Chapter 14: Chemical Equilibrium CHE 124: General Chemistry II Dr. Jerome Williams, Ph.D. Saint Leo University

Overview Hemoglobin & The Equilibrium Condition Reaction Dynamics Dynamic Equilibrium Law of Mass Action: Equilibrium Constants Homogeneous Equilibrium

Hemoglobin Hemoglobin is a protein (Hb), found in red blood cells, that reacts with O 2 – enhances the amount of O 2 that can be carried through the bloodstream Hb + O 2  HbO 2 – the Hb represents the entire protein it is not a chemical formula – the  is used to describe a process that is in dynamic equilibrium 3 Tro: Chemistry: A Molecular Approach, 2/e

Hemoglobin Equilibrium System Hb + O 2  HbO 2 The concentrations of Hb, O 2, and HbO 2 are all interdependent The relative amounts of Hb, O 2, and HbO 2 at equilibrium are related to a constant called the equilibrium constant, K – the larger the value of K, the more product is found at equilibrium Changing the concentration of any one of these necessitates changing the other concentrations to restore equilibrium 4 Tro: Chemistry: A Molecular Approach, 2/e

O 2 Transport Hb + O2O2  In the lungs, with high concentration of O 2, the equilibrium shifts to combine the Hb and O 2 to make more HbO 2 In the cells, with low concentration of O 2, the equilibrium shifts to break down the HbO 2 and increase the amount of free O 2 HbO 2 O 2 in lungs Hb HbO 2 O 2 in cells 5 Tro: Chemistry: A Molecular Approach, 2/e

HbF Hb Fetal Hemoglobin, HbF HbF + O 2  HbFO 2 Fetal hemoglobin’s equilibrium constant is larger than adult hemoglobin’s Because fetal hemoglobin is more efficient at binding O 2, O 2 is transferred to the fetal hemoglobin from the mother’s hemoglobin in the placenta Hb + O2O2  HbO 2 O2O2 O2O2 HbF + O2O2  HbFO 2 O2O2 6 Tro: Chemistry: A Molecular Approach, 2/e

Oxygen Exchange between Mother and Fetus 7 Tro: Chemistry: A Molecular Approach, 2/e

Arrow Conventions Chemists commonly use two kinds of arrows in reactions to indicate the degree of completion of the reactions A single arrow indicates all the reactant molecules are converted to product molecules at the end A double arrow indicates the reaction stops when only some of the reactant molecules have been converted into products –  in these notes 8 Tro: Chemistry: A Molecular Approach, 2/e

Reaction Dynamics When a reaction starts, the reactants are consumed and products are made – forward reaction = reactants  products – therefore the reactant concentrations decrease and the product concentrations increase – as reactant concentration decreases, the forward reaction rate decreases Eventually, the products can react to re-form some of the reactants – reverse reaction = products  reactants – assuming the products are not allowed to escape – as product concentration increases, the reverse reaction rate increases Processes that proceed in both the forward and reverse direction are said to be reversible – reactants  products 9 Tro: Chemistry: A Molecular Approach, 2/e

Hypothetical Reaction 2 Red  Blue The reaction slows over time, but the Red molecules never run out! At some time between 100 and 110 sec, the concentrations of both the Red and the Blue molecules no longer change – equilibrium has been established. Notice that equilibrium does not mean that the concentrations are equal! Once equilibrium is established, the rate of Red molecules turning into Blue is the same as the rate of Blue molecules turning into Red 10 Tro: Chemistry: A Molecular Approach, 2/e

Hypothetical Reaction 2 Red  Blue 11 Tro: Chemistry: A Molecular Approach, 2/e

Eventually, the reaction proceeds in the reverse direction as fast as it proceeds in the forward direction. At this time equilibrium is established. Reaction Dynamics Time Rate Rate Forward Rate Reverse Initially, only the forward reaction takes place As the forward reaction proceeds it makes products and uses reactants Because the reactant concentration decreases, the forward reaction slows. As the products accumulate, the reverse reaction speeds up. Once equilibrium is established, the forward and reverse reactions proceed at the same rate, so the concentrations of all materials stay constant 12 Tro: Chemistry: A Molecular Approach, 2/e

Dynamic Equilibrium As the forward reaction slows and the reverse reaction accelerates, eventually they reach the same rate Dynamic equilibrium is the condition wherein the rates of the forward and reverse reactions are equal Once the reaction reaches equilibrium, the concentrations of all the chemicals remain constant – because the chemicals are being consumed and made at the same rate 13 Tro: Chemistry: A Molecular Approach, 2/e