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, CH 4, in the bunsen burner. CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  H = -891 kJ Iron rusting 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)  H = kJ

Some Spontaneous Processes are Endothermic Dissolving NH 4 NO 3 (s) in water is an example of a spontaneous endothermic process. NH 4 NO 3 (s)  NH 4 + (aq) + NO 3 - (aq) Ice melting is another spontaneous, endothermic process. H 2 O(s)  H 2 O(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 = S final – S initial or S products – S reactants Nature wants to increase S, so S final > S initial Nature wants  S to be positive.

Predicting Changes in Entropy S gas > S liquid > S solid S Mixture > S Pure Substance S Dissolved Solid > S Solid but S gas > S Dissolved Gas

Predicting Entropy Which has more entropy, 1 mole of dry ice or 1 mole of CO 2 gas? 1 mole of CO 2 gas

Predicting Changes in Entropy 2SO 3 (g)  2SO 2 (g) + O 2 (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 - T  S If  G is negative, rxn is spontaneous. If  G is positive, rxn is not spontaneous.

 G =  H + (-T  S) HH SS-T  S GG ++-?  G = f(T) +_+Never negative NOT spont. -+-Always negative SPONT. --+?  G = f(T)

 G =  H - T  S  H is negative;  S is positive.  H is negative;  S is negative.  H is positive;  S is positive.  H is positive;  S is negative. Always spontaneous. Never spontaneous. Spontaneity depends on temperature.

 S is neg.  in chaos.  S is pos.  In chaos.  H is neg.  in energy. Can’t tell.  G = f(T).  G < 0. Always spontaneous  H is pos.  In energy.  G > 0. Never spontaneous Can’t tell.  G = f(T).  G =  H - T  S  G is negative for spontaneous processes.

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) 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 N 2 (g) + 2 O 2 (g) kJ  2 NO 2 (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 H 2 (g) + O 2 (g)  2 H 2 O(l) 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.