Chemical Equilibrium and Energetics of Chemical Reactions PDF

Chemical Equilibrium and Energetics of Chemical Reactions assoc. prof. Zuzana Tatarkova, PhD. Law about spontaneity of chemical processes (2. law of TD ) DEFINITION OF BASIC TERMS ThermoDynamics (TD) – the study of relationships between differen...

Chemical Equilibrium and Energetics of Chemical Reactions assoc. prof. Zuzana Tatarkova, PhD. Law about spontaneity of chemical processes (2. law of TD ) DEFINITION OF BASIC TERMS ThermoDynamics (TD) – the study of relationships between different forms of Energy involved in chemical and physical processes. State functions – characterize state of the system (E, p, T, c), – their change depends only on initial and final state of the system. System and surroundings Thermodynamic Systems According to their interactions with surroundings: Open system – exchanges energy and matter with its surroundings Closed system – can exchange energy but not matter Isolated system – cannot exchange either energy or matter According to physical state of reactants and products: heterogenous system – consist of physically different parts homogenous system – physically uniform  CHEMICAL EQUILIBRIUM - rates of forward and opposite reactions are equal, - there is no observable change in the reaction mixture. Definition of the equilibrium constant (Law of Guldberg-Waage) [C ]c.[ D]d aA + bB  cC + dD K= [ A]a.[ B]b Significance of the equilibrium constant K: - estimation of the composition of reaction mixture, - estimation of the direction of reaction. 1080 K»1 at equilibrium the concentrations of products are larger than concentration of reactants, and reaction favors the formation of products 102 K1 concentrations of products and reactants are roughly equal 10-2 K«1 at equilibrium the concentrations of products are lower than concentration of reactants, and reaction occurs to only limited extend 10-80 Le Chatelierꞌs Principle Any change in concentration (c), temperature (T) or pressure (p) of the reaction mixture in equilibrium will move the reaction in such direction that eliminates the change. N2 (g) + 3H2 (g) 2NH3 (g) ΔH° = - 92 kJ EXOTHERMIC What is the effect of each of the following changes on equilibrium? 1) change in concentration of reactants or products ↑ Substrate added – shifts equilibrium in the direction of NH3 formation ↑ Product added – shifts equilibrium in the direction of NH3 decomposition 2) change in temperature ↑ T – shifts equilibrium in the direction in which the heat is absorbed = Endothermic ↓ T – shifts equilibrium in the direction in which the heat is released = Exothermic 3) change in pressure ↑ p – shifts equilibrium in the direction of NH3 formation ↓ p – shifts equilibrium in the direction of NH3 decomposition  Equilibrium and Steady State Chemical equilibrium Dynamic (steady-state) occurs in isolated to maintain constant properties system, does not occur of the system, supply of energy in living systems is required (living systems) Although both states can create a situation where the concentration does not change, they are different. Chemical equilibrium Dynamic equilibrium (static) (steady state) The state in which reactants and products are in concentrations that have no General definition further tendency to change with time. Movement No movement of substances Substances are on movement The rate of forward and reverse reaction is equal The reaction does not stop after Reaction rate The reaction stops after reaching reaching equilibrium. The reaction equilibrium is proceeding in a way that keeps the amounts unchanged The 1st Law of Thermodynamics Energy cannot be created or destroyed, it can only be converted from one form to another. The total internal energy (E) of an isolated system is constant. E = Q+W Q – heat disordered form of Energy Q0 heat is absorbed Q0 heat is liberated W – work arranged form of Energy (mechanical, chemical, electric,...) W  0 work is done on the system by its surroundings W  0 work is done by the system ENTHALPY - from the Greek enthalpein, meaning “to warm”, - state function, - reaction heat or heat effect of the reaction that occurs at constant pressure.  H = Hproducts - Hreactants H  0 H  0 - exothermic reaction - endothermic reaction - heat is released - heat is absorbed https://chem.libretexts.org  ENTHALPY Enthalpy changes of forward and opposite reactions have equal absolute value, and they differ only in sign (Lavoisier and Laplace). 2H2 (g) + O2 (g) → 2H2O (l) H = - 57 kJ 2H2O (l) → 2H2 (g) + O2 (g) H = + 57 kJ Hessꞌs law Enthalpy change H depends only on the initial and final state! It does not depend on the reaction. When the reaction canbreaking Bond be expressed as the ALWAYS sum ofan2 input requires or more reactions, then the H of of Energy reaction is the sum of the enthalpies of these and reactions 2S (s)bond + 3O making 2 releases (g) → 2SO 3 (g) the  Energy! H = -792 kJ or Study of enthalpy change in biological processes: 2S (s) + 2O2 (g) → 2SO2 (g)  H1 = -594 kJ - protein denaturation, 2SO2 (g) + O2 (g) → 2SO3 (g)  H2 = -198 kJ - spiral-ball transition in DNA, H1 + H2 = -792 kJ - study of ligand binding to biomacromolecules. ENTROPY - state function, - to measure of the disorder or randomness. Qrev S = [T, reversible] T Definition of the 2nd law of thermodynamics: The total entropy of an isolated system always increases in a spontaneous process (Heat cannot be spontaneously transferred from the colder matter to the hotter one.) S  0 spontaneous process that tends towards higher disorder S  0 nonspontaneous process S = 0 system at equilibrium Negative value of S = negetropy – the measure of order (information)  Spontaneous processes and 2nd Law of Thermodynamics Spontaneous process: - a process, which occurs itself and proceeds until equilibrium is reached, - a process that goes spontaneously in one direction cannot go spontaneously in the opposite direction. ICE CUBE WATER Crystal structure High order Higher disorder The effect of Temperature! T > 0 – ice becomes water spontaneously T < 0 – water becomes ice spontaneously Principles which determine spontaneity of processes: 1) Tendency towards minimum energy (enthalpy, H  0) 2) Tendency towards maximum disorder (entropy, S  0)  GIBBS FREE ENERGY - state function, - measure of a system's maximum potential for doing work. G = Gfinal – Ginitial G = H - TS H and S - partial criteria of spontaneity G - direct criterion of spontaneity H2O (s) → H2O (l) T  0°C 2SO2 (g) + O2 (g) → 2SO3 (g) Requires continual T  786°C input of energy GIBBS FREE ENERGY G  0 spontaneous - exergonic reaction G  0 nonspontaneous - endergonic reaction G = 0 system at equilibrium The value of ΔG determines the intensity of the "driving" force of the process but does not provide any information about the speed of this process. Gibbs Free Energy and Equilibrium For the reaction A + B  C + D [C].[D] G = ΔG + RT ln R = 8.314 J.K-1.mol-1 (gas constant) [A].[B] T in Kelvins (0°C = 273.15 K) G - standard Gibbs free energy change, - defined at standard state (concentration equal to 1 mol/L, T= 25C) For equilibrium: [C].[D] G = 0 and =K [A].[B] thus G = - RT lnK Significance: ✓ enables calculation of G from K and vice versa ✓ G – estimation of the spontaneity of the process at standard conditions Termodynamics and Living Systems Living System – an open system – continual exchange of matter and energy with its surroundings to maintain physiological functions. Energy (matter) obtained from environment is converted in living systems into the different forms: 1) Green plants and photosynthesizing bacteria Light energy → chemical energy of biomolecules 2) All other organisms Chemical energy obtained from food is converted into: a) chemical energy of biomolecules (proteins, lipids, saccharides), which have various functions in organism, b) mechanical work (muscle contraction) c) electrical work (signal transmission in nervous system) d) osmotic work (maintenance of gradients of ions and molecules) e) heat – released into surroundings The 1st Law of Thermodynamics and Living Systems In living system, energy can be transformed from one form to another, but cannot be created or destroyed. Limitations of 1st Law of TD: 1. Does not say anything about whether the process is possible or not 2. Does not say whether the process will occcur on its own or not 3. Does not provide any sufficient condition for a certain process to take place. The 2nd Law of Thermodynamics and Living Systems Synthesis of biopolymers and their assembly into higher structures  entropy decrease Living systems = open systems - entropy change must be considered not only in the living organism/system but also in its surroundings Ssurr + Sinside  0  Sinside - entropy change inside the living system  Ssurr - entropy change in the surroundings A living cell is in a low-entropy, non-equilibrium state characterized by a high degree of structural organization. To maintain this state, a cell must release some of the energy as heat, thereby increasing Ssurr sufficiently that the 2nd law of TD is not violated. Ssurr + Sinside  0 Thermodynamics and living systems Major source of energy in higher chemotrophic organisms: 2H + ½ O2 H2O G = - 237 kJ.mol-1 Gibbs Free Energy and Biochemical Reactions BIOPOLYMERS: - molecules with high Gibbs free energy, - in comparison to their building blocks they have high value of H and low value of S. Gibbs Energy and Coupling of Chemical Reactions in Living Systems Phosphorylation of glucose – an initial step in glycolysis and other pathways of glucose metabolism Phosphorylation of glucose with H3PO4 - endergonic reaction G 0 (1) glucose + H3PO4 → glucose 6-phosphate + H2O G = +14 kJ/mol !!! Coupling of endergonic reaction with exergonic reaction !!! - utilization of conserved chemical energy, liberated in catabolic processes, for anabolic processes. Requirements for coupling of reactions: Total Gibbs free energy change must be  0, Coupled reactions must have common intermediate, which is produced in one reaction and is consumed in the second one. This intermediate is an energy carrier. Hydrolysis of adenosine triphosphate (ATP): (2) ATP + H2O → ADP + H3PO4 G = -31 kJ/mol Gibbs Energy and Coupling of Chemical Reactions in Living Systems Coupling of reaction (1) with reaction (2): glucose + H3PO4 → glucose 6-phosphate + H2O G = +14 kJ/mol ATP + H2O → ADP + H3PO4 G = - 31 kJ/mol glucose +ATP → glucose 6-phosphate + ADP G = -17 kJ/mol  MACROERGIC COMPOUNDS - compounds allowing the release of a large amount of energy in a simple reaction Zlúčenina Makroergická väzba kJ/mol Fosfoenolpyruvát Enol-fosfátová - 61,9 1,3-bisfosfoglycerát Acyl-fosfátová - 49,3 Kreatínfosfát Guanidín-fosfátová - 43,1 Acetyl-CoA/Acyl-CoA Tioesterová - 31,4 ATP na ADP + Pi -32,2 Pyrofosfátová ATP na AMP + PPi - 30,5  ATP (adenosine triphosphate) 1941 F. Lipmann and H. Kalckar a major carrier of chemical energy in the living systems continuous ATP cleavage / recovery Factors which determine high G of ATP hydrolysis Electrostatic repulsion at pH = 7 ATP has 4 negative charges, which are strongly repelled. Resonance stability ADP and Pi have higher resonance stability than ATP. Stabilization due to hydration. Ionization of molecules = increase in the number of particles = higher disorder, ΔS > 0