Medical Chemistry Lecture 2024 PDF

Week 3 Lecture 6 Medical Chemistry Lecture 2024 Chemical Equilibrium Gen. Med.: Dr. Ferenc Erdődi Dentistry: Dr. Andrea Kiss Department of Medical Chemistry UD Faculty of M...

Week 3 Lecture 6 Medical Chemistry Lecture 2024 Chemical Equilibrium Gen. Med.: Dr. Ferenc Erdődi Dentistry: Dr. Andrea Kiss Department of Medical Chemistry UD Faculty of Medicine  Equilibrium Physical equilibrium The Leaning Tower of Pisa H2O (l) H2O (g) Equilibrium is a state in which there are no observable changes as time goes by.  Chemical Equilibrium Major questions concerning chemical reactions : - what happens (reaction mechanism) ? Reaction kinetics - how fast does it happen ? - to what extent does it happen ? Chemical Equilibrium  General reaction: A + B C A B C A AB A B B C A B A A A A C A A B B B C C C B C C B C B C B B B A A B A B A B A B A A A A A A A B B A B A C B C C B C C B C B C only reactants no further changes (both reactants formation of the product and product are present) 0 Reaction time equilibrium Characterization of the equilibrium with the ratio of components: 5C concentration of products = at equilibrium = concentration of reactants 3A x 3B We need to define this approach more precisely !!! Chemical equilibrium: the state reached in a chemical reaction when the concentrations of reactants and products remain constant over time.  Chemical Equilibrium Equilibrium state is a dynamic one in which reactant and product concentrations remain constant, not because the reaction stops, but because the rates of the forward and reverse reaction are equal. General reaction: vf A + B vr C v = reaction rate; depends on the concentration of the reactants ( [A],[B] or [C]) Forward reaction: vf = kf [A] [B] vf = kf when [A] = [B] = 1 mol/L Reverse reaction: vr = kr [C] vr = kr when [C] = 1 mol/L k = reaction rate constant; the rate at 1 mol/L concentration of the reactants The relation between chemical equilibrium and the forward and reverse reaction rates At the beginning of reaction: [A] and [B] maximal At equilibrium: so vf maximal vf = vr vr ~ 0 kf [A] [B] = kr [C] v, reaction rate concentration [C] vf [A], [B] vr time time After proceeding the forward If vf = vr equilibrium is reached reaction: no further change in [A], [B] and [C] [A] and [B] decreased kf [C] while [C] increased, K (equilibrium kr [A] [B] so vf decreased constant) vr increased  kf [C] kr [A] [B] K (equilibrium constant) a. completion b. favoured in forward kf>>kr direction: kf>kr [C] [C] concentration concentration [A], [B] [A], [B] time c. favoured in concentration reverse direction [A], [B] kf K then the reverse reaction must occur to reach equilibrium (i.e., products are If Q < K then the forward consumed, reactants are formed, the reaction must occur to reach numerator in the equilibrium constant equilibrium. expression decreases and Q decreases until it equals K).  INFLUENCES ON CHEMICAL EQUILIBRIUM LE CHATELIERS’S PRINCIPLE: If a system at equilibrium is subjected to a stress, the system will react in a way to relieve the stress. Henry Le Chatelier (1850-1936) „If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure the equilibrium will shift in order to minimize that change” What are the stresses? The stresses applied to a system may be changes in: (a) concentration of the reactants and the products (b) pressure (c) temperature (a) changes in concentration A+B C+D [C] [D] will increase in [C] and [D] In order to reestablish equilibrium K at new equilibrium concentrations [A] [B] for each components increase in [A] and [B] Q < K, a reaction proceeds toward product formation to reach equilibrium If after reaching the equilibrium we increase [A] and [B]: the reaction should go forward to reestablish equilibrium at higher concentration of products and reactants  LE CHATELIER’S PRINCIPLE (b) changes in pressure: applicable to reaction where the reaction components are in gas phase or at least one of the reaction partners is in gas phase. Pressure will affect reactions only where there is a change in the number of gas moles as a consequence of reaction. (The number of moles of gas on the reactant and on the product sides are different). Example: PCl3 (g) + Cl2 (g) PCl5 (g) 1 mol 1 mol 1 mol 2 mol gas If the pressure increases then to relieve the stress (by decreasing the volume) the number of moles of gas should decrease If the reaction is shifted toward the forward direction, it leads to a decrease in the number of gas moles.  LE CHATELIER’S PRINCIPLE (c) the effect of temperature Chemical reactions could be exothermic (heat producing) or endothermic (requires heat). Looking at heat whether it is needed (endothermic) or it is produced (exothermic) the reactions could be written in general cases: Raising temperature A + heat B endothermic favors endothermic reaction Decreasing B A + heat exothermic temperature favors exothermic reactions  Altering an Equilibrium Mixture: Changes in Temperature In general, when an equilibrium is disturbed by a change in temperature, Le Châtelier’s principle predicts that the equilibrium constant for an exothermic reaction (negative Ho) decreases as the temperature increases. the equilibrium constant for an endothermic reaction (positive Ho) increases as the temperature increases. Chapter 13/23  N2(g) + 3H2(g) 2NH3(g) (a) If we start with a mixture of nitrogen and hydrogen (in any proportions), the reaction will reach equilibrium with a constant concentration of nitrogen, hydrogen and ammonia. (b) If we start with just ammonia and no nitrogen or hydrogen, the reaction will proceed and N2 and H2 will be produced until equilibrium is achieved.  Le Châtelier’s Principle N2(g) + 3H2(g) 2NH3(g) - Change in reactant or product concentration - If H2 is added while the Q 1, then products dominate at equilibrium and equilibrium lies to the right If K K 2+ - As NaF is added to the system, the solubility of CaF2 decreases. Solubility is decreased when a common ion is added: this is an application of Le Châtelier’s principle. Common-Ion Effect  Solubility and pH Again we apply Le Châtelier’s principle: if the F- is removed, then the equilibrium shifts towards the products and CaF2 dissolves. CaF2(s) Ca2+(aq) + 2F-(aq) F- can be removed by adding a strong acid: HCl H+ + Cl- - + Q