Entropy and Gibbs Free Energy

Enthalpy Heat of reaction There is a tendency for processes to occur that lead to a lower energy state and greater stability heat energy released or absorbed a chemical reaction exothermic = favorable

Entropy Describes the disorder of a system Systems spontaneously go from an ordered state to a disordered state S

Spontaneous Processes Spontaneous processes are those that can proceed without any outside intervention. The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously

Spontaneous Processes Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

Spontaneous Processes Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures. Above 0C it is spontaneous for ice to melt. Below 0C the reverse process is spontaneous.

Entropy Like total energy, E, and enthalpy, H, entropy is a state function. Therefore, S = Sfinal  Sinitial

Entropy on the Molecular Scale Ludwig Boltzmann described the concept of entropy on the molecular level. Temperature is a measure of the average kinetic energy of the molecules in a sample.

Entropy on the Molecular Scale Molecules exhibit several types of motion: Translational: Movement of the entire molecule from one place to another. Vibrational: Periodic motion of atoms within a molecule. Rotational: Rotation of the molecule on about an axis or rotation about  bonds. 

Entropy and Physical States Entropy increases with the freedom of motion of molecules. Therefore, S(g) > S(l) > S(s)

Solutions Dissolution of a solid: Ions have more entropy (more states) But, Some water molecules have less entropy (they are grouped around ions). Usually, there is an overall increase in S. (The exception is very highly charged ions that make a lot of water molecules align around them.)

Entropy Changes In general, entropy increases when Gases are formed from liquids and solids. Liquids or solutions are formed from solids. The number of gas molecules increases. The number of moles increases.

Entropy Changes Entropy changes for a reaction can be calculated the same way we used for H: S° for each component is found in a table. Note for pure elements:

Gibbs Free Energy If DG is negative, the forward reaction is spontaneous. If DG is 0, the system is at equilibrium. If G is positive, the reaction is spontaneous in the reverse direction.

Gibbs Free Energy Used to predict a spontaneous reaction G = H - T S If G = 0, the system is at equilibrium If G = -, a spontaneous change will occur If G = +, no change will occur

G = H - T S It is favorable for a reaction to occur spontaneously if the reaction is exothermic and there is an increase in entropy A reaction will not occur spontaneously if the reaction is both endothermic and there is a decrease in entropy

G = H - T S If the reaction is exothermic and there is an decrease in entropy, the reaction can occur spontaneously at a low temperature. This will produce a - G. If the reaction is endothermic and there is an increase in entropy, the reaction can occur spontaneously at a high temperature.

Prentice Hall’s online resources http://cwx.prenhall.com/petrucci/medialib/media_portfolio/16.html http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swf http://www.chemcollective.org/applets/equilib.php

Gibbs free energy and the equilibrium constant ΔGo = - R T ln K At the standard condition, activities of all reactants and products are unity (all equal to 1). In this system, Q = 1. If K > 1, then the forward reaction is spontaneous, Q   ->   K. http://www.science.uwaterloo.ca/~cchieh/cact/applychem/gibbsenergy.html

Gibb's Energy and Electric Energy For redox reactions, Gibb's energy is the electric energy, ΔG = - n F E