Equilibrium in Acid-Base Systems 16.4a: Interpreting pH curves

Titrations equivalence point- when the exact theoretical amount of titrant has been added (reaction is at completion) equivalence point- when the exact theoretical amount of titrant has been added (reaction is at completion) end point- an experimental amount that causes an observable change (colour, pH conductivity…) end point- an experimental amount that causes an observable change (colour, pH conductivity…) ideally the equivalence point and endpoint should be the same ideally the equivalence point and endpoint should be the same indicator- chemical substance that changes colour near equivalence point indicator- chemical substance that changes colour near equivalence point equivalence point is usually marked by the color change (end point) of an indicator equivalence point is usually marked by the color change (end point) of an indicator

Buffering and the pH curve the ability of some solutions of counteracting any significant change in pH when an acid or base is added the ability of some solutions of counteracting any significant change in pH when an acid or base is added there are two buffering regions in a monoprotic pH curve there are two buffering regions in a monoprotic pH curve at the start and end of the reaction at the start and end of the reaction in the middle is a sudden change in pH in the middle is a sudden change in pH the equivalence & end points should be found on this vertical portion the equivalence & end points should be found on this vertical portion

pH curve also called a titration curve also called a titration curve plotting of pH of the sample as a function of the amount of titrant added plotting of pH of the sample as a function of the amount of titrant added

Strong Acid and Strong Base very gradual changes in pH until close to the equivalence pt.

Weak Base and Strong Acid

Comparing Shapes

Acid-Base Indicators is a conjugate weak acid-weak base pair is a conjugate weak acid-weak base pair formed when an indicator dissolves in H 2 O formed when an indicator dissolves in H 2 O used to mark end of titration (equivalence point) used to mark end of titration (equivalence point) only works if you choose one whose end point is the same as equivalence point only works if you choose one whose end point is the same as equivalence point colour change occurs when H + is removed colour change occurs when H + is removed phenolphthalein HIn + H 2 O  H 3 O + + In - WA WB CWA CWB yellow blue

Polyprotic Entities acids/bases that can donate/accept more than one H + acids/bases that can donate/accept more than one H + always dissociates one H + at a time always dissociates one H + at a time Example: carbonic acid Example: carbonic acid H 2 CO 3  H + + HCO 3 - K a1 = 4.3x10 -7 H 2 CO 3  H + + HCO 3 - K a1 = 4.3x10 -7 HCO 3 -  H + + CO 3 2- K a2 = 5.6x HCO 3 -  H + + CO 3 2- K a2 = 5.6x K a1 > K a2 > K a3 for K a1 > K a2 > K a3 for typical weak polyprotic acid

Polyprotic pH curves - Polyprotic pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na 2 CO 3 + HCl ——> NaHCO 3 + NaCl Step 2 NaHCO 3 + HCl ——> NaCl + H 2 O + CO 2 Overall Na 2 CO 3 + 2HCl ——> 2NaCl + H 2 O + CO 2 Volume of HCl added is the same in both steps since mole ratio is the same in both. There are two sharp pH changes

Polyprotic pH curves - Polyprotic pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na 2 CO 3 + HCl ——> NaHCO 3 + NaCl Step 2 NaHCO 3 + HCl ——> NaCl + H 2 O + CO 2 Overall Na 2 CO 3 + 2HCl ——> 2NaCl + H 2 O + CO 2 First rapid pH change around pH = 8.5 due to the formation of NaHCO 3. Can be detected using phenolphthalein There are two sharp pH changes Second rapid pH change around pH = 4 due to the formation of acidic CO 2. Can be detected using methyl orange.

Homework Textbook p754 #1,2 Textbook p754 #1,2 Textbook p759 #5-9 Textbook p759 #5-9 LSM 16.4 A&F LSM 16.4 A&F