molecules in this rxn. So how can I be sure whether entropy of the system increases, or that of the surroundings

Last edited: Jul 5, 2007