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chap-19-2011W-b

# Nh4no3s nh4no3aq g rxn h rxn tsrxn 257 2730109

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Unformatted text preview: l-K J/molNH4NO3(aq):∆Hf° = –339.9 kJ/mole ;S° = 259.8 J/mol-K J/molPredict whether the reaction is spontaneous at this T ∆ rG = ∆ r H − T ∆ r S ∆Hrxn° = –339.9 – (–365.6) = + 25.7 kJ/mol ∆Srxn° = 259.8– (151.1) = 108.7 J/mol =0.1087 kJ/mole 259.8– ∆Grxn° = ∆Hrxn° - T ∆Srxn° = (+ 25.7) – (298)×(0.109) = - 6.7 kJ/mole < 0 Spontaneous Assuming ∆H° and ∆S° do not change with temp, (b) determine whether the following reaction is spontaneous at 0°C. NH4NO3(s) → NH4NO3(aq). ∆G rxn =∆H rxn - T∆Srxn = ( 25.7) - (273)(0.109) = - 4.1 kJ/mole Spontaneous at 0 ºC (c) determine the temperature range at which the reaction is non-spontaneous non∆G rxn =∆H o rxn - T∆So rxn = 0 at equilibrium Assuming ∆H° and ∆S° do not change with temp T = ∆H ∆S = 236 K = - 37 °C ° ° ∆G = ∆H − T ∆S ° ∆H o > 0 ∆So > 0 Lower than this temp. it will be Non-spontaneous Non- “entropy-driven” reaction entropy- All such reactions will become spontaneous at relative high temp. relative Free Energy (∆G) and Temperature (T) When ∆H < 0 : ∆S > 0 : ∆S < 0 : Enthalpy-driven process When ∆H > 0 : ∆S > 0 : EntropyEntropy-driven process ∆S < 0 : ∆G = ∆H − T ∆S spontaneous at all temperatures, all both factors favor spontaneity Spontaneous when T is LOW enough is Enthalpy factor favors spontaneity Spontaneous when T is HIGH enough is Entropy factor favors spontaneity NonNon-spontaneous at all temp., both factors favor non-spontaneity non- Equilibrium Equilibrium constant (K) The magnitude of Equilibrium : R eaction end point K valume Equilibrium position K valume → 10-5 Reaction Hardly occurs 10-3 1 103 Reactants & products Co-exist in equilibrium Co- 105 Goes to completion Reaction Reaction Quotient, Q aA + bB kf kr cC + dD (C / c ) c ⋅ ( D / c ) d Q= ( A / c )a ⋅ ( B / c )b from Q < K system proceeds from left to right to reach equilibrium high high reactant concentration Q = K the system is at equilibrium at all concentrations remain constant Q > K system proceeds from right to left to reach equilibrium from high product concentration Free Free Energy (∆G) and equilibrium constant (K) equilibrium Most...
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