CHEM 120 Lecture Notes - Lecture 20: Weak Base, Acid Dissociation Constant, Equivalence Point

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carmineminnow41

15 Jun 2018

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Activity: effective ionic concentration (a), applied to all terms

- Activity of pure water is constant

- Activity of dilute solutions: aHA → [HA]aq/co where c0 = 1 M

5% approximation: simplify equations by ignoring x when the dissociating constant is very small,

less than 5% decomposition/ionization change ( )

Levelling Effect: the inability of a solvent to differentiate among the relative strengths of acids

- All strong acids/bases are equally strong in water because the strongest acid possible is

H3O and the strongest base is OH-

- H20 exerts a levelling effect on any strong base/acid

- Differentiating Effect: same as levelling; in a solution with a weak base, such as acetic acid,

protonation doesnt occur to completion so strong acid particles remains (as a result,

relative strength of the acid can be calculated)

Common Ion Effect: the shift in eq. caused by the addition of a compound having an ion in common

with a dissolved substance; additions are common to eq. & suppresses the ionization of a weak

acid/base

When a salt supplies the

common anion CH3COO- the eq. shifts to form more CH3COOH

- The addition of CH3COO reduces the %dissociation of the acid

- Reaction shifts to the left when a dissolved substance is added to a solution with a common

ion

o Reaction always shifts where the concentration is almost 0

- If you add NH4 to a solution, reaction will shift to form more NH3

Buffers: systems that resist pH change by adding:

- weak base & conj. acid OR weak acid & conj. base or incomparable (equimolar amounts)

Henderson – Hasselbalch Equation: pH = pKa + log[(conj.base)/(acid)] → pH = pKa

- equation only works when, 0.1 < [A-]/[HA] < 10, [A-] > 10 Ka, [HA] > 10 Ka

A buffers ability to convert a strong acid to a weak acid minimizes the effect of the addition on the

pH; water can not work as a buffer too low

Steps for a Buffer Solution:

- weak acid

- weak acid & salt

- salt of a weak acid

- vis-versa for weak bases

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Activity: effective ionic concentration (a), applied to all terms. Activity of dilute solutions: aha [ha]aq/co where c0 = 1 m. 5% approximation: simplify equations by ignoring (cid:494)x(cid:495) when the dissociating constant is very small, less than 5% decomposition/ionization change ( []0 ) Levelling effect: the inability of a solvent to differentiate among the relative strengths of acids. All strong acids/bases are equally strong in water because the strongest acid possible is. H20 exerts a levelling effect on any strong base/acid. Differentiating effect: same as levelling; in a solution with a weak base, such as acetic acid, protonation doesn(cid:495)t occur to completion so strong acid particles remains (as a result, relative strength of the acid can be calculated) Common ion effect: the shift in eq. caused by the addition of a compound having an ion in common with a dissolved substance; additions are common to eq. & suppresses the ionization of a weak acid/base.