Thermodynamics Part II

Entropy, S Entropy is a measure of the disorder or randomness of a system. The entropy of the universe is increasing. When: DS is positive disorder increases (favors spontaneity) DS is negative disorder decreases (disfavors spontaneity) In general: Sgas> Sliquid > Ssolid 107

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 1) Melting of ice at room temperature

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 1) Melting of ice at room temperature Entropy increases: a phase change from an organized solid to a more disorganized liquid ∆S (+)

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 2) Condensation of water vapor on a window

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 2) Condensation of water vapor on a window Entropy decreases: a phase change from a disorganized gas to a more organized liquid ∆S (-)

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 3) An iron rusting 4Fe(s) + 3O2(g)  2Fe2O3(s)

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 3) An iron rusting 4Fe(s) + 3O2(g)  2Fe2O3(s) Entropy decreases: there are 4 moles of a solid reacting with 3 moles of a gas to make only 2 moles of the product ∆S (-)

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 4) Baking soda reacting with vinegar NaHCO3(s) + CH3COOH(l)  NaCH3COO(aq) + H2O(l) + CO2(g)

Predicting ∆S Determine whether the entropy value increases or decreases for the following situations Ex. 4) Baking soda reacting with vinegar NaHCO3(s) + CH3COOH(l)  NaCH3COO(aq) + H2O(l) + CO2(g) Entropy increases: a solid and liquid yield an aqueous soln, a liquid, and a gas (Also, there are 2 moles of reacts making 3 moles of product) ∆S (+)

Entropy changes for reactions can be determined similarly to DH for reactions. As with DH, entropies have been measured and tabulated in for So298.

Ex. 5) Find the ∆S for liquid ethanol, C2H5OH, undergoing combustion: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) Does this reaction create order or disorder?

Enthalpy and entropy can sometimes reinforce each other – this makes the reaction really go or really not go. Ex. Dynamite has a neg Δ H & a pos Δ S so the rxn really goes once started… If the signs don’t reinforce does a rxn occur? This is where Gibbs Free energy addresses the spontaneity of rxns. Ex. Liquid water to water vapor Δ H is + and Δ S is + Δ G is – (neg),the reaction will go on its own once started to make the products. Ex. gas burning. Δ G is + (pos), the rxn won’t go on it’s own, wants to stay as reactants. Ex. batter  cake

The change in the Gibbs Free Energy is a reliable indicator of spontaneity of a physical process or chemical reaction. When: DG is > 0 reaction is not thermodynamically favored (nonspontaneous) reactant favored DG is < 0 reaction is thermodynamically favored (spontaneous) product favored Changes in free energy obey the same type of relationship we have described for enthalpy and entropy changes.

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) Ex. 6) Find the ∆Grxn for liquid ethanol, C2H5OH, undergoing combustion: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) Is this reaction spontaneous or nonspontaneous?

Using ∆H and ∆S to solve ∆G The relationship between ∆H, ∆S and temperature can help describe the spontaneity of a system. Whether the reaction is thermodynamically favored or not thermodynamically favored

Ex. 7) Determine if a reaction is spontaneous at room temperature, 25oC, when ∆H = -176 kJ and ∆S = -285 J/K Units must match!!!

Ex. 8) What happens if the reaction is now at 550oC, both ∆H = -176 kJ and ∆S = -285 J/K are the same. Units must match!!!

Ex. 8) What happens if the reaction is now at 550oC, both ∆H = -176 kJ and ∆S = -285 J/K are the same. Units must match!!! * At what temperature would ∆G = 0?

This general relationship gives us 4 possibilities among the signs DH DS DG Therefore - + - forward rxn spontaneous at all T’s - - ? forward rxn spontaneous at low T’s + + ? forward rxn spontaneous at high T’s + - + forward rxn nonspontaneous at all T’s

Copy this into your notes State function + - Δ H Endothermic (taking in heat) ice melting *Exothermic (giving off heat) dynamite Δ S *Towards disorder Towards order Δ G not thermodynamically favored nonspontaneous *thermodynamically favored spontaneous Enthalpy (heat changes) H Entropy (order and disorder) S Gibbs Free energy (spontaneity) G The universe prefers exothermic reactions that cause disorder and are spontaneous