# College Chemistry : Gibb's Free Energy

## Example Questions

### Example Question #1 : Gibb's Free Energy

How do we know if a reaction is spontaneous?

The sign of

The first law of thermodynamics

The sign of

The second law of thermodynamics

The value of  (the rate constant) tells us

The sign of

Explanation:

A reaction is spontaneous if and only if . The sign of only tells us if a reaction is exothermic or endothermic. The rate constant only tells us the rate at which a given chemical reaction proceeds based on the reactants and the products. The first law of thermodynamics only tells us that energy is conserved, and can neither be created nor destroyed. The second law of thermodynamics states that the entropy of the universe is spontaneously increasing, however, under the right conditions, these reactions may proceed.

### Example Question #1 : Thermodynamics

What is the standard change in free energy for a reaction run at  with an equilibrium constant of ?

Explanation:

In this question, we're told of a reaction that is run at a given temperature and with a certain equilibrium constant. We're asked to evaluate the standard change in free energy for this reaction.

First, right off the bat, we can see that the equilibrium constant for the reaction is greater than one. As a result, we know that under standard conditions, equilibrium of this reaction will favor products. Hence, we know right away that  will be negative, which allows us to rule out any answer choices that are positive.

To actually calculate the value of , we'll need to use an equation that relates this quantity with temperature and the equilibrium constant. We can do so by using the following expression.

If we plug in the values that we know, we can solve for the correct answer.

### Example Question #1 : Thermodynamics

Calculate the change in free energy, in kilojoules, of the reaction at  by using the free energies of formation.

values, in , for molecules are as follows:

Explanation:

Recall how to find the change in free energy for a reaction from using free energies of formation:

Where  is the number of moles of the molecule.

Plug in the given values to find the change in free energy for the reaction.

### Example Question #1 : College Chemistry

Suppose that a given chemical reaction has an enthalpy change of  and an entropy change of . At what temperature would this chemical reaction be in equilibrium?