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Spontaneity Recap of Enthalpy • Describes chemical potential energy stored in matter. • Can only measure changes in enthalpy. • Enthalpy is arithmetical. – Reverse equation, reverse sign of H – Multiply equation by some #, multiply H by that number. – Add two equations, add the H’s. Spontaneous Processes • A physical or chemical change that occurs with no outside intervention. • Some energy may be supplied to get the process started – activation energy Examples of Exothermic Spontaneous Processes • Burning methane gas, CH4, in the bunsen burner. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -891 kJ • Iron rusting 4Fe(s) + 3O2(g) 2Fe2O3(s) H = -1625 kJ Some Spontaneous Processes are Endothermic • Dissolving NH4NO3(s) in water is an example of a spontaneous endothermic process. • NH4NO3(s) NH4+(aq) + NO3-(aq) • Ice melting is another spontaneous, endothermic process. • H2O(s) H2O(l) Nature is lazy & disorganized • 2 driving forces in nature – Get to the lowest energy state – Get to the most chaotic state • For a system to get lower in energy, it must release energy. • So nature favors exothermic processes with -H. • But spontaneous endothermic processes occur, so something else is important, too. Entropy, S • A measure of the disorder or randomness of the particles of a system. • Law of Disorder: spontaneous processes always occur in the direction that increases the chaos of the universe. Unmelt.mov Unmix.mov Unshatr.mov Chaos • The more degrees of freedom a system has, the more chaotic it can be. • Degrees of freedom = ways you can move & places you can be. • Gases have much more entropy than liquids & liquids have more than solids. Entropy = 0? • A pure crystal with no imperfections at 0K. • Every atom is where it’s supposed to be. • Nothing is moving. Changes in Entropy, S • S = Sfinal – Sinitial or Sproducts – Sreactants • Nature wants to increase S, so Sfinal > Sinitial • Nature wants S to be positive. Predicting Changes in Entropy • Sgas > Sliquid > Ssolid • SMixture > SPure Substance • SDissolved Solid > SSolid • Sgas > SDissolved Gas but Predicting Entropy • Which has more entropy, 1 mole of dry ice or 1 mole of CO2 gas? • 1 mole of CO2 gas Predicting Changes in Entropy • 2SO3(g) 2SO2(g) + O2(g) S > 0 • 2 moles of gas on the reactant side. • 3 moles of gas on the product side. • Plus, the product side is a mixture. Temperature & Entropy • An increase in temperature increases the random motion of the particles, so entropy increases with temperature. Predicting Spontaneity • Use Gibbs free energy expression • G = H - TS • If G is negative, rxn is spontaneous. • If G is positive, rxn is not spontaneous. G = H + (-TS) H S -TS G + + - ? + _ + - + - - - + G = f(T) Never NOT negative spont. Always SPONT. negative ? G = f(T) G = H - TS • H is negative; S is positive. Always spontaneous. • H is negative; S is negative. Spontaneity depends on temperature. • H is positive; S is positive. Spontaneity depends on temperature. • H is positive; S is negative. Never spontaneous. G = H - TS G is negative for spontaneous processes. S is neg. in chaos. S is pos. In chaos. H is neg. in energy. Can’t tell. G = f(T). H is pos. In energy. G > 0. Never spontaneous G < 0. Always spontaneous Can’t tell. G = f(T). Summary • Nature has 2 driving forces • Tends to minimize enthalpy (potential energy). Wants H to be negative. • Tends to maximize entropy (chaos). Wants S to be positive. Summary • Spontaneity is determined by the combination of the enthalpy change and the entropy change. Predict the sign of G • LiBr(s) Li+(aq) + Br-(aq) + 48.83 kJ • The reaction is exothermic so H is negative. The chaos increases so S is positive. Both enthalpy and entropy are going in the direction preferred by nature. This reaction is always SPONTANEOUS. Predict the sign of G • N2(g) + 2 O2(g) + 66.4 kJ 2 NO2(g) • The reaction is endothermic so H is positive. The chaos decreases (3 moles of a gas to 2 moles of a gas) so S is negative. Neither the enthalpy nor the entropy goes in the direction that nature prefers, so this reaction is NEVER spontaneous. Predict the sign of G • 2 H2(g) + O2(g) 2 H2O(l) + 571.6 kJ • • • • • The reaction is exothermic: H is negative. Chaos decreases: S is negative. One driving force, enthalpy, is with nature. Entropy is going against nature. CANNOT predict the spontaneity of this rxn.