Buffers and Titration

CHEM Buffers and Titration Lecture 20 Today’s agenda z Equilibrium in acid-base systems z z Buffers z z Finish comparing strong vs. weak acids (and strong vs. weak bases) When approximately equal amounts of HA and A– are present in solution Titration z Predict what happens to pH as you add an acid to a base, or vice-versa Announcements z There is a graded extra credit assignment during discussion sections on Thursday Nov 20 and Tuesday Nov 25. You must attend discussion to do it. z Next exam is Exam 3 on Tuesday, Dec 2 (first lecture after Thanksgiving break) – note date change from syllabus z Final exam has been scheduled for Tuesday, Dec 16, 3:00pm There are only three type of acid-base problems Predict the pH: given the amount of acid, base and/or salt added to some water, predict the pH of the solution. 1. z z z How you approach calculating the pH depends on what you added to the water, so that’s the first thing you have to figure out Variables are: strong or weak, acid or base “Salts” in the Bronsted-Lowry scheme are actually acids or bases – their conjugates are more familiar to you Equilibrium: given the measured pH of a solution, figure out how much acid, base or salt must have been added to some water to make the pH be that value. 2. z These are always equilibrium problems Titration: given a solution of unknown (acid or base) concentration, neutralize it with a known amount of (base or acid) to figure out the unknown concentration. 3. z z Involves stoichiometry since a neutralization reaction is occurring If a weak acid or base is involved, it will also involve equilibrium calculations Comparing strong and weak acids Strong acid 0.020 M HCl solution z Acid dissociates completely Weak acid 0.020 M CH3COOH solution z z z z z z [H+] is equal to [HCl] [H+] = 0.020 M pH = 1.70 z z Acid does not dissociate completely Need to know Ka to solve Must use equilibrium calculation to solve [H+] ≈ C A ⋅ K a = 0.00060 M pH = 3.22 How to recognize strong vs. weak acids Memorize the strongest acids z All halides except fluoride: HCl, HBr, HI z Nitric acid: HNO3 z Sulfuric acid (only the first H+): H2SO4 z Perchloric acid: HClO4 Weak acids are listed in the Ka table Acids and Bases in general: What you (will) need to be able to do z z z z Identify conjugate acid-base pairs and predict reactions Equilibrium Titration Buffers z Equations to use as shortcuts for solving problems Strategies to master: Using the math tricks to solve problems z Deciding on the right approach to solving a problem: recognizing acid-base equilibrium problems z Recognizing hydrolysis reactions – “hydrolysis” is a fancy name for adding a weak acid or weak base to water (unfortunately referred to as a “salt” because it’s the conjugate that happens to be more familiar) z Adding a “salt” to water z z z Is the salt a conjugate of a strong acid/base or of a weak acid/base? If it is a salt of a strong acid or base, then nothing will happen (like adding table salt to water – no change in pH). If it is a conjugate of a weak acid or base, then the “salt” is itself also a weak base or acid. So it hydrolyzes and makes some H+ or OH-, which changes the pH. Acid-base properties of salt solutions: hydrolysis pH 7.0 NaCl (aq) pH 3.5 NH4Cl (aq) pH 9.5 NaClO (aq) When you add a salt to water, if it is soluble to any extent, it breaks apart into its constituent + and – ions. These ions can be weak acids or weak bases themselves. If they are, they “hydrolyze” to form either H+ or OH ¯, which changes the pH away from neutral pH 7 of the water. Hydrolysis of a salt: comparing weak vs. strong Salt of a strong acid z What is the pH of a 0.020 M solution of NaBr? z Is Na+ a conjugate of anything? No. z Is Br– a conjugate of anything? Yes. Of HBr. z Is HBr strong or weak? z HBr is a strong acid, so Br– is a very weak base. Salt of a weak acid z What is the pH of a 0.020 M solution of NaBrO? z Is Na+ a conjugate of anything? No. z Is BrO– a conjugate of anything? Yes. Of HBrO. z Is HBrO strong or weak? z HBrO is a weak acid, so BrO– is a weak base, but not very weak. Br – + H2O ' HBr + OH– BrO – + H2O ' HBrO + OH– z z Br- Ka for HBr is very large, so Kb for is very small. Equilibrium lies so strongly to the left that OH– does not get produced in significant enough quantity to rival 1.0×10–7 M that exists in water. z z Ka for HBrO is 2.5 ×10–9, so Kb for BrO- is 4.0×10–6. Rxn occurs to enough extent that OH– gets produced in significant enough quantity to make solution basic. Hydrolysis example Exercise similar to 16.17, p. 701 Which of the following salts, when added to water, would produce the most acidic solution? a) b) c) d) KBr NH4NO3 AlCl3 Na2HPO4 Acid-base equilibrium Acid HA or HB+ can be Strong: HCl HNO3 HClO4 H2SO4 can be Weak dissociates calculations involve To figure out [H+]: Determine how many moles of strong acid dissolved in how many liters of water calculations involve B + H2O To figure out [H+]: Set up ICE table Either solve using quadratic formula, or make approximations to get [H + ] ≈ Weak A dissociates partially ' K b equilibrium dissociates completely HA + H2O ' A– + H3O+ A– + H3O+ can be Strong: has an OH– ion Ka partially equilibrium a few others Base A– or B conjugate partner of can be dissociates completely HA + H2O Buffer if conjugate weak partners are present in approx equal molar quantities ⋅ Ka HB+ + OH– [OH − ] ≈ CB ⋅ K b To figure out [OH–]: Determine how many moles of strong base dissolved in how many liters of water pOH pH pH + pOH=14 pH pH pOH pH