Brønsted-Lowry Theory of Acids and Bases An acid is a proton donor. A base is a proton acceptor. baseacid base acid conjugate acid conjugate base NH 3 + H 2 O NH 4 + + OH - NH 4 + + OH - NH 3 + H 2 O conjugate acid conjugate base
Acid Ionization Constant Ka=Ka=Ka=Ka= = 1.8 x 10 -5 [CH 3 CO 2 H] [CH 3 CO 2 - ][H 3 O + ] CH 3 CO 2 H + H 2 O CH 3 CO 2 - + H 3 O + baseacid conjugate acid conjugate base
Relative Strengths of Some Brønsted- Lowry Acids and Bases
pH and pOH The potential of the hydrogen ion was defined in 1909 by Søren Sørensen as the negative of the logarithm of [H + ]. pH = -log[H 3 O + ] pOH = -log[OH - ] -logK W = -log[H 3 O + ]-log[OH - ]= -log(1.0 x 10 -14 ) K W = [H 3 O + ][OH - ]= 1.0 x 10 -14 pK W = pH + pOH = 14
pH and pOH Scales Despite stopping at 0 and 14, the scales are actually open-ended
pH Scale Range Calculate the pH of a 0.020 M HCl solution pH = -log [H 3 O + ] pH = 1.70 (Note that this must actually be measured)
Neutralization Reactions and Titration Curves Equivalence point:Equivalence point: –The point in the reaction at which both acid and base have reacted in stoichiometric amount. –Neither acid nor base is present in excess. End point:End point: –The point at which the indicator changes color. Titrant:Titrant: –The solution of known concentration added to the solution of unknown concentration. Titration Curve:Titration Curve: –The plot of pH vs. volume.
Titration of a Strong Acid with a Strong Base 10 mL of 0.10 M HCl titrated with 0.10 M NaOH
Titration of a Strong Acid with a Strong Base The pH has a low value at the beginning.The pH has a low value at the beginning. The pH changes slowlyThe pH changes slowly –until just before the equivalence point. The pH rises sharplyThe pH rises sharply –perhaps 6 units per 0.1 mL addition of titrant. The pH rises slowly again.The pH rises slowly again. Common acid-base indicator is phenolphthaleinCommon acid-base indicator is phenolphthalein
Buffer Solutions A mixture of a weak acid and its conjugate base that changes pH only slightly on addition of acid or base.A mixture of a weak acid and its conjugate base that changes pH only slightly on addition of acid or base. …Or, a weak base and its conjugate acid…Or, a weak base and its conjugate acid
Titration of a Weak Acid with a Strong Base— Determination of Equivalence Point Equivalence point is ½ the distance between tangent lines
Lab This Week Set-up LabQuest for pH determinationsSet-up LabQuest for pH determinations Use a base of known concentration (~0.10 M NaOH) to titrate an unknown acid. Base is already standardized.Use a base of known concentration (~0.10 M NaOH) to titrate an unknown acid. Base is already standardized. Determine the concentration and K a of each acidDetermine the concentration and K a of each acid Note that in buffer regions you can use 0.5-1.0 mL increments but in non-buffer regions use 3-5 dropsNote that in buffer regions you can use 0.5-1.0 mL increments but in non-buffer regions use 3-5 drops
Proposal How will you determine the molar concentration of the unknown acid?How will you determine the molar concentration of the unknown acid? How will you determine K a ?How will you determine K a ?