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OUTCOMES FOR U4TE: IT IS EXPECTED THAT YOU WILL
1) determine whether an amphiprotic ion will act as an acid or a base in solution.
2) write a dissociation equation for a salt in water.
3) write net ionic equations representing the hydrolysis of salts.
4) predict qualitatively whether a salt solution will be acidic, basic or neutral.
Ka, Kb AND AMPHIPROTIC IONS
I) The Ka and Kb can be used to determine whether an AMPHIPROTIC ION will form an acidic or basic solution. An
AMPHIPROTIC substance is one, which can act as either an acid or a base depending upon the conditions of the
solution.
A) Consider these examples.
1) The HSO3− ion is amphiprotic because it can act as an acid by donating a proton to form SO32−, or it can act
as a base by accepting a proton to form H2SO3 ions.
acting as an acid: HSO3−(aq) + H2O(l)  SO32−(aq) + H3O+(aq)
acting as a base: HSO3−(aq) + H2O(l)  H2SO3(aq) + OH−(aq)
2) The HS− ion is amphiprotic because it can act as a base by accepting a proton to form H2S, or it can act as
an acid by donating a proton to form S2− ions.
acting as a base: HS−(aq) + H2O(l)  H2S(aq) + OH−(aq)
acting as an acid: HS−(aq) + H2O(l)  S2−(aq) + H3O+(aq)
B) Since an amphiprotic substance can act as either an acid or a base, it has both a Ka and a Kb. i.e. The
amphiprotic ion HC2O4− has both a Ka and a Kb: Ka(HC2O4−) = 6.4 x 10−5, Kb(HC2O4−) = 1.6 x 10−13
1) If its Ka is greater than its Kb, it will tend to act as an acid.
2) If its Ka is less than its Kb, it will tend to act as a base.
3) Since Ka(HC2O4−) is greater than Kb(HC2O4−), HC2O4− will hydrolyze as an acid.
C) Use these steps to determine whether a solution containing an amphiprotic ion will be acidic, basic or neutral.
1: Obtain the Ka of the AMPHIPROTIC ION from the Relative Strength of Brönsted-Lowry Acids and
Bases table on page 6 of your Data Booklet.
2: Determine the formula of its CONJUGATE ACID by writing the HYDROLYSIS EQUATION for the ion
when it acts as a base.
3: Use the Relative Strength of Brönsted-Lowry Acids and Bases table on page 6 of your Data Booklet
to obtain the Ka of the conjugate acid of the AMPHIPROTIC ION, list it, then use the formula below to
calculate the Kb for the AMPHIPROTIC ION.
Kw = Ka • Kb
4: Compare the Ka to the Kb.
a: If the Ka is greater than the Kb, the ion acts as an acid and the solution is acidic.
b: If the Ka is less than the Kb, the ion acts as a base and the solution is basic.
c: If the Ka is equal to the Kb, the ion is neither an acid nor a base; the solution is neutral.
D) SAMPLE PROBLEMS 1
1. Will a solution containing 0.020 M HSO3− be acidic, basic or neutral?
1: Obtain the Ka of the AMPHIPROTIC ION from the Relative Strength of Brönsted-Lowry Acids and
Bases table on page 6 of your Data Booklet.
Ka(HSO3−) = 1.0 x 10−7
2: Determine the formula of its CONJUGATE ACID by writing the HYDROLYSIS EQUATION for the ion
when it acts as a base.
HSO3−(aq) + H2O(l)  H2SO3(aq) + OH−(aq)
© Rob Ashby 2014. Duplication by permission only.
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3: Use the Relative Strength of Brönsted-Lowry Acids and Bases table on page 6 of your Data Booklet
to obtain the Ka of the conjugate acid of the AMPHIPROTIC ION, list it, then use the formula below to
calculate the Kb for the AMPHIPROTIC ION.
Kw = Ka • Kb = Ka(H2SO3) x Kb(HSO3−)
(
)
K b HSO3− =
Kw
K a ( H 2SO3)
−14
= 1.0 x 10 −2 = 6.6666 x 10−13 = K b ( HSO3−) = 6.7 x 10−13
1.5 x 10
4: Compare the Ka to the Kb.
a: If the Ka is greater than the Kb, the ion acts as an acid and the solution is acidic.
b: If the Ka is less than the Kb, the ion acts as a base and the solution is basic.
c: If the Ka is equal to the Kb, the ion is neither an acid nor a base; the solution is neutral.
Ka(HSO3−) = 1.0 x 10−7 is greater than Kb(HSO3−) = 6.7 x 10−13
Therefore the solution is acidic.
