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Chemical Equilibrium Farah Mustafa Department of Biochemistry Rm: IE-179; Office: 03-713-7509 Mobile: 050-760-5516 Lecture Outline • • • • • • Introduction to Equilibrium Equilibrium Constant Free Energy and Equilibrium Le Chatlier’s Principle Reaction Speed and Catalysis Metabolism Equilibriu...

Chemical Equilibrium Farah Mustafa Department of Biochemistry Rm: IE-179; Office: 03-713-7509 Mobile: 050-760-5516 Lecture Outline • • • • • • Introduction to Equilibrium Equilibrium Constant Free Energy and Equilibrium Le Chatlier’s Principle Reaction Speed and Catalysis Metabolism Equilibrium • All reactions are reversible • Under suitable conditions, they reach a state of equilibrium • At equilibrium, the concentrations of products and reactants no longer change • Thus, equilibrium is a state of a chemical reaction in which there are no observable changes as time goes by • However, chemical equilibrium is a dynamic state • Chemical Equilibrium is achieved when: • the rates of the forward and reverse reactions are equal and • the concentrations of the reactants and products remain constant • Reactions continue to occur, but because they occur at the same rate, no net change is observed on the macroscopic level Reaching equilibrium on the macroscopic and molecular levels cool N2O4(g) colorless heat 2NO2(g) dark brown Reaching equilibrium on the macroscopic and molecular levels N2O4(g) 2NO2(g) Equilibrium Summary • Thus, equilibrium applies to the extent of a reaction, the concentration of product that has appeared given unlimited time, or when no further change occurs. At equilibrium: rateforward = ratereverse • A system at equilibrium is dynamic on the molecular level but static at macroscopic (observable) level • No further net change is observed because changes in one direction are balanced by changes in the other. Rate of a Chemical Reaction The rate of a reaction is variable. It depends on: – concentration of the reacting species – reaction temperature – presence or absence of catalysts – the nature of the reactants The concentration of A and B decreases with time lowering the rate of the forward reaction. The concentration of C and D increases with time increasing the rate of the reverse reaction. Forward Reaction A + B → C + D Reverse Reaction C + D → A + B 16.2 Equilibrium Constant • At equilibrium, the rates of the forward and reverse reactions are equal, and the concentrations of the reactants and products are constant. • The equilibrium constant (Keq) is a value representing the unchanging concentrations of the reactants and the products in a chemical reaction at equilibrium. For the general reaction: aA + bB → cC + dD → At a given temperature: c K eq d [C] [D] = a b [A] [B] For the reaction: 3H2 + N2 → 2NH3 → 2 K eq [NH3 ] = 3 [H2 ] [N2 ] • If K = 1, the rxn is at Equilibrium • If K > 1 then it favors the Products • If K < 1 then it favors the Reactants For the reaction: 4NH3 + 3O2 → 2N2 + 6H2O → 2 K eq 6 [N2 ] [H2O] = 4 3 [NH3 ] [O2 ] When the molar concentrations of all species in an equilibrium reaction are known, the Keq can be calculated by substituting the concentrations into the equilibrium constant expression. The Meaning of ΔG for a Chemical Reaction • A system reaches the lowest possible free energy by going to equilibrium, not by going to completion. Complete Reaction: Complete consumption of reactants. Equilibrium: Reaction achieves equilibrium by reaching the lowest state of energy. Change in Free Energy to Reach Equilibrium Progress of reaction G is decreasing and reaction proceeds in the forward direction. Progress of reaction G is increasing and reaction proceeds in the reverse direction. Progress of reaction G is not changing and has reached its lowest point. There is no net change in the reaction. What is the ∆G at Equilibrium? The ∆G at equilibrium is zero (0). • The process is at equilibrium • No net flow in either the forward or the reverse direction • Neither process is favored • Enthalpic and entropic changes are exactly balanced Relationship Between Free Energy and the Equilibrium Constant Le Châtelier’s Principle If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position. • Changes in Concentration N2 (g) + H2 (g) Equilibrium shifts left to offset stress NH3 (g) Add NH3 Le Châtelier’s Principle: Changes in Concentration Change Shifts the Equilibrium Increase concentration of product(s) Decrease concentration of product(s) Increase concentration of reactant(s) Decrease concentration of reactant(s) left right right left Le Châtelier’s Principle: Changes in Pressure and Volume A (g) + B (g) Change Increase pressure Decrease pressure Increase volume Decrease volume C (g) Shifts the Equilibrium Side with fewest moles of gas Side with most moles of gas Side with most moles of gas Side with fewest moles of gas Le Châtelier’s Principle: Change in Temperature Change Exothermic Rx Endothermic Rx Increase temperature K decreases K increases Decrease temperature K increases K decreases Keep in mind: If K > 1 then it favors the products If K < 1 then it favors the reactants If K = 1, the reaction is at equilibrium • In an endothermic reaction where heat is absorbed/consumed, if the temperature is raised, it will push the reaction towards the products so as to decrease the effect of the added temperature by increasing products with higher enthalpy. • In an exothermic reaction where heat is released, if the temperature is decreased, it will push the reaction towards the products as a counter measure to increase the temperature. Le Châtelier’s Principle: Catalysis Adding a Catalyst:  does not change K  does not shift the position of an equilibrium system  system will reach equilibrium sooner (speeds up the reaction) Thus, adding a catalyst will have no effect on the equilibrium position as both the forward and reverse reactions rates will be increased equally. http://www.chem1.com/acad/webtext/chemeq/Eq-02.html Rates of Reactions • The rate of a reaction is the speed at which a reaction happens. • If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. • Few things affect the overall speed of the reaction and the number of collisions that can occur. Catalysis • A chemical reaction involving a material which promotes or increases the rate of the reaction without itself undergoing any permanent chemical change. Energy Diagram for an Exothermic Reaction A catalyst does not change the energy of a reaction. Enzymes: Biological Catalysts • Permit reactions to ‘go’ at conditions that the body can tolerate. • Enzymes are typically very large proteins. • Can process millions of molecules every second. • Very specific – only react with one or a few types of molecules. Classification of Enzymes Based on Type of Reaction • Oxidoreductase Catalyzes an oxidation-reduction reaction • Transferase Transfers a functional group • Hydrolase Causes hydrolysis reactions • Lyase Causes formation of double bonds • Isomerases Rearranges functional groups • Ligase Joins 2 molecules by forming C-C, C-O, C-S, C-N bonds Metabolism   The sum of all chemical reactions that occur within a living organism is defined as metabolism Metabolism can be subdivided into two contrasting categories: 1. 2. Anabolism Catabolism Anabolism vs Catabolism  Anabolism is the process by which simple substances are synthesized (built up) into complex substances   Anabolic reactions usually involve carbon reduction and consume cellular energy Catabolism is the process by which complex substances are broken down into simpler substances  Catabolic reactions usually involve carbon oxidation and produce energy for the cell Equilibrium: The Angel of Death for Living Beings • Chemical equilibrium is something one definitely should avoid for one self as long as possible. • The countless chemical reactions in living organisms are constantly moving toward equilibrium, but are prevented from getting there by input of reactants and removal of products. • So rather than being in equilibrium, we try to maintain a "steady-state" condition which physiologists call homeostasis — maintenance of a constant internal environment. • For more on this, see this Wikipedia article, Equilibrium is Death! http://www.chem1.com/acad/webtext/chemeq/Eq-01.html#SEC2