CHEM 120 Lecture Notes - Lecture 19: Acid Strength, Ammonium Chloride, Weak Base
15 Jun 2018
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%Dissociation: same as every time you do the 5% approximation
- At low concentration, weak acids are highly ionized
- Higher concentration does not favour equilibrium
Most bases are OH- group connected to R molecule → e.g. most amine groups, CN ion,
Weak Acids & Conjugate Bases: KB =
- The conjugate of a weak acid is a weak base
- A- + H2O OH- + HA (anion is base)
- HA is acidic while A- is basic; HF + H20 → H30+ F- {Ka = Kw/KB}
o HF is a weak acid, so eq. is to the left
o [HF]>>[F-] & [H3O]>> [OH-] solution is acidic
- F- + H2O → OH- + HF
o F is a weak base, eq. is also to the left
o [F-]>> [HF] & [OH-] >>[H3O] solution is basic
Salt Solutions:
- Salts of strong bases/acids (e.g. NaCl) they do not react with water so do not hydrolyze (i.e.
pH= 7)
- Salts of strong bases/weak acids (e.g. NaCH3CO2) will hydrolyze: pH > 7 (anion is base)
- Salt of a weak base/strong acid (e.g. NH4Cl) hydrolyze: pH < 7 (cation is acid)
IF the initial [CH3CO2-] anions and [NH4+] cations equal, overall pH WILL DEPEND on eq. of base
hydrolysis and acid dissociation reactions
- Kb > Ka, pH > 7 more basic
- Ka > Kb, pH < 7 more acidic
Polyprotic Acids: yields >1 Hydrogen molecules
- Stepwise ionization; loses 1 proton at a time
- Ionization constant written for each stage
- Two eq. constants used to calculate the concentration of species
o E.g. H2CO3 HCO3- + H+
- Ka1>Ka2 the pH is solely determined by [H3O] recorded in Ka1 process
- CO3 is a polyprotic base; can for into CO2 + H as well
Strong Acids/Bases:
- Acid strength increases as the polarity of the bond increases (left to right of p.table)
- Acid strength increases as the length of the bond increase (down a group)
o E.g. HF << HCl < HBr < HI
Oxoacids: binary acids - occur when H is attached to O attached to essential non-metal
o Acid strength increases with increasing EN (up a group) HClO > HBrO
Case 1: acids with different central atoms have the same oxidation number
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CHEM 120 Full Course Notes
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Document Summary
%dissociation: same as every time you do the 5% approximation. At low concentration, weak acids are highly ionized. Most bases are oh- group connected to r molecule e. g. most amine groups, cn ion, Weak acids & conjugate bases: kb = [+][ ] A- + h2o oh- + ha (anion is base) The conjugate of a weak acid is a weak base. Ha is acidic while a- is basic; hf + h20 h30+ f- {ka = kw/kb: hf is a weak acid, so eq. is to the left. F- + h2o oh- + hf: f is a weak base, eq. is also to the left. [f-]>> [hf] & [oh-] >>[h3o] solution is basic. Salts of strong bases/acids (e. g. nacl) they do not react with water so do not hydrolyze (i. e. Salts of strong bases/acids (e. g. nacl) they do not react with water so do not hydrolyze (i. e. ph= 7)
