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Chapter 19

Lecture and Textbook Collaborated Notes - Chapter 19 - CHEM 1A03

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Aadil Merali Juma

Chem 1A03 Chapter 19: Spontaneous Change: Entropy and Gibbs Energy 19.1 Spontaneity: The Meaning of Spontaneous Change Spontaneous Processes  Rusting o 4 Fe + 3 O  2 Fe O (s) 2(g) 2 3(s) o Forward process is spontaneous o Fe O2 3(s)will not decompose to Fe(s)d O 2(g)  The reverse process is non-spontaneous  Making Fe (s)omFe O2 3(s)equires carbon  The oxygen is removed as CO 2(g)  Melting o H O  H O 2 (s) 2 (l) o Spontaneous above 0°C Conclusions about Spontaneity  If a process is spontaneous, the reverse process is nonspontaneous  Both spontaneous and nonspontaneous processes are possible, but only spontaneous processes will occur without intervention o Non spontaneous processes require the system to be acted on by an external agent  Some spontaneous processes occur very slowly and others occur rapidly Spontaneity and ΔH  Although many exothermic reactions are spontaneous, there is not a direct correlation between ΔH and spontaneity  Thermite o Exothermic and spontaneous o Fe O2 3(s)+ 2 Al(s)  2Fe(l)Al 2 3(s)  Cold Packs o Endothermic and spontaneous  NH 4O 3(s) NH 4 (aq) NO 3 (aq)  1870; P. Berthelot and J. Thomsen proposed that the direction of spontaneous change is the direction in which the enthalpy of a system decreases (exothermic)  incorrect 19.2 The Concept of Entropy Entropy (S): The Boltzmann Equation  Two Marbles  Confined to 1 compartment o W = 9 x 1 = 9  Different compartments o W = 9x 8 = 72 Chem 1A03  No restriction o W = 9 x 9 = 81  S = klnW o S = Entropy o k = Boltzmann Constant o W = number of “microstates”  the number of ways that the particle can be positioned in the states available and still give rise to the same total energy  Associate the number of energy levels in the system with the number of ways of arranging the particles in these energy levels o Microscopic levels  states o Particular way a number of particles is distributed among these states  microstates  More states more microstates  greater entropy Entropy  The greater the number of configurations (or microstates) consistent with “macrostate”, the greater the entropy of the system Entropy Change (ΔS)  Gas expansion or mixing causes an increase in entropy: o A(g) + B(g)  mixture of A & B o ΔS = S – (S + S ) > 0 A&Bmixed A(g) B(g) o ΔS = q rev T o Like H, S is state function; S is an extensive property  ΔS is directly proportional to the quantity of heat; the more energy added to a system (as heat) the greater the number of energy levels available to the microscopic particles  ΔS is inversely proportional to the Kelvin temperature; raising the temperature increase the availability of energy levels, but for a given quantity of heat the proportional increases in number of energy levels is greatest at lower temperatures Entropy Increases, ΔS > 0 When  Solid  Liquid  Solids or Liquids  Gas  Number of molecules of gas increases as a result of a chemical reaction  The temperature of a substance increases o Increased temperature means increased number of accessible energy levels for the increases molecular motion) Chem 1A03 19.3 Evaluating Entropy and Entropy Changes  ΔS =trH /T tr tr o tr – transition; can be replaced by fus (fusion) or vap (vaporization) etc Absolute Entropies  To establish an absolute value of the entropy of a substance, we look for the zero-point energy ; a condition in which the substance is in its lowest possible energy state o The evaluate entropy changes as the substance is brought to other conditions of temperature and pressure o Add together entropy changes and obtain a numerical value of the absolute entropy o Entropy of Phase Changes Calculating ΔS°, standard molar entropy  Third Law of Thermodynamics o The entropy of a pure, perfect crystal at 0K is zero  We have absolute entropies  ΔS° = Σν Δp°(products) – Σν ΔS°rreactants) o Σ = sum o ν = stoichiometric coefficients o Calculations follow strategies used for ΔH° (Ch 7) Entropy and Molecular Structure  Molecular entropy increases o With increasing molecular complexity  Ie. With greater number of vibrational and rotational degrees of freedom available to the molecule (recall – heat capacity) o For molecules of the same complexity, the more massive the molecule the greater the entropy  Eg/ S (CH 3r) > S (CH 3l) Entropy of Dissolution  Recall: the endothermic yet spontaneous dissolution of NH NO i4 3 water – Cold Packs Chem 1A03 Air Bags: Entropy Increase  Chemistry of Sodium Azide: NaN 3  Impact detector detonates – timescale = 50ms  Air Bag contents: NaN , K3O SiO3, 2  1) NaN  Na + 3/2 N 3 (s) 2(g) o N g2s  130 g Sodium Azide produ
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