2. Will a solution of 0.10 M HPO42− be acidic, basic or neutral?
1: Obtain the Ka of the AMPHIPROTIC ION from the Relative Strength of Brönsted-Lowry Acids and
Bases table on page 6 of your Data Booklet.
Ka(HPO42−) = 2.2 x 10−13
2: Determine the formula of its CONJUGATE ACID by writing the HYDROLYSIS EQUATION for the ion
when it acts as a base.
HPO42−(aq) + H2O(l)  H2PO4−(aq) + OH−(aq)
3: Use the Relative Strength of Brönsted-Lowry Acids and Bases table on page 6 of your Data Booklet
to obtain the Ka of the conjugate acid of the AMPHIPROTIC ION, list it, then use the formula below to
calculate the Kb for the AMPHIPROTIC ION.
Kw = Ka • Kb = Ka(H2PO4−) x Kb(HPO42−)
(
)
K b HPO42− =
−14
Kw
= 1.0 x 10 −8 = 1.612 x 10−7 = K b ( HPO42−) = 1.6 x 10−7
−
6.2 x 10
K a ( H 2PO4 )
4: Compare the Ka to the Kb.
a: If the Ka is greater than the Kb, the ion acts as an acid and the solution is acidic.
b: If the Ka is less than the Kb, the ion acts as a base and the solution is basic.
c: If the Ka is equal to the Kb, the ion is neither an acid nor a base; the solution is neutral.
Ka(HPO42−) = 2.2 x 10−13 is less than Kb(HPO42−) = 1.6 x 10−7
Thus the solution is basic.
E) REQUIRED PRACTICE 1: Will these solutions be acid or basic? {Answers are on page 8}
1. 1.0 M HCO3−
2. 0.0045 M H2PO4−
3. 3.49 x 10-4 M HC2O4−
4. 2.000 M HC6H5O72−
REVIEW OF SALTS [REF: pg. 598]
I) Definition of a SALT: According to ARRHENIUS THEORY, a SALT is any ionic compound whose cation is not H+
and whose anion is not OH−.
e.g. NaCl, NH4F, Ca3(PO4)2, CuCO3, PbO, PbO2, etc.
A) Formation of salts [20-17]: salts are formed through three major reactions.
© Rob Ashby 2014. Duplication by permission only.
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1) The reaction between two elements; a metal (M) with a non-metal (N-M):
M
N-M
salt
2Na(s) + Cl2(g) → 2NaCl(aq)
2) The reaction between a metal (M) and an acid (A)
M
A
salt
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
3) Neutralization reactions between acids (A) and bases (B).
a) Strong Acids with Strong Bases
SA
SB
salt
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
b) Strong Acids with Weak Bases
SA
WB
salt
HBr(aq) + NH3(aq) → NH4Br(aq)
c) Weak Acids with Strong Bases
WA
SB
salt
CH3COOH(aq) + KOH(aq) → KCH3COO(aq) + H2O(l)
BEHAVIOR OF SALTS IN WATER [20-18]
I) When a salt dissolves in water, it dissociates to produce positive ions called cations and negative ions called
anions.
NaCl(aq) → Na+(aq) + Cl−(aq)
NH4Br(aq) → NH4+(aq) + Br−(aq)
KCH3COO(aq) → K+(aq) + CH3COO−(aq)
A) REMEMBER: SALTS are very often produced as a result of a neutralization reaction between an acid and a
base!! This means, the anion produced in solution is the conjugate of an acid, and the cation produced in
solution is often the conjugate of a base.
1) NaCl dissociates to produce Na+(aq) + Cl−(aq) ions: NaCl(aq) → Na+(aq) + Cl−(aq)
a) Na+ is the conjugate of the strong base NaOH: NaOH(aq) → Na+(aq) + OH−(aq)
b) Cl− is the conjugate of the strong acid HCl: HCl(aq) → H+(aq) + Cl−(aq)
2) NH4Br dissociates to produce NH4+(aq) + Br−(aq) ions: NH4Br(aq) → NH4+(aq) + Br−(aq)
a) NH4+ is the conjugate acid of the weak base NH3: NH4+(aq) → H+(aq) + NH3(aq)
b) Br− is the conjugate of the strong acid HBr: HBr(aq) → H+(aq) + Br−(aq)
3) KCH3COO dissociates to produce K+(aq) + CH3COO−(aq) ions: KCH3COO(aq) → K+(aq) + CH3COO−(aq)
a) K+ is the conjugate of the strong base KOH: KOH(aq) → K+(aq) + OH−(aq)
b) CH3COO− is the conjugate base of the weak acid CH3COOH: CH3COOH(aq) → H+(aq) + CH3COO−(aq)
II) HYDROLYSIS of SALTS [20-18]
A) When a salt dissociates in water, the cation, anion or both may take part in a hydrolysis reaction.
