Exam (elaborations) CHEM 220 (CHEM 220)

MODULE 8: TEXTBOOK NOTES (CHAPTERS 9, 10 and 11) CHAPTER 9 → MONOPROTIC ACIDS AND BASES IN AQUEOUS SOLUTIONS The solvent is both acidic and basic: 2H O ⇌ H O+ + OH- K = 1.0x10-14 at 25℃ pKw = -logKw = 14 Is activity important? - Activity can be important. Normally we neglect activities in pH calculations unless it is clearly needed (ex. Titration in a 0.1M salt solution which we will not be considering) At 25℃ neutral pH (=-log[H+]) = 7 because [H+]=[OH-] = √Kw =7 - Table 6.1 will show the neutral pH at different temperatures (ex. 35℃ → body temperature) Acid-base titrations involve these reactions: Strong or weak acid or base + strong acid or base → products (analyte in the flask) + (titrant in the burette) → products (full arrow implies a very large K value for the reaction) Strong acid + strong base → salt + H2O Ex. HCl (analyte) + NaOH (titrant) → NaCl + H2O - Because the products are neither acidic or basic, the pH at the equivalence point, when we have added exactly enough titrant to consume all of the analyte, must be equal to 7 at 25℃ . - We always choose a strong acid or base for the titrant, but the analyte could be strong or weak. A weak acid → CH3COOH (acetic acid) is a weak acid used in weed killers and in vinegars. Compare: CH COOH ⇌ CH COO- + H+ K = 1.8x10-5 HCl ⇌ H+ + Cl- K = 104 **The larger the Ka value, the stronger the acid. The smaller the Ka value, the weaker the acid Titration with a strong base: CH COOH + NaOH ⇌ CH COO- + Na+ + H O - Na+ is the spectator ion Calculate Ktitration for the titration above: A weak base → NH3 (ammonia) used in cleaning products, smelling salts Compare: NH + H O ⇌ NH + + OH- K = 10-4.75 NaOH → Na+ + OH- K = infinity Titration with a strong acid: NH + HCl ⇌ NH + + Cl- - Cl- is the spectator ion NH + H+ ⇌ NH + K =1/K = K /K So what’s with pKa values and why does everyone use them? - A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water - For a weak acid or weak base we can calculate -log(Ka) for the acid or the conjugate acid of a weak base. The pKa is useful because above the pKa (pH pKa), the functional group or ion is deprotonated. Below the pKa (pHpKa), the function group is protonated. Strong base titrated with Strong acid - The curve is derived in an analogous manner, but before the equivalence point excess OH_ is present (thus the solution is initially basic). After the equivalence point, excess H+ is present (thus the solution is acidic) - 3 regions of the curve (before the equivalence point, at the equivalence point and after the equivalence point) Titration of a Weak acid with a strong base Ex. acetic acid titrated with NaOH There are now 4 region sto the titration curve: 1) Before the NaOH is added, only CH3CO2H is present (treat as a weak acid) 2) Before the equivalence point, CH3CO2H + CH3CO2Na (treat as buffer) 3) At the equivalence point, CH3CO2Na online (treat as weak base) 4) After the equivalence point, CH3CO2Na + NaOH (treat as strong base) Region 1 → before titrant is added [H+] = √Ka[CH3COOH] = √KaC CH3COOH , thus Ka = x2 (where x is small) Region 2 → before equivalence point - A mixture of CH COOH and CH COO- (a buffer) [CH3CO2H] = (mmol of CH3CO2H initially present - mmol NaOH added)/initial volume + volume NaOH added) [CH CO -] = mmol NaOH added/initial volume + volume NaOH added Then we can use the H-H equation: pH = pK + log[CH CO -]/[CH CO H] .......................................Continued..............................