Thermodynamics Kinetics and Equilibrium Dr Mullins 1 How We Study Biochemical Processes Thermodynamics predicts if the process is possible spontaneous nonspontaneous Reactions Favorable or non favorable reactions Kinetics expresses how fast the process will occur Equilibrium How far the reaction will go 2 Equilibrium State in which forward and reverse reactions occur at the same rate Concentration of reactants and products remain constant over time Dynamic Equilibrium All chemical reactions proceed until they reach equilibrium The equilibrium level for a reaction is intrinsic to that specific reaction Le Chatelier s Principal Any stress placed on an equilibrium system will cause the system to shift to minimize the effect of the stress Stress can be placed on the system by adding or removing something from either side 3 k1 aA bB Change the temperature change Keq Equilibrium Constant cC dD k 1 At equilibrium rate of the forward rxn rate of back rxn k1 A aeq B beq k 1 C ceq D deq C ceq D deq k1 Keq A aeq B beq k 1 P eq Keq R eq 4 Equilibrium Constant A B If Keq 1 At equilibrium the reactants predominate over products Assuming the starting concentrations of A and B are equal Then the reaction proceeds to the left as it approaches equilibrium If Keq 1 At equilibrium the products predominate over reactants Assuming the starting concentrations of A and B are equal Then the reaction proceeds to the right as it approaches equilibrium 5 A Biochemist s View of Thermodynamics Life obeys the law of thermodynamics Living organisms require ENERGY Biochemists study the processes by which energy is extracted channeled and consumed by living organisms Relationship between the structure and functions of bio macromolecules Metabolism Thermodynamics or bioenergetics gives us a way to describe or characterize the energy changes in any biochemical reaction 6 Energy Changes Thermodynamics is concerned with only the initial and final energy states of the system or reactions components It is not concerned with the mechanism of the process or how long the it takes to perform the process E Ef EI 7 The Thermodynamic Parameters of Interest to Biochemists H Enthalpy Represents the amount of heat absorbed or released by the reaction S Entropy Represents the degree of order or randomness of the system G Free Energy Represents the amount of useful work that can be done by a chemical reaction 8 Energy 1st Law For any process the energy E of the system and its surroundings is constant We can only convert energy from one type to another We can not make NEW energy Energy flows in or out of a system in the form of heat or work so E E f E I q w q is heat absorbed BY the system w is work done ON the system Thus both q and w are positive when energy flows into a system Mechanical work w P V E q P V If there is no change in volume then E q or E is approximated by the heat exchanged when there is no change in volume 9 Enthalpy However most biological systems operate under constant pressure conditions So we are going to define a new function that is used under constant pressure Enthalpy H H E PV or H E P V Remember E q P V so H q P V P V or H q Therefore under constant pressure H is can be approximated by the Heat evolved or absorbed 10 Enthalpic Reactions During a chemical reaction Old bonds break New Bonds form Energy is either consumed or released Exothermic Reactions H 0 Heat is evolved by the system New bonds are more stable Endothermic Reactions H 0 Heat is absorbed by the system New bonds are less stable 11 Thermodynamic Standard States In order to compare the thermodynamic parameters of different reactions it is convenient to define a reference or standard state Standard states apply to only ONE defined set of conditions Generally 1M reactants and products A specific temperature usually 298 K Standard state conditions are denoted by a superscript degree sign H or G G is known as the Standard Free Energy change 12 Biological Standard States Standard States assumes a concentration of 1 M if H 1 M then pH 0 but the pH in most cells is near the neutral range Biochemical reactions are buffered so that the H concentration does not vary Therefore use a value of 1 for H Additionally in biochemical reactions the concentration of water is very high 55 5 M Therefore use a value of 1 for water Since we modified the standard state to reflect these changes it is given the symbol G 13 Entropy 2nd Law For any process the entropy S of the system and its surroundings always increases A measure of disorder An ordered state is low entropy A disordered state is high entropy S 0 when the final state is more order than the initial state Products more complex and more ordered S 0 when the final state is less order than the initial state Products are less complex and more disordered 14 What direction will the reaction proceed A B or B A What direction is favorable or spontaneous A favorable or spontaneous reaction proceed from a state of high energy to low energy A NON favorable or a non spontaneous reaction proceed from a state of low energy to high energy Neither H nor S alone is sufficient to determine spontaneity But the Change in free energy G can used to determine the favorability of a process So for any process at constant pressure and temperature G H T S 15 How much work can be done G H T S H represents the total energy S represents the wasted energy G represents the useful energy H total energy G useful energy T S wasted energy G useful energy H total energy T S wasted energy 16 Free Energy Change and Equilibrium Remember all reactions proceed until they reach equilibrium Once a reaction reaches equilibrium no work can be done The free energy change G of any reaction tells us how much useful work can be accomplished So if a reaction is at equilibrium and no work can be done Then at equilibrium G 0 But what if the reaction is not at equilibrium 17 Free Energy If G 0 the system is at equilibrium S H T Entropic and Enthalpic Changes are equal Favorable reactions proceed to a state of lower free energy High energy to low If G 0 then the reaction proceeds in the direction it is written exergonic reaction and energy is released If G 0 then the reaction proceeds in the opposite direction endergonic reaction and energy is required 18 What direction will the reaction go If the forward reaction is favorable then the reverse reaction is NOT favorable If A B has a G 0 then B A would have G 0 Remember sign tells direction But how do we know