HYDROLYSIS is defined as the reaction of a substance with water.
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U4TE – ACID-BASE PROPERTIES OF SALTS
CH 12
+
1) The cation NH4 will hydrolyze as follows: NH4
+
(aq)
+ H2O(l)  NH3(aq) + H3O
+
4
(aq)
a) Thus HYDROLYSIS of CATIONS produces acidic solutions.
2) The anion CH3COO− will hydrolyze as follows: CH3COO−(aq) + H2O(l)  CH3COOH(aq) + OH−(aq)
b) Thus HYDROLYSIS of ANIONS produces basic solutions.
B) ONLY IONS THAT ARE CONJUGATE ACIDS OF WEAK BASES, OR CONJUGATE BASES OF WEAK
ACIDS HYDROLYZE !!
1) These anions NEVER hydrolyze because they are conjugate anions of strong acids.
a) Cl− because it is the conjugate anion of the strong acid HCl.
b) Br− because it is the conjugate anion of the strong acid HBr.
c) I− because it is the conjugate anion of the strong acid HI.
d) NO3− because it is the conjugate anion of the strong acid HNO3.
e) ClO4− because it is the conjugate anion of the strong acid HClO4.
i) Each of the above anions has a Kb while each acid has a Ka. These anions do not hydrolyze, react
with water, because their Kb values are very much smaller than the Ka values of their conjugate acids,
for example, the Kb(Cl−) is very much smaller than the Ka(HCl). This means that Cl− is far too weak a
base to hydrolyze, react with water.
2) These cations NEVER hydrolyze because they are conjugate cations of strong bases.
a) Group 1 elements: ALKALI METALS: Li+, Na+, K+, Rb+, Cs+, Fr+. i.e. Li+ comes from the strong base
LiOH; K+ comes from the strong base KOH.
b) Group 2 elements: ALKALINE EARTH METALS: Mg2+, Ca2+, Sr2+, Ba2+, Ra2+.
NOTE: Be2+ IS THE ONLY GROUP 2 ION THAT WILL HYDROLYZE.
C) REMEMBER: When a SALT dissociates, its
1) anions produced are conjugates of either a strong or a weak acid.
a) If the acid is weak, its ANION is the conjugate base of the acid.
b) If the acid is strong, its ANION is not a base.
c) ANIONS FOUND IN STRONG ACIDS DO NOT HYDROLYZE.
2) cations produced are conjugates of either a strong or weak base.
a) If the base is weak, its CATION is the conjugate acid of the base.
b) If the base is strong, its CATION is not an acid.
III) Predicting qualitatively whether a salt solution be acidic, basic or neutral.
1) Use this flow chart to determine whether or not a substance will hydrolyze as an acid or base.
SALT
CATION is a Group 1
or 2 ion. {except Be2+}
YES
CATION will
NOT HYDROLYZE.
is the conjugate
of a strong acid.
ANION
NO
YES
CATION WILL
HYDROLYZE as a
WEAK ACID. Remember
NO
ANION is
AMPHIPROTIC.
ANION WILL
HYDROLYZE. Is it
AMPHIPROTIC?
the Complex Ions.
NO
YES
ANION will NOT
HYDROLYZE.
YES
Compare its Ka
to its Kb.
NO
ANION WILL
HYDROLYZE as
a WEAK BASE.
© Rob Ashby 2014. Duplication by permission only.
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B) Use these steps to predict whether a salt will be acidic, basic or neutral.
1: Write the SALT’S dissociation equation to identify its CATION and ANION.
2: Use the above flow chart to predict which ion will hydrolyze.
a: If the cation does hydrolyze, write its hydrolysis equilibrium.
b: If the anion does hydrolyze, write its hydrolysis equilibrium.
3. Compare the results of the flow chart.
a: If the cation hydrolysis, the salt will be acidic.
b: If the anion hydrolysis, the salt will be basic.
c: If BOTH the cation and the anion hydrolyze, compare the Ka(cation) to the Kb(anion).
i: If the Ka is greater than the Kb, the salt will be acidic.
ii: If the Kb is greater than the Ka, the salt will be basic.
iii: If the Kb is equal to the Ka, the salt will be neutral.
