Entropy & Spontaneity Chapter 17

Review Enthalpy – ∆H=q - heat of reaction Exothermic vs. endothermic Exothermic is generally favored in nature Kinetics – rates of reaction – how fast or slow System refers to the reactants and products Surroundings - everything around it Heat flows from one to the other Laws of thermodynamics: First Law = conservation of energy = energy in universe is constant Second Law = universe is constantly increasing in disorder (randomness) = increasing in entropy Third Law = entropy of a perfect crystal is zero at 0K = (absolute entropy can be determined for any temp higher than 0K)

Spontaneous Means it occurs without outside intervention - on its own. Does not mean fast. Must follow the laws of thermodynamics Tells us about direction but not speed Helps predict if the process will occur but not how it will Alternate way to describe spontaneous process is to say it is thermodynamically favored. Common characteristic of spontaneous processes is entropy Entropy = measure of molecular disorder or randomness Driving force behind processes

Entropy Represented as S ∆S = change in entropy (final – initial) ∆S can be positive or negative Atoms love going to lower energy states Atoms love to get all messed up (move around randomly) Things naturally tend to get messy unless you keep them in check. Entropy is closely associated with probability (likelihood) The more ways a state/condition can be achieved the more likely (probable) it is to occur. Spontaneous processes proceed toward states that have highest probability of existing. Larger number of arrangements or energy levels available to a system More disorder or randomness = more entropy

Entropy Think about states of matter Solids – tightly packed, rigid structure (lots of order) Liquids – ability to flow, move a little Gases - spread out, move far apart (little to no order) Solids have lowest entropy; gases have highest entropy Prefer to go to state with less order = more entropy What about mixtures/solutions? Tendency of materials to mix is due to increased volume available to the particles of each component of a mixture More complex the molecule = generally more entropy EX: When 2 liquids are mixed, the molecules of each liquid have more available total volume, therefore more available positions/interactions available.

Relating entropy & temperature

Entropy Changes in Reactions Whether a reaction will occur spontaneously can be determined by looking at ∆S and ∆H. Table 17.5, p798 Also look at the number of molecules/moles of each reactant and product in the equation Fewer molecules means fewer possible configurations = lower entropy More molecules/moles on product side = entropy increases More molecules/moles on reactant side = entropy decreases Standard entropy - ∆S° - listed in Appendix 4 ∆S° reaction = ∑n p ∆S° products - ∑n r ∆S° reactants

Free Energy Symbol – G G = H – TS ∆G = ∆H – T∆S ∆G° = ∆H° – T∆S° Remember this ° indicates in standard states Calculating ∆G° is just like determining ∆H° and ∆S° ∆G° reaction = ∑n p ∆G° products - ∑n r ∆G° reactants ∆G = ∆G° + RT ln Q ∆G° = -RT ln K (system at equilibrium so ∆G = 0) ∆G° = - nFE° (F = faradays constant; E° = standard voltage)

Summary ∆G positive = not spontaneous ∆G negative = spontaneous ∆H∆SResult Negativepositivespontaneous at all temperatures Positivepositivespontaneous at high temperatures Negativenegativespontaneous at low temperatures Positivenegativenot spontaneous, ever ∆GK 0at equilibrium; K = 1 Negative> 1; products favored Positive < 1; reactants favored