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Transcript
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 ∆Ssurr = - ∆𝐻 𝑇 • The sign of ∆S depends directly on direction of heat flow • whether endothermic or exothermic reaction is important • Endothermic (positive) q > 0; exothermic (negative) q < 0 • The magnitude of ∆S depends inversely on the temperature • Units: J/K • If reaction is exothermic, ∆Ssurr = positive • If reaction is endothermic, ∆Ssurr = negative • ∆Ssurr = positive is favored 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 = ∑np∆S°products - ∑nr∆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 = ∑np∆G°products - ∑nr∆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 Negative Positive Negative Positive ∆S positive positive negative negative Result spontaneous at all temperatures spontaneous at high temperatures spontaneous at low temperatures not spontaneous, ever ∆G 0 Negative Positive K at equilibrium; K = 1 > 1; products favored < 1; reactants favored