Second+Law+of+Thermodynamics,+Entropy,+and+Free+Energy

=The Second Law of Thermodynamics =


 * **Second Law of Thermodynamics** states that if no energy enters or leaves the system, the energy of the state will ALWAYS be less than it was in the initial state.

=Entropy = = = Second Law of Thermodynamics is also known as **Entropy**.
 * is symbolized by "S"
 * measures disorder
 * energy is transferred from one form to another; as the energy decreases, there is more disorder and entropy increases.

When entropy is positive, there is an increase in disorder. When entropy is negative, there is a decrease in disorder or an increase in order.

An increase in entropy has a spontaneous reaction. A decrease in entropy has a non spontaneous reaction.

Measuring Entropy
 * A solid compound has the most order
 * Liquids --> grams has more order, therefore, +
 * Grams --> liquids has more disorder, therefore, -

Take the following example:

N 2 (g) + 3H 2 (g) -->2NH 3 (g) In this reaction, although all of the states are gases, there is a decrease in entropy. This is because there are 4 moles of gas in the reactants side but only 2 moles of gas on the product side. So, because the moles of gas decreases, there is less disorder, and the entropy decreases.

Calculating Entropy Changes
ΔS = S°(final) -S°(initial)

to solve the equation above, one needs to know the standard molar entropies, (S°),of each of the reactants and products to then calculate the change in entropy. 

Entropy Changes in Chemical Reactions
ΔS° = £nS°(products) - £mS°(reactants) The entropy change in a chemical reaction such as this one: Al2O3 + 3H2 --> 2Al +3H20
 * S° || standard entropies ||
 * £ || Add ||
 * n || multiply coefficients for products ||
 * m || multiply coefficients for reactants ||

ΔS° = [S°(Al2O3) +3S°(H2)] - [2S°(Al) + 3S°(H2O)]

ΔS° = (51.00)+(3mol)(130.58) - (2mol)(28.32) + (3mol)(188.83) ΔS° = -180.39J/K

Entropy is measured in Joules/Kelvins

=Gibb's Free Energy =

J. Willard Gibbs (1839-1903)

 * mathematician who came up with free energy.
 * Free energy is a way to predict the spontaneity of a system by using ΔH and ΔS.
 * In every spontaneous process, some of the energy is available to do work, that energy is called Gibbs free-energy change.
 * energy that can do useful work

If ΔG is negative, the reaction is **spontaneous** If ΔG is zero, the reaction is at **equilibrium** If ΔG is positive, the reaction is **nonspontaneous**

With the equation G = H - TS, finding G would be very hard because it is impossible to find exact values for G, H, T, & S. Therefore, the equation is modified to measuring the change of G, **ΔG = ΔH - TΔS**. This equation can be used to find ΔG or predict the if G is spontaneous or not spontaneous by a negative or positive sign.

-Often times, a question on the AP Exam will ask if a reaction is favorable under certain conditions. If a reaction is spontaneous (with a negative ΔG), the reaction will be favorable. If the reaction is non spontaneous, the reaction will be unfavorable.

Works Cited
 * http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEner1.html