Section 18.4 Entropy

What you need to know - Entropy -Gibbs Free Energy -Enthalpy -Calculating Gibbs Free Energy -Determine if a rxn is spontaneous or non- spontaneous.

First a Quick Review: Enthalpy Enthalpy (H) is the heat released or absorbed during a rxn at constant pressure Calculating Enthalpy of Formation ΔHrxn = Σ(ΔH products) – Σ(ΔH reactants) Get the values for change in enthalpy from a table( equation and table will be given)

Entropy (S) Entropy can be thought of as a measure of the randomness or disorder of a system. – Entropy increases with the freedom of motion of molecules. SOLID < LIQUID < GAS

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. -Temperature increases -Volume increases

For the following, is entropy increasing or decreasing? 1) Ice Melting 2) C (s) + O 2(g)  CO 2(g) 3) Cleaning a bedroom 4) Dropping a deck of cards

Law of Disorder: natural tendency for the universe is for systems to move in direction of maximum disorder because it takes the least amount of energy to maintain

Entropy Changes Entropy changes for a reaction can be calculated the same way we used for  H: o = standard state ( 25C and kPa) ΔS° rxn = Σ(ΔS° products) – Σ(ΔS° reactants) S° for each component is found in a table. Note for pure elements:

Practice Problem For the following reaction, calculate the change in Entropy using the table given to you at STP ( all in gas phase below) 2 H 2(g) + O 2(g)  2H 2 O (l) Entropy should decrease (-) Products = (2 * 69.94) = Reactants = ( 2 * 130.6) + (205.0)= ΔS° = ( ) – ( ) = J/K*mol

Gibbs Free Energy (G) Gibbs Free Energy (G) : energy available to do work After entropy, and enthalpy, Gibbs free energy is the energy left over that is available. ΔG° rxn = Σ(ΔG° products) – Σ(ΔG° reactants) ΔG° rxn = - rxn is spontaneous ΔG° rxn = 0 rxn is at equilibrium ΔG° rxn = + rxn is non-spontaneous

Putting it all together ΔG o = ΔH o - TΔS o Calculate the Gibbs free energy change for the formation of methane at 298 K C (s) + 2H 2(g)  CH 4(g) ΔH o = KJ/mol ΔS o = J/Kmol /1000 = KJ/mol T = 298 K ΔG o = (-74.86KJ/mol) - (298K)( KJ/mol) = kJ/mol spontaneous rxn!

Two Types of Reactions Calculating ΔG can be used to determine if a reaction is spontaneous or not. SpontaneousNon-spontaneous -releases free energy (-ΔG) -Favors products -Occurs naturally -Increase in entropy (S) Example) Iron Rusting -absorbs free energy (+ΔG) -Favors reactants -Does not occur naturally -Decrease in entropy (S) Example) Photosynthesis

Free Energy and Temperature By knowing the sign (+ or -) of  S and  H, we can get the sign of  G and determine if a reaction is spontaneous.