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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
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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) + O2(g) CO2(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 101.3 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 H2(g) + O2(g) 2H2O(l) Entropy should decrease (-) • Products = (2 * 69.94) = 139.88 • Reactants = ( 2 * 130.6) + (205.0)= 466.2 • ΔS° = ( 139.88 ) – ( 466.2 ) = -326.32 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 ΔGo = ΔHo - TΔSo Calculate the Gibbs free energy change for the formation of methane at 298 K C(s) + 2H2(g) CH4(g) ΔHo = -74.86 KJ/mol ΔSo= -80.69 J/Kmol /1000 = -0.08069 KJ/mol T = 298 K ΔGo = (-74.86KJ/mol) - (298K)(-0.08069KJ/mol) = -50.81 kJ/mol spontaneous rxn!
Two Types of ReactionsCalculating ΔG can be used to determine if a reaction is spontaneous or not.
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.