Ch Three (Segel) Notes - Thermodynamics PDF

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Chem 161 - General Biochemistry w/ Fairclough
Lecture on thermodynamics, coupled reactions, enthalpy, and entropy. ...

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Chapter Three Notes Biochemical Energetics The Law of Thermodynamics  first law of thermodynamics – energy can neither be created nor destroyed o one form of energy may be converted into another but the total energy of the system plus its surroundings remains constant o conservation of energy  second law of thermodynamics – all naturally occurring processes proceed in a direction that leads to minimum potential energy level (equilibrium) o spontaneous reactions release energy as they progress toward equilibrium and the energy can be harnessed and made to do work  spontaneous reactions:  heat flows from a warm body into a cooler body (never the opposite)  a wound spring unwinds (unwound doesn’t wind itself)  water flows downhill (never the opposite)  gases diffuse from a region of high pressure and concentration to a region of low pressure and concentration o energy is conserved  heat lost by the warm body is gained by the cooler body  what has been “lost” is the capacity or potential to do more work (transfer more energy) o the entropy (S) of the universe is constantly increasing  entropy: a measure of randomness or orderliness of the energy and matter in a system  the more random, disordered, disorganized, or chaotic the system, the higher the entropy, and vice versa  only organized, nonrandom energy is useful (can be made to do work)  increase in entropy represents a loss of organization and a decrease in the potential to do further work  third law of thermodynamics – at a temperature of absolute zero (0°K), where all random motion ceases, the entropy of a perfect crystal of every substance is zero; all atoms are maximally ordered  thermodynamic functions of state: o the change in free energy  measure of the maximum useful work that a reaction could perform at constant temperature and pressure  depends on the displacement of the system from equilibrium o the change in enthalpy (heat content)  measure of the heat flow that accompanies a reaction as it proceeds toward equilibrium at constant temperature, pressure, and volume

Coupled Reactions  chemical reactions may be exergonic or endergonic o exergonic: yields energy (capable of doing work) o endergonic: utilize/requires energy (work must be done to make the reaction happen)  coupled reactions o exergonic reactions are catalyzed and some of the energy is trapped in “energy-rich” compounds, which then drive endergonic reactions o living cells maintain their integrity over long periods of time, grow, and multiply, by coupled reactions o adequately illustrates the principle of energy conservation and utilization in living cells o actual mechanisms seldom involve the simultaneous catalysis of two reactions – the net effect is generally obtained by catalyzing two consecutive reactions involving a commonplace intermediate  ex. an endergonic reaction is supplied with energy by the hydrolysis of ATP; unused energy is conserved through the ATP supplying a phosphate group to ADP, an exergonic reaction supplies more energy while simultaneously forming ATP, repeat…(?)  the potential energy is used to transfer a portion of the energy-rich molecule to an acceptor that “activates” the acceptor o ATP supplies energy and donates phosphate, pyrophosphate, AMP, and adenosine o energy values are additive - ∆G unknowns can be calculated if the reaction in question can be expressed as the sum of two or more reactions whose ∆G values are known

Free Energy Change (∆G)  free energy difference or Gibbs free energy exchange (∆G) – the difference between the energy contents of the products and the reactants in a chemical reaction (energy released or utilized) o maximum potential of a reaction for performing useful work o ∆G = Gproducts – Greactants o exergonic reactions have negative ∆G values o endergonic reactions have positive ∆G values

Relationship Between ∆G and the  

S} ¿ [P] ¿

Ratio

S is the reactant (substrate) and P is the product in a reaction, S is converted to P until equilibrium is attained o the energy released depends on how far from equilibrium the original S/P ratio is  the greater the ratio, the more work that can be done  an S/P ratio at the start of a reaction is greater than the ratio at equilibrium; the reaction proceeds “spontaneously” (∆G is negative; S → P)  the S/P ratio at the end of the reaction equals the equilibrium ratio; no further net reaction occurs (P/S = Keq, ∆G = 0) o the mathematical state of the ∆G of a reaction must contain two terms: one that indicates the concentrations of the substrates and products and one that states the equilibrium concentrations

o



 R = gas constant = 8.315 x 10-3 kJ/mol  T = absolute temperature (K)  [P], [S] = molar concentrations of the reactants and substrates  a, b, c, etc = the coefficients of P, S, in the balanced chemical equation ∆G under standard state conditions (all reactants and products are considered to be maintained at steady-state concentrations of 1M) is designated as ∆G° o both the ∆G° and Keq values impart the same information – which direction and how far a reaction will proceed when all substrates and products are 1 M o ∆G < 0 (negative) means the reaction will proceed from left to right toward a state of minimum energy (“spontaneous”) o ∆G > 0 (positive) means the reaction will proceed from right to left o



∆G = -RT ln Keq + RT ln

P2 ¿b ¿ d S2 ¿ S 1 ¿c ¿ ¿ a P1 ¿ ¿ ¿ ¿

∆G = 0 means the system is at equilibrium and Keq =

P S

o the magnitude will tell us how far to the right the reaction will go ∆G says nothing about the rate at which the reaction will approach equilibrium, only the difference between the free energy contents of the products and reactants (will be the same regardless of the path taken or the number of steps involved)

Effect of Non-Standard [H+]  if the H+ ion appears as a substrate or product, a modified standard-state in which all substrates and products except H+ are considered to be 1M  the H+ ion concentration is taken to be 10-7 M  Keq is designated as Keq’ and ∆G° is designated as ∆G°’ o Keq’ is the equilibrium constant of the reaction at the specified pH 7 b



∆G = ∆G°’ + RT ln

o

P2 ¿ ¿ d S2 ¿ c S1 ¿ ¿ ¿ a P1 ¿ ¿ ¿ ¿

if Keq’ or ∆G°’ is specified, there is no need to include H+ or pH in the calculations unless ∆G°’ is being calculated at another pH

Enthalpy and Entropy



the first and second laws of thermodynamics relate the ∆G of a reaction to the heat evolved in the following way: o ∆G = ∆H - T∆S  ∆H is the enthalpy change – represents the quantity of heat released or absorbed at constant temperature, pressure, and volume  ∆S is the entropy change – measure of the change in the randomness of the system o ∆H = ∆G + T∆S o



∆S =

∆ H −∆G T

enthalpy and entropy, like free energy, are functions of state o depend only on the initial and final states of the system and not the mechanism or path of the reaction...