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 FormationΔHrxn = 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 whenGases 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 ChangesEntropy 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.

Spontaneous Non-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.