Chemical Equilibrium_updated 30oct.pptx PDF

CHEMICAL EQUILIBRIUM Equilibrium  The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at Equilibrium the same rate. The Concept of Equilibrium As a system approaches equilibrium,...

CHEMICAL EQUILIBRIUM Equilibrium  The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at Equilibrium the same rate. The Concept of Equilibrium As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate. Equilibrium  A System at Equilibrium Once equilibrium is achieved, the amount of each reactant and product remains constant. Equilibrium  A System at Equilibrium Rates become equal Concentrations become constant Equilibrium  Depicting Equilibrium In a system at equilibrium, both the forward and reverse reactions are running simultaneously. We write the chemical equation with a double arrow: Equilibrium Equilibrium The Equilibrium Constant Equilibrium  The Equilibrium Constant Forward reaction: Reverse reaction: Rate law Rate Law Equilibrium  The Equilibrium Constant At equilibrium Rearranging gives: Equilibrium  The Equilibrium Constant The ratio of the rate constants is a constant (as long as T is constant). The expression becomes Equilibrium  The Equilibrium Constant To generalize, the reaction: Has the equilibrium expression: This expression is true even if you don’t know the elementary reaction mechanism. Equilibrium Equilibrium Can Be Reached from Either Direction Kc, the final ratio of [NO2]2 to [N2O4], reaches a constant no matter what the initial concentrations of NO2 and N2O4 are (with const T). Equilibrium Equilibrium Can Be Reached from Either Direction This graph shows data from the last two trials from the table. QuickTime™ and a QuickTime™ and a QuickTime™ and a TIFF (LZW) decompres or TIFF (LZW) decompressor TIFF (LZW) decompressor are needed to see this picture. are needed to see this picture. are needed to see this picture. Equilibrium Equilibrium Can Be Reached from Either Direction It does not matter whether we start with N2 and H2 or whether we start with NH3. We will have the same proportions of all three substances at equilibrium. What is the equilibrium expression? Equilibrium What Does the Value of K Mean? If K >> 1, the reaction is product-favored; product predominates at equilibrium. Equilibrium What Does the Value of K Mean? If K >> 1, the reaction is product-favored; product predominates at equilibrium. If K K, there is too much product and the equilibrium shifts to the left. Equilibrium  If Q < K, there is too much reactant, and the equilibrium shifts to the right. Equilibrium Le Châtelier’s Principle Equilibrium  Le Châtelier’s Principle “If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.” Systems shift from “Q” towards “K”. Equilibrium What Happens When More of a Reactant Is Added to a System? Equilibrium  The Haber Process The transformation of nitrogen and hydrogen into ammonia (NH3) is of tremendous significance in agriculture, where ammonia-based fertilizers are of utmost importance. Equilibrium The Haber Process If H2 is added to the system, N2 will be consumed and the two reagents will form more NH3. Equilibrium The Haber Process This apparatus helps push the equilibrium to the right by removing the ammonia (NH3) from the system as a liquid. Equilibrium Effects of Pressure Change on Equilibrium If the volume of a gas mixture is compressed, the overall gas pressure will increase. In which direction the equilibrium will shift in either direction depends on the reaction stoichiometry. However, there will be no effect to equilibrium if the total gas pressure is increased by adding an inert gas that is not part of the equilibrium system. Equilibrium  Reactions that shift right when pressure increases, and shift left when pressure decreases Consider the reaction: 2SO2(g) + O2(g) ⇄ 2SO3(g), 1. The total moles of gas decreases as reaction proceeds in the forward direction. 2. If pressure is increased by decreasing the volume (compression), a forward reaction occurs to reduce the stress. 3. Reactions that result in fewer moles of gas favor high pressure conditions. Equilibrium Reaction that shifts left when pressure increases, but shifts right when pressure decreases Consider the reaction: PCl5(g) ⇄ PCl3(g) + Cl2(g); 1. Forward reaction results in more gas molecules. 2. Pressure increases as reaction proceeds towards equilibrium. 