A stress is anything that causes a change in the equilibrium concentrations. LE CHATELIER S PRINCIPLE EQUILIBRIUM SHIFTS A system in equilibrium tends to respond so as to relieve the effect of any stress upon the system. EQUILIBRIUM EXPRESSION aa + bb cc + dd c d c a b K= Pure solids and solvents do not appear in the K c expression. Equilibrium constants relate the extent of the equilibrium Kc>1 products favoured Kc<1 reactants favoured Kc=1 neither reactants nor products favoured. Equilibrium can be approached from either direction. Equilibrium can only be reached in a closed or isolated system. Le Chatelier s principle EQUILIBRIUM LAW (expressions & constants) Equilibrium reactions must show evidence of reversibility Macroscopic properties (temp, pressure, ph, colour) are constant. Calculating equilibrium concentrations from initial. Calculating equilibrium values at equilibrium 3.
is an anode and the object is the cathode.Ĥ EQUILIBRIA & LE CHATELIER S SUMMARY SKILLS TO MASTER 1. Connect a stronger reducing agent metal to the object. CORROSION PROTECTION Methods to prevent corrosion (unwanted spontaneous redox reaction) Paint the object or coat with nonreactive (inert) coating like plastic. Chloride anomaly: Cl 2 (g) is produced when electrolyzing an aqueous ionic chloride compound. Weak reducing agents (nonspontaneous with water) can be made in an aqueous cell Strong reducing agents (spontaneous reaction with water) made in molten cell. Electrorefining: producing pure elements using electrolysis. Net cell voltage (E o net ) is negative For electroplating, the object to be coated with metal must be the cathode. Net cell voltage (E o net ) is positive Cell notation: electrode/electrolyte//electrolyte/electrode ELECTROLYTIC CELLS Same chemistry as the voltaic cell, except Anode is + and cathode is - No porous boundary/salt bridge needed since redox reaction is nonspontaneous. Anions move toward anode Cations move toward cathode Remember A or C: Anode is Oxidation Reduction is Cathode Porous boundary or salt bridge is required to keep solutions separate but allow ion migration prevents cell polarization. or OIL RIG Reducing agents get oxidized Oxidizing agents get reduced VOLTAIC CELLS Reduction occurs at the cathode (+ electrode) Oxidation occurs at the anode ( electrode) Electrons move from anode to cathode via the external wire. or O.I.L.) Reduction is the gain of electrons (G.E.R. Corrosion protection Recognizing Redox Reactions Oxidation is the loss of electrons (L.E.O. Balancing redox reactions: skeleton & half-reaction method 3. Exothermic Reactionģ ELECTROCHEMISTRY SUMMARY Skills to Master 1. Catalysts decrease time to reach equilibrium but do not change K c or equilibrium position. Catalysts provide an alternative reaction pathway with a lower activation energy. The activation energy is the minimum amount of collision energy required to force both reactant molecules together into a single entity called an activated complex. This is the minimum amount of energy required to cause a chemical reaction to occur. Endothermic Reaction Activation Energy An energy barrier between reactants and products. Combustion reactions in open systems produce H 2 O(g), while in closed systems H 2 O(l) is produced. Calorimetry n rh=mc t where: n = moles of chemical specified (mol) calculated either by n=cv for solutions or n=m/m for solids r H = molar enthalpy of reaction (KJ/mol) m = total (volume) mass of water in calorimeter (Kg) c = 4.19 KJ/Kg o C t = temperature change ( o C) t i t f Standard Enthalpies of Formation Used to calculate enthalpy change for a chemical reaction Where: H = enthalpy change kj Σ means sum of n = moles (mol) r H = molar enthalpy of formation (kj/mol) from data book pages 6 & 7 *Notes: r H of elements in their natural state is 0 kj/mol. r for general reaction, c for combustion, f for formation. or r H = molar enthalpy of reaction for specified chemical (kj/mol) Subscripts can be used to denote the reaction type: e.g. given in problem as moles of chemical statement. 2 THERMOCHEMISTRY SUMMARY SKILLS TO MASTER Carrying out calorimetry calculations Relating enthalpy & molar enthalpy calculations Using standard enthalpies of formation Activation energy FORMULAS MUST BE MEMORIZED Enthalpy change Hn r H H = enthalpy change (kj) n = amount of substance (moles), from either: - the balancing number from the equation.
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