Weak Acid-Strong Base And Strong Acid Weak Base

Arrhenius Acids and Bases Acids are a source of H + ions HCl (aq)  H + (aq) + Cl - (aq) Bases are a source of OH - ions NaOH (aq)  Na + (aq) + OH - (aq) Acid-Base Neutralizations reactions always have the same net ionic reaction H + (aq) + OH - (aq)  H 2 O (l)

Strong and Weak Acids Acids are very corrosive How much [H + ](aq) determines the strength Depends on molarity of the acid and how much it ionizes Strong acids ionize 100% into H + ions HCl (aq)  H + (aq) + Cl - (aq) Weak acids only partially ionize into ions CH 3 COOH(aq) CH 3 COO - (aq) + H + (aq)

Strong and Weak Acids We measure the strength of an acid using K a, or pK a Acetic Acid has a pKa = log 10 (1.66E-4) = 4.78 The pK a is a way to identify what your weak acid is CH 3 COOH(aq) CH 3 COO - (aq) + H + (aq)

Measuring [H + ]: pH Scale Since at the end of the day the strength of an acid comes down to [H + ] we need a measure for it Since for weak acids the amount of [H + ] also depends on K a we cannot just use molarity of the acid Since [H + ] = M (15 orders of magnitude) we use a logarithmic scale pH = -log 10 [H + ]

Measuring [H + ]: pH Scale pH = -log 10 [H + ] or [H + ]=10 -pH The smaller the pH the stronger the acid pH water = 7 (neutral) pH 7 basic (alkaline) [H + ]1M1M 0.1 M 0.01 M M M M M M M log 10 [H + ] pH Increasing Acidity (pH decreases) Increasing Basicity (pH decreases)

Measuring [H + ]: pH Scale pH = -log 10 [H + ] or [H + ]=10 -pH We can measure pH with a pH meter A pH meter has a semi-permeable membrane that allows H + ions to pass through. The bigger [H+] is, the bigger the current/voltage is Software converts the voltage to pH

Titration Curve A titration curve is obtained when an acid is titrated with a base, and the pH of the resulting solution graphed as a function of the amount of titrant added via the burette The equivalent point (where the acid is completely neutralized by the base) occurs at the center of the characteristic S curve A Titration curve can be used to measure K a of a weak acid or base

Measuring pK a of a weak acid CH 3 COOH (aq) + NaOH (aq)  NaCH 3 COO (aq) + H 2 O (l)

Tonight Groups involve up to 4 people Titrate 2O mL of your unknown acid with the standardized NaOH Phenolphthalein indicator (2 trials) Equivalence point (just before the solution turns permanently pink One titration curve per group Calibrate with two buffer solutions Determine K a and identify the unknown acid mL of your acid / 20 mL of deionized H 2 O Magnetic stirrer 5 drops phenolphthalein Standardized NaOH