1 Enthaply vs. entropy in chemical reaction: S solid < S liquid - PDF document

There are two fundamental tendencies in nature that help determine whether a reaction will occur: Topic 9.4 1. Enthalpy Enthalpy, entropy and spontaneity •Energy change during reaction = heat of reaction ( Δ H) •Reactions tend toward lower energy state = exothermic On the basis of energy alone, we expect reactions to go in the exothermic direction 2. Entropy spontaneous Entropy tends to ↑ 2. Entropy •A measure of disorder • Symbolized by ‘S’ nonspontaneous + Δ S means ↑ in disorder - Δ S means ↓ in disorder Entropy tends to ↓ On the basis of entropy alone, we expect reactions to go in the direction of greater randomness 1

Enthaply vs. entropy in chemical reaction: S solid < S liquid << S gas • Spontaneous reactions favored by : • ↑ Entropy = More disordered ↓ or ↑ entropy reaction • ↓ Enthalpy = Heat released from reaction A ( s ) � A ( l ) Increases Four different scenarios: + Δ S 1. Δ H is (-): Exothermic, favorable spontaneous Decrease A ( g ) � A ( s ) Δ S is (+): More disordered, favorable - Δ S Endothermic, unfavorable 2. Δ H is (+): A ( g ) + B ( g ) � C ( g ) Decrease nonspontaneous Δ S is (-): Less disordered, unfavorable - Δ S favorable 3. Δ H is (-): 2 A ( g ) + 3 B ( s ) � 3 C ( g ) + 2 D ( s ) Increases ? + Δ S Δ S is (-): unfavorable NaCl ( s ) � Na + ( aq ) + Cl � Increases 4. Δ H is (+): unfavorable ( aq ) ? + Δ S Δ S is (+): favorable For situations 3 and 4 use Gibb’s Free Energy ( Δ G ): Will the following reactions be spontaneous or a measure of how spontaneous a reaction is nonspontaneous? Δ G = Δ H − T Δ S Free E Entropy � S = + � S = change 1) 2X (g) � Y (g) + 2Z (g) 2 to 3 moles Heat of Temp. gas Reaction in K � H = - � H = exothermic If Δ G is (–), spontaneous Δ G = Δ H − T Δ S – – –(+) If Δ G is (+), nonspontaneous If Δ G is = 0, reaction is at equilibrium spontaneous � S = + � S = 3) Y (s) � A - (aq) + B + (aq) 1 solid to 2 ions � H = + � H = = - endothermic � S = X (g) + Z (g) � Y (g) + U (s) 2 gas to 1 gas 2) and 1 solid � H = = + Δ G = Δ H − T Δ S endothermic + –(+) – Δ G = Δ H − T Δ S + + –(–) - or + depending on T Δ G = + � H > T � S At low T: Δ G = (+), nonspontaneous nonspontaneous At high T: Δ G = (–), spontaneous T � S > � H 2

The stability of compounds can be determined by comparing heats of formation values = - � S = Δ H f 4) X (g) + X (g) � X 2(g) 2 to 1 moles gas C 2 H 2 +227 Most unstable - � H = exothermic C 2 H 4 +52 Bond forming is always exothermic C 2 H 6 -84 Most stable Δ G = Δ H − T Δ S –(–) – • The more endothermic the heat of formation, the + more unstable a compound is. Spontaneous at low T Nonpontaneous at high T • The more exothermic, the more stable 3