Thermodynamics - Biochemistry - Lecture Slides, Slides of Biochemistry

Download Thermodynamics - Biochemistry - Lecture Slides and more Slides Biochemistry in PDF only on Docsity! Thermodynamics and chemical equilibria • Lecture 2 1/15/2003 • Chapter 1 Voet, Voet and Pratt Docsity.com Thermodynamics: Allows a prediction as to the spontaneous nature of a chemical reaction: Will this reaction proceed in a forward direction as the reaction is written: A + B C Will A react with B to form C or not? Is this reaction going from higher to lower energy? ENERGY not necessarily just heat!!! Docsity.com Direction of heat flow by definition is most important. q = heat absorbed by the system from surroundings If q is positive reaction is endothermic system absorbs heat from surroundings If q is negative exothermic system gives off heat. A negative W is work done by the system on the surroundings i.e expansion of a gas Docsity.com First Law of Thermodynamics: Energy is Conserved Energy is neither created or destroyed. In a chemical reaction, all the energy must be accounted for. Equivalence of work w and energy (heat) q Work (w) is defined as w = F x D (organized motion) Heat (q) is a reflection of random molecular motions (heat) Docsity.com U = Ufinal - Uinitial = q - w Exothermic system releases heat = -q Endothermic system gains heat = +q U = a state function dependent on the current properties only. U is path independent while q and w are not state functions because they can be converted from one form of energy to the other. (excluding other forms of energy, e.g. electrical, light and nuclear energy, from this discussion.) Docsity.com Enthalpy (H) When considering only pressure/volume work H = qp - PV + PV = qp H = qp when other work is 0 qp is heat transferred at constant pressure. In biological systems the differences between U and H are negligible (e.g. volume changes) Docsity.com The change in enthalpy in any hypothetical reaction pathway can be determined from the enthalpy change in any other reaction pathway between the same products and reactants. This is a calorie (joule) “bean counting” Hot Cold Which way does heat travel? This directionality, is not mentioned in First Law Docsity.com Other examples Considering the random thermal motion of molecules, why at any particular time doesn‟t all the atoms in the air in this room end up into the upper right corner of the room? or When a driver jumps into the water, energy from that person is dissipated to the water molecules. Have you ever seen the water molecules act in unison and retransfer this energy back to the driver and push him out of the water? In all cases Energy is conserved. Docsity.com N = 4 particles, 2 orientations or 24 = 16 different arrangements of distinguishable particles 1 { 4 { 6* { 4 { 1 { } N+1 different states * The most probable state is the one with the most arrangements Docsity.com The state that is the most probable is one with the highest value of WL= N/2 in one bulb and N/2 in the other. The probability that L is equal to N/2 increases when N is large For N=10 the probability that L lies within 20% of N/2 = .66 N=50 probability L lies within 20% of N/2 = .88 N=1023 , L = 1 For a mole of gas in the two bulbs 6.0221 x 1023 The probability of a ratio of 1 in 1010 (one in ten billion) difference or 10,000,000,000 vs. 10,000,000,001 on one side to the other is 10-434 = 0 Docsity.com The reason that direction occurs is not by the laws of motion, but the aggregate probability of all other states is So low!! or insignificant that only the most probable state will occur. Entropy for N = 1023 This number is greater than the number of atoms in the universe!! NLn2 2 N 10W  22 22 107 5x10 10W x Possible arrangements Docsity.com Entropy is the arrow of time The entropy of the universe is always increasing. What does this mean in a closed system? What does this mean in an open system? What if the universe starts to collapse, does time go backwards? Docsity.com It is difficult or (impossible) to count the number of arrangements or the most probable state! So how do we measure entropy? It takes 80 kcal/mol of heat to change ice at zero °C to water at zero °C 80,000 = 293 ev‟s or entropy units 273 A Reversible process means at equilibrium during the change. This is impossible but makes the calculations easier but for irreversible process  final inital T dq S T q S  Docsity.com At constant pressure we have changes in qp (Enthalpy) and changes in order - disorder (Entropy) A spontaneous process gives up energy and becomes more disordered. G = H - TS Describes the total usable energy of a system A change from one state to another produces: G = H - TS = qp - TS If G is negative, the process is spontaneous Docsity.com Now if we are at equilibrium or G = 0 Then ba dc o [B][A] [D][C] RTln G 0G  ba dc o [B][A] [D][C] RTln G  eq o RTlnKG  So what does Go really mean? OR Docsity.com Keq Go G ba dc o [B][A] [D][C] RTln G G  If Keq = 1 then G = 0 Go equates to how far Keq varies from 1!! Docsity.com Keq can vary from 10 6 to 10-6 or more!!! Go is a method to calculate two reactions whose Keq’s are different However The initial products and reactants maybe far from their equilibrium concentrations so ba dc [B][A] [D][C] Must be used Docsity.com Keq G o (kJ·mole-1 106 -34.3 104 -22.8 102 -11.4 101 -5.7 100 0.0 10-1 5.7 10-2 11.4 10-4 22.8 10-6 34.3 The Variation of Keq with G o at 25 oC Docsity.com   )(reactantsG - (products)G G of o f o The f = for formation. By convention, the free energy of the elements is taken as zero at 25 oC The free energies of any compound can be measured as a sum of components from the free energies of formation. Docsity.com Standard State for Biochemistry Unit Activity 25 oC pH = 7.0 (not 0, as used in chemistry) [H2O] is taken as 1, however, if water is in the Keq equation then [H2O] = 55.5 eqK oG G The prime indicates Biochemical standard state Docsity.com Coupled Reactions A + B C + D G1 (1) D + E F +G G2 (2) 0 G1 If reaction 1 will not occur as written. However, if 2G is sufficiently exergonic so 0 G G 21  Then the combined reactions will be favorable through the common intermediate D A + B + E C + F + G G3 0 G GG 213  Docsity.com As long as the overall pathway is exergonic, it will operate in a forward manner. Thus, the free energy of ATP hydrolysis, a highly exergonic reaction, is harnessed to drive many otherwise endergonic biological processes to completion!! Docsity.com Life obeys the Laws of Thermodynamics • Living organisms are open systems • Living things maintain a steady state • Enzymes catalyze biological reactions Docsity.com