Chapter 15Aqueous Equilibria: Acids and Bases
15.8 Acid-Base Properties of Salts
pH of a salt solution is determined by the acid-base properties of the consistuent cations and anions
◦In an acid-base reaction, the influence of the stronger partner is predominant
◦Strong acid + Strong Base Neutral solution
◦Strong acid + Weak Base Basis solution
◦Weak acid + Strong Base Acidic solution
Neutral Salt
A salt of a strong base and a strong acid. E.g NaCl
Neutral cation + neutral anion neutral salt
Na+Cl- NaCl
Na+(aq) + H2O(l) NR
Cl –(aq) + H2O(l) NR
Cations from strong bases: group 1A and 2A metals (Ca2+, Sr2+, Ba2+)
- Anions from strong monoprotic acids: Cl-, Br-, I-, NO3- and ClO4-
Basic Salts
A salt of a strong base and a weak acid.
E.g NaCN
Neutral cation + basic anion basic salt
Na+ CN-NaCN
Na+(aq) + H2O(l) NR
CN-(aq) + H2O(l) HCN(aq) + -OH(aq)
Acidic Salts: A salt of a weak base and a strong acid.
E.g NH4Cl
Acidic cation + neutral anion Acidic salt
NH4+ + Cl- NH4Cl
NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq)
Cl –(aq) + H2O(l) NR
Acid-Base Properties of Salts
A salt of a weak base and a weak acid
Acidic cation + basic anion (50 :50 mixture) must compare Ka and Kb
•KaKb: The solution will contain an excess of H3O1+ ions (pH < 7).
•KaKb: The solution will contain an excess of OH1- ions (pH > 7).
•Ka = Kb: The solution will contain approximately equal concentrations of H3O1+ and OH1- ions (pH ≈ 7).
Determine each of the following salt solutions as acidic, basic or neutral. Write the hydrolysis of the salt solution
KNO2KBrNH4F
Determining the pH of a salt solution
Find the pH of a 0.100 M NaCHO2 solution. The salt completely dissociates into Na+(aq) and CHO2- and Na+ has no acidic or basic properties.
Predict whether 0.24M NH4Br salt solution is acidic, basic or neutral then determine the pH of the salt solution.
Acid-Base Properties of Salts
Ionization in Polyprotic Acids
•has a separate Ka.
•Ka1Ka2Ka3
•Generally, the difference in Ka values is great enough so that the second ionization does not happen to a large enough extent to affect the pH.
–Most pH problems just do first ionization.
–Except H2SO4 uses [H2SO4] as the [H3O+] for the second ionization.
•[A2−] = Ka2as long as the second ionization is negligible.
Ionization in H2SO4
•The ionization constants for H2SO4 are as follows:
H2SO4 + H2O HSO4 + H3O+strong
HSO4 + H2O SO42 + H3O+Ka2 = 1.2 × 10−2
•For most sulfuric acid solutions, the second ionization is significant and must be accounted for.
•Because the first ionization is complete, use the given [H2SO4] = [HSO4−]initial = [H3O+]initial.
15.9Strengths of Binary Acids
Relationship between Bond Strength and Acidity
The strength of H—A bond also affects the strength of the corresponding acid. The stronger the bond, the weaker the acid. The more tightly the hydrogen atom is held, the less likely it is to come off.
*Don’t confuse with bond polarity.
Acid / Bond Energy (kJ/mol) / Type of AcidHF / 565 / weak
HCl / 431 / strong
HBr / 364 / strong
Strengths of Oxyacids, H–O–Y
•The more electronegative the Y atom, the stronger the oxyacid.
–HClO > HIO
–Acidity of oxyacids decreases down a group.
•Same trend as binary acids
–Helps weaken the H–O bond.
•The larger the oxidation number of the central atom, the stronger the oxyacid.
–H2CO3 > H3BO3
–Acidity of oxyacids increases to the right across a period.
•Opposite trend of binary acids
•The more oxygens attached to Y, the stronger the oxyacid.
–Further weakens and polarizes the H–O bond
–HClO3 > HClO2
AcidH─O─Y / Electronegativity of Y / Ka
H─O─Cl / 3.0 / 2.9 × 10−8
H─O─Br / 2.8 / 2.0 × 10−9
H─O─I / 2.5 / 2.3 × 10−11
15.10 Lewis Acids and Bases
Lewis Acid: An electron-pair acceptor.
• Include cations and neutral molecule having vacant valence orbitals that can accept a share in a pair of electrons from a Lewis Base
Lewis Base: An electron-pair donor.
• All Lewis bases are Bronsted-Lowry bases
Examples
For each of the following reactions, identify the Lewis acid and the Lewis base
◦CO2(g) + -OH(aq) HCO3-(aq)
◦B(OH)3(aq) + H2O(l) B(OH)4-(aq) + H+ (aq)
What Is Acid Rain
•Natural rain water has a pH of 5.6.
–Naturally slightly acidic due mainly to CO2
•Rain water with a pH lower than 5.6 is called acid rain.
Acid rain is linked to damage in ecosystems and structures
•Many natural and pollutant gases dissolved in the air are nonmetal oxides.
–CO2, SO2, NO2
•Nonmetal oxides are acidic.
CO2(g) + H2O(l) H2CO3(aq)
2 SO2(g) + O2(g) + 2 H2O(l) 2 H2SO4(aq)
4 NO2(g) + O2(g) + 2 H2O(l) 4 HNO3(aq)
•Processes that produce nonmetal oxide gases as waste increase the acidity of the rain.
–Natural – volcanoes and some bacterial action
–Man made – combustion of fuel
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