Ch 19 & 20 (Ch 19 Spontaneous Change Entropy and Free Energy, Ch 20 Electrochemistry)

Ch 19 & 20 (Ch 19 Spontaneous Change Entropy and Free Energy, Ch 20 Electrochemistry)

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1 Ch. 19: Spontaneous Change Entropy & Free Energy Themes: Life-Sustaining Spontaneity Air Bags, Refrigeration & Coupled Reactions 1 Spontaneous Processes Rusting 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) forward process is spontaneous, the reverse is non-spontaneous 2 recall Extraction of Fe(s) from Iron Ore (steelmaking, blast furnace) non-spontaneous! Melting: H 2 O(s) H 2 O(l) - spontaneous at RT Refer to Ch. 19-1, pg 780-782 Spontaneity & H • Although many exothermic reactions are spontaneous, there is NOT a direct correlation between H and spontaneity • Thermite exothermic & spontaneous 3 Fe 2 O 3 (s) + 2Al(s) 2Fe(l) + Al 2 O 3 (s) • Cold Packs endothermic & spontaneous NH 4 NO 3 (s) NH 4 + (aq) + NO 3 (aq) • What criteria do we have for determining spontaneity? 2 () HOl  Refer to Ch. 19-1, pg 780-782 Air Bags • Chemistry of Sodium Azide: NaN 3 • Impact detector detonates – timescale = 50ms NaN 3 Na(s) + 3/2N 2 (g) 4 130g Sodium Azide produces 73L N 2 gas! What happens to Na metal? #&$@%^*#$ … Now What? Air bag contents (NaN 3 + KNO 3 + SiO 2 ) + coffee in your lap… NaN (s) Na(s) + 3/2N (g 5 NaN 3 (s) Na(s) + 3/2N 2 (g) Na(s) + KNO 3 mixture of Na 2 O + K 2 O + N 2 (g) Metal oxides are still very reactive final step involves silicates Na 2 O + K 2 O + SiO 2 (s) alkaline glass Entropy Change ( S) • Gas expansion or mixing causes an increase in entropy: A(g) + B(g) mixture of A & B 6 S = S mix of gases –( S A(g) + S B(g ) ) > 0 rev q S T  Refer to Ch. 19-2, pg 782-787 Like H, S is a state function S is an extensive property
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2 Entropy • The greater the number of configurations, the greater the entropy of the system 7 Refer to Ch. 19-2, pg 782-787 Entropy of Phase Changes Melting: S liquid > S solid Solid Liquid tr tr H S  8 Refer to Ch. 19-3, pg 787-791 Liquid Vapor Vaporization: S vapour > S liquid tr T tr = transition Entropy of Phase Changes Molar entropy increases with increasing temperature CH 3 Cl 9 rev q S T Refer to Fig 19-7 Entropy & Molecular Structure Propane Ethane Methane 10 Molar entropy increases • with increasing molecular complexity • i.e., with greater number of vibrational and rotational degrees of freedom available to the molecule (recall heat capacity) S o = 270.3 J mol -1 K -1 S o = 229.6 J mol -1 K -1 S o = 186.3 J mol -1 K -1 Refer to Ch. 19-3, pg 787-791 Entropy of Dissolution 11 • Recall: The endothermic, yet spontaneous dissolution of NH 4 NO 3 in water – Cold Packs Solute Solvent Solution Dissolving: S soln > (S solvent + S solute ) Refer to Figure 19-4 iclicker - Goward: Increasing Entropy Which of the following processes lead to a decrease in entropy? S< 0 a) Melting of a pure solid to form a pure liquid 12 b) Dissolving a salt into solution c) Decreasing number of moles of gas in a chemical reaction ( n gases (products) - n gases (reactants) < 0 d) Increasing the temperature of a substance Refer to Ch. 19-3, pg 787-791
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3 iclicker - Hatala: Increasing Entropy Which of the following processes lead to the greatest increase in entropy?
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Ch 19 &amp; 20 (Ch 19 Spontaneous Change Entropy and Free Energy, Ch 20 Electrochemistry)

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