Lecture Notes on Thermodynamics - Definition, Equations | CHGN 124, Study notes of Chemistry

Download Lecture Notes on Thermodynamics - Definition, Equations | CHGN 124 and more Study notes Chemistry in PDF only on Docsity! ~ 'I THERMODYNAMICS • DEFINITIONS 0.. • Spontaneous Processes - A process that occurs without outside help. Spontaneous Nonspontaneous Pants fall to floor Pants hang themselves in closet Raw egg becomes hard boiled in water Hard-boiled egg becomes raw egg Reversible Processes b. - A reversible process is a process that is always in equilibrium. Example Ice melting at 0 cC - Equilibrium is adjusted by adding or subtracting heat. - Water can go from solid to liquid to solid to liquid etc... .. .~ G· Irreversible Processes .- A process not in equilibrium Example Ice melting at 25 cC - Process only happens in one direction d. Jollie Calorie - 1 cal = 4.184 J (exact)(new definition of cal.) -defined as - old definition ofcalorie: energy needed to raise the temperature :of 1 gofHzO 1 °C .. 1 2 - aside: 1 dietary calorie = 1 Cal = 1kcal-KE'=2'mv - SI unit e.Heat-q =m.e.boT - choatic molecular motion --reae ~ ~r e - heating increases (or decreases) molecular motion in all directions +q = heat gained by system - q =heat lost by system • Work-w - concerted motion - Work = Force x distance => w =F x d - movement against force is work - work increases (or decreases) molecular mojo . a specific direction - in gases, work is related to changes in volUIIl~ f tf5boV) ~ ,/ ..... :; ~ ~ ..;;..- - I d'()... +w = work done on system (compression) - w =work done by system (expansion) Internal Energy is the sum ofkinetic ~d potential energy in a thermodynamic system. . ~e.&c_E..fI.~rgy - energy ofmovement - example - thermal energy (molecules moving) - K.E. = ~~ to~zE'!Lgp.~~ - energy stored that is released through motion - example - ball in air 11. FIRST LAW OF THERMODYNAMICS - The change in the energy of a system is due to heat and/or work. ItJ3, = q + w II q represents heat and w represents work. In other words, the sum of the kinetic energy change and the potential energy change of a system is equal to the heat and the work. ­ (a), the gas is confined to the right half of the cylinder by a partition. When the partition is removed (b), the gas spontaneously (irreversibly) expands to fill the whole cylinder. No work is done by the system dUring this expansion. In (c), we can use the piston to compress the gas back to its original state. Doing so requires that the Vacuum Gas surroundinqs do work on the (a) ,(b) (c) system • Note that pressure is a force divided b ~ to define ~£.r~ (w) as w = - P 1:1V • lWtha'!pl.at constant pressure is the measurement of.h,!l~: .qp =AH= mx.CXAiJ Movable partition Piston For an expanding gas, 1:1 V is a positive quantity because the volume is increasing. Thus 1:1 V and w must have opposite signs, so we have a negative sign in front of our equation, because it is doing work on our surroundings. an area and that the change in volume is a distance. These can be Elements are not included in the calculation because elements require no change in form. • BESS:J..-AV:!. The total heat of reaction can be written as a sum of "mini-reaetions". @ ISlibstance . SO,J/mol-K Gases H:z(g) 130.6 N2(g) 191.5 02(g) 205.0 • Unlike enthalpiesof formation, the standard molar entropies of elements H2O(g) 188.8 NH3(g) 192.5are not zero. CHsOH(g) 237.6# The standard molar entropies genercilly increase with increasing molar mass CJf6(g) 269.2of the substance. -Liquids • The standard molar entropies generally increase with the number of atoms \ H2O(l) 69.9in th~ formula ~f the ~ub~tance. ~_ the number and importance of the vibrational \ CH3OH(l) 126.8degrees of freedom of molecules increase with increasing mass and increasing! C6~(l) 172.8number of atoms. / Solids Fe(s) 27.2 Fe03(s) 142.3 NaC1(s) 72.3 \oJ. THIRD LAW OF THERMODYNAMICS As less energy is stored, the entropy of the system decreases. If we keep lowering the temperature, cia we reach a state in which these motions are essentially shutdo"YIl, a point of perfect order? The entropy of a perfect crystal is zero at absolute zero (0 K). Consequences of the Third Law - Absolute zero is unattainable. . - Entropy of all substances at absolute zero is zero. - At temperature above zero, crystal will not be petfect - Vibrational motion introduces imperfections " To remove imperfection takes some sort ofmotion Reconsider 2nd Law of Thermo. Using the thermodynamic definition of entropy, the entropy change of the surroundings can be related to the heat of the system. ,6,S = qsurr = - qsys SUIT T T At constant pressure: qsys = LlHsys Therefore the second law can be rewritten as • LlHsys ~ ---~o ~ T~S-MI~O sys T II => ~H-T~S ~O ~ ~G ~ 0At cODSbml temperature: IlJ,G ~ Lfl-T til ~G is useful to decide ifa reaction occurs IJ,G < 0 rxn is spontaneous IJ,G =0 system is at equilibrimn Recall ~Suniv = 0 for reversible process IJ,G > 0 rxn is nonspontaneous; i.e., rxn does not happen ---- ------ Note: ..6J-I: is the change of energy that occurs in a chemical reaction = difference between the ~ energy required to break the chemical bonds in the reactants and the energy produced by the formation of the chemical bonds of the products. - 6G : it is the portion of the energy change ofa spontaneous reaction that is free to do useful work. The remainder of the energy enters the environment as heat. (T tJ. S sys = q sys ) _ Temperature dependence of free energy can be summarized in table AlI /1S /1G - + - spontaneous - - - or+ spontaneous below specific temp. + + - or+ sPontaneous above specific temp. + - + never spontaneous Standard Free-Energy Changes . State Standard of Matter State Solid Pure solid ,~GO = 2: n~GJ (products) :- 2: m~GJ (reactants) . Liquid Pure liquid Gas 1 atm pressure Solution 1 M cOIicentration For a general reaction: a A + b B !::; c C + d D ; We define the reaction quotient Q = [cJt. rD]! [AJD-(BJ" Reaction 6G direction of Rx Q/K In (QIK) ~l ---­ Spontaneous make more product <1 Non spontaneous make more reactants >1 >0r~ Equilibrium 1__-0 I , no change =1 =0 ---' • There is a direct relationship between AG and In (QIK) D.G= RT In(Q /)G= RT InQ - RT InK .4? - A}' h. (,?f.J ;JL' ') (0) We generally refer to reactions where reactants and products are in their standard states (cc= 1m; P= latm) L1 17°;:. A 6 f- R-r L ! 11 a) 6. GO = RT In 1 - RTInK = Q - RT InK = J::..Go = - RT In K \ ..makes it possible to calculate K knowing GO. \. ."Makes it possible to calculate GO knowing K. - K >1 means 00° <0 : the reaction starting with reactants and products in their standard states is Spontaneous to make more products. .. K <1 means~Go >0: the reaction starting with reactants and products in their standard states is Non spontaneous to make more products. Equilibrium mixture Spontaneous .v bG < 0 If a mixture has too much N2 and H2 relative to NH3J the equilibrium.lies too far to the left (Q < K) and tile mixture will react to formNH3 spontaneously. If there is too much NH3 in the mixture, the equilibrium lies to the right (Q > K) and the NH3 will decompose .spontaneously .into N2 and H2• At equilibrium Q =K and the free energy is at a minimum (6.G = 0). Pure NH3 , ]c:; (Q = K,6.G.= 0) / 1erJc9!/ ;