3. If mixture is compressed, pressure increases, and reverse reaction occurs to reduce pressure; 4. If volume expands and pressure drops, forward reaction occurs to compensate. 5. This type of reactions favors low pressure condition Equilibrium  Reactions not affected by pressure changes Consider the following reactions: 1. CO(g) + H2O(g) ⇄ CO2(g) + H2(g); 2. H2(g) + Cl2(g) ⇄ 2HCl(g); 1. Reactions have same number of gas molecules in reactants and products. 2. Reducing or increasing the volume will cause equal effect on both sides – no net reaction will occur. 3. Equilibrium is not affected by change in pressure. Equilibrium The Effect Temperature on Equilibrium Consider the following exothermic reaction: N2(g) + 3H2(g) ⇄ 2NH3(g); DHo = -92 kJ, The forward reaction produces heat => heat is a product. When heat is added to increase temperature, reverse reaction will take place to absorb the heat; If heat is removed to reduce temperature, a net forward reaction will occur to produce heat. Exothermic reactions favor low temperature conditions. Equilibrium The Effect Temperature on Equilibrium Consider the following endothermic reaction: CH4(g) + H2O(g) ⇄ CO(g) + 3H2(g), DHo = 205 kJ 1. Endothermic reaction absorbs heat  heat is a reactant; 2. If heat is added to increasing the temperature, it will cause a net forward reaction. 3. If heat is removed to reduce the temperature, it will cause a net reverse reaction. 4. Endothermic reactions favor high temperature condition. Equilibrium Catalysts increase the rate of both the forward and reverse reactions. Equilibrium  Equilibrium is achieved faster, but the equilibrium composition remains unaltered. Equilibrium Chemical Equilibria in Industrial Processes The production of hydrogen gas: Reaction: CH4(g) + H2O(g) ⇄ CO(g) + 3H2(g); This reaction is endothermic with DH = 206 kJ Increasing the reaction temperature will increase both the rate and the yield. This reaction favors high temperature and low pressure conditions. Equilibrium Examples Equilibrium  Solubility Product Assume complete dissociation of AgCl in equilibrium between the solid and its saturated solution, AgCl ( s )  Ag  (aq )  Cl  (aq) The equilibrium equation could be written as, [ Ag  ][Cl  ]  K C' [ AgCl ] The concentration of [AgCl] is a constant and the convention is to combine it with Kc’ to give [Ag+][Cl-] = Ksp Ksp is the solubility product. For greater precision, activities are used: a Ag  aCl   K sp Equilibrium 53  Factors Influencing Solubility 1. Solute solvent interactions - the stronger force of interactions between molecule and solvent, the higher the solubility. - polar liquid will dissolve polar solvent. - non-polar liquid will not dissolve in polar liquid. - materials with the same force of molecular attraction will readily dissolve in one another. Equilibrium 54 2. Pressure effects - solubility of gas will increase proportionally with the partial pressure of the solution. - the relation between pressure and solubility of gas follows the Henry’s law, Sg = kPg Equilibrium 55 3. Temperature Effects - the solubility of most solid solute in water will increase when the temperature of the solution increases. - the solubility of gas in water decreases with the increase of temperature. Equilibrium 56  Ideal Solution: Raoult’s Law Raoult’s Law: Vapor pressure P1 of the solvent 1 is equal to its mole fraction in the solution multiplied by the vapor pressure P1* of the pure solvent. Mathematically, For substance 1 in the mixture P1  x1 P1* For substance 2 in the mixture P2  x2 P2* Equilibrium 57  Ideal Solution: Henry’s Law - that mass of gas m2 dissolved by a given volume of solvent at constant temperature is proportional to the pressure of the gas in equilibrium with the solution. - Mathematically stated: m =kP 2 2 2 Since mass per unit volume is equal to a concentration term, P2 k'x 2 or P2 k"c2 - where for dilute solutions the concentration c2 is proportional to the mole fraction of dissolved substances. Equilibrium 58 Homogeneous and Heterogeneous Homogeneous system  a single phase which is uniform throughout both in chemical composition and physical state. Heterogeneous system  consisting of more than one phase; the phases are distinguished from each other through separation by distinct boundaries. Equilibrium 59 Phase Diagram for Water Equilibrium 60 Phase Diagram of Pure Solvent and Solution of non-Volatile Solute CECAIR PEPEJAL GAS Equilibrium 61 Ternary Phase Diagram Equilibrium Equilibrium