C) SAMPLE PROBLEMS 2
1. Predict whether Ba(NO2)2 will be acidic, basic or neutral. Write hydrolysis equations for each ion that
undergoes hydrolysis.
1: Write the SALT’S dissociation equation to identify its CATION and ANION.
Ba(NO2)2(aq) → Ba2+(aq) + 2NO2−(aq)
2: Use the above flow chart to predict which ion will hydrolyze.
a: If the cation does hydrolyze, write its hydrolysis equilibrium.
b: If the anion does hydrolyze, write its hydrolysis equilibrium.
Ba2+ is a GROUP 2 ion, ∴ IT WILL NOT HYDROLYZE.
−
ANION: NO2 is the conjugate base of a weak acid, ∴ IT WILL HYDROLYZE AS A BASE.
NO2−(aq) + H2O(l)  HNO2(aq) + OH−(aq)
CATION:
3. Compare the results of the flow chart.
a: If the cation hydrolyzes, the salt will be acidic.
b: If the anion hydrolyzes, the salt will be basic.
c: If BOTH the cation and the anion hydrolyze, compare the Ka(cation) to the Kb(anion).
i: If the Ka is greater than the Kb, the salt will be acidic.
ii: If the Kb is greater than the Ka, the salt will be basic.
iii: If the Kb is equal to the Ka, the salt will be neutral.
Only the ANION HYDROLYZES, so the solution is BASIC.
2. Predict whether NH4ClO4 will be acidic, basic or neutral. Write hydrolysis equations for each ion that
undergoes hydrolysis
1: Write the SALT’S dissociation equation to identify its CATION and ANION.
NH4ClO4(aq) → NH4+(aq) + ClO4−(aq)
2: Use the above flow chart to predict which ion will hydrolyze.
a: If the cation does hydrolyze, write its hydrolysis equilibrium.
b: If the anion does hydrolyze, write its hydrolysis equilibrium.
CATION:
ANION:
NH4+ is NOT A GROUP 1 OR 2 ion, ∴ IT WILL HYDROLYZE AS AN ACID.
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
ClO4− is the conjugate anion of a strong acid, ∴ IT WILL NOT HYDROLYZE.
3. Compare the results of the flow chart.
a: If the cation hydrolyzes, the salt will be acidic.
b: If the anion hydrolyzes, the salt will be basic.
Continued on the next page.
© Rob Ashby 2014. Duplication by permission only.
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c: If BOTH the cation and the anion hydrolyze, compare the Ka(cation) to the Kb(anion).
i: If the Ka is greater than the Kb, the salt will be acidic.
ii: If the Kb is greater than the Ka, the salt will be basic.
iii: If the Kb is equal to the Ka, the salt will be neutral.
Only the CATION HYDROLYZES so the solution is ACIDIC.
D) REQUIRED PRACTICE 2: Predict whether or not the following salts will be acidic, basic, or neutral. Write
hydrolysis equations for each ion that undergoes hydrolysis. {Answers are on page 8}
1. NH4I
2. Ca(NO3)2
3. KCl
4. KCN
5. LiCH3COO
E) SAMPLE PROBLEMS 3
1. Predict whether (NH4)2SO3 will be acidic, basic or neutral.
1: Write the SALT’S dissociation equation to identify its CATION and ANION
(NH4)2SO3(aq) → 2NH4+(aq) + SO32−(aq)
2: Use the above flow chart to predict which ion will hydrolyze.
a: If the cation does hydrolyze, write its hydrolysis equilibrium.
b: If the anion does hydrolyze, write its hydrolysis equilibrium.
CATION:
ANION:
NH4+ is NOT A GROUP 1 OR 2 ion, ∴ IT WILL HYDROLYZE AS AN ACID.
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
SO32- is the conjugate base of a WEAK ACID, ∴ IT WILL HYDROLYZE AS A BASE.
SO32−(aq) + H2O(l)  HSO3−(aq) + OH−(aq)
3. Compare the results of the flow chart.
a: If the cation hydrolyzes, the salt will be acidic.
b: If the anion hydrolyzes, the salt will be basic.
c: If BOTH the cation and the anion hydrolyze, compare the Ka(cation) to the Kb(anion).
i: If the Ka is greater than the Kb, the salt will be acidic.
ii: If the Kb is greater than the Ka, the salt will be basic.
iii: If the Kb is equal to the Ka, the salt will be neutral.
Ka(cation): Ka(NH4+) = 5.6 x 10−10
calculate Kb(anion