1 © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 1 Experiment 9 Experiment 9 Acid Acid Base Base Equilibria Equilibria CH 204 CH 204 Spring 2008 Spring 2008 Dr. Brian Anderson Dr. Brian Anderson © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 2 Last Week Last Week Heat in chemical reactions: Heat in chemical reactions: heat is a measurable quantity heat is a measurable quantity produced and consumed in produced and consumed in stoichiometric stoichiometric amounts amounts Heat Capacity (J/K): Heat Capacity (J/K): how much heat is required to raise the temperature how much heat is required to raise the temperature of something by one degree Celsius (or 1 Kelvin) of something by one degree Celsius (or 1 Kelvin) Specific Heat Capacities (J/ Specific Heat Capacities (J/gK gK): ): Lead 0.128 Iron 0.449 Water 4.184 Lead 0.128 Iron 0.449 Water 4.184 Hess’s Law, Enthalpy of Formation Hess’s Law, Enthalpy of Formation © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 3 This week This week Weak acid titration. Weak acid titration. Determine K Determine K a a of acetic acid by a couple of acetic acid by a couple different methods. different methods. Witness the awesome power of a buffer Witness the awesome power of a buffer solution to resist changes in pH. solution to resist changes in pH.2 © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 4 Non Non Equilibrium Reaction Equilibrium Reaction Reaction goes to completion. Reaction goes to completion. HCl HCl → → H H + + + + Cl Cl – – KNO KNO 3 3 → → K K + + + NO + NO 3 3 – – CH CH 4 4 + 2O + 2O 2 2 → → CO CO 2 2 + 2H + 2H 2 2 O O © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 5 Equilibrium Reaction Equilibrium Reaction Products react with each other Products react with each other to re to re form the reactants. form the reactants. PbSO PbSO 4 4 Pb Pb 2+ 2+ + SO + SO 4 4 2 2– – CH CH 3 3 COOH CH COOH CH 3 3 COO COO – – + H + H + + N N 2 2 (g) (g) + 3H + 3H 2 2 (g) (g) 2 NH 2 NH 3 3 (g) (g) © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 6 Equilibrium Expression Equilibrium Expression K K a a = = [H [H + + ][A ][A – – ] ] [HA] [HA] For any equilibrium reaction, For any equilibrium reaction, aA aA+ + bB cC bB cC + + dD dD For a weak acid dissociation, For a weak acid dissociation, HA H HA H + + + A + A – – K K eq eq = = [C] [C] c c [D] [D] d d [A] [A] a a [B] [B] b b products products reactants reactants3 © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 7 Strong Acids Strong Acids H H 2 2 SO SO 4 4 HNO HNO 3 3 HCl HCl HBr HBr HClO HClO 3 3 HClO HClO 4 4 HI HI Weak Acids Weak Acids All the rest! All the rest! CH CH 3 3 COOH R COOH R COOH COOH H H 3 3 PO PO 4 4 HF NH HF NH 4 4 + + “HA” “HA” © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 8 Calculating [H Calculating [H + + ] and pH ] and pH If you are given the [H If you are given the [H + + ] ] pH = pH =  log[H log[H + + ] ] If you are given the pH If you are given the pH [H [H + + ] = 10 ] = 10  pH pH © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 9 Four Four Part Lab Part Lab 1) 1) Calibrate pH meter Calibrate pH meter Make sure you’re in CALIBRATION mode. Make sure you’re in CALIBRATION mode. Calibrate the pH meter in the order in the lab Calibrate the pH meter in the order in the lab manual: manual: pH 7 pH 7 first, then first, then pH 4 pH 4, then , then pH 10 pH 10. . Press ENTER or CON to confirm calibration. Press ENTER or CON to confirm calibration.4 © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 10 Last Two Last Two Titrations Titrations of Your Life of Your Life 2) 2) Titrate Titrate 25 ml of 0.1 M acetic acid using 0.1 M 25 ml of 0.1 M acetic acid using 0.1 M NaOH NaOH DO NOT add water! No indicator this time. DO NOT add water! No indicator this time. Titrate Titrate in a beaker, in a beaker, not a flask, because you need room for the pH electrode. not a flask, because you need room for the pH electrode. Record pH after the addition of every 1.0 ml of Record pH after the addition of every 1.0 ml of NaOH NaOH at first, at first, and as the pH begins to change more quickly, record and as the pH begins to change more quickly, record smaller volume increments, down to 0.2 or 0.1 ml. Try to smaller volume increments, down to 0.2 or 0.1 ml. Try to catch points on the vertical portion of the graph. catch points on the vertical portion of the graph. Switch roles with your lab partner and repeat the titration a Switch roles with your lab partner and repeat the titration a second time. second time. Graph pH (y Graph pH (y axis) versus ml added (x axis) versus ml added (x axis) in Excel. axis) in Excel. © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 11 At the Equivalence Point At the Equivalence Point All of the HA has been reacted away. All of the HA has been reacted away. If the solution was initially 0.1M acetic acid, If the solution was initially 0.1M acetic acid, it is now 0.05 M acetate it is now 0.05 M acetate At the half At the half equivalence point, half of the HA equivalence point, half of the HA has been reacted away, and the HA and has been reacted away, and the HA and A A – – concentrations are equal. concentrations are equal. © 2003 By Default! A Free sample background from www.awesomebackgrounds.com Slide 12 Half Half Equivalence Point Equivalence Point [H [H + + ][A ][A – – ] ] [HA] [HA] K K a a = = K K a a = [H = [H + + ] ] [A [A – – ] ] [HA] [HA] pH = pH = pK pK a a  log K log K a a = =  log [H log [H + + ] ] At the half At the half equivalence point, [HA] = [A equivalence point, [HA] = [A – – ]. ]. x5 © 2003 By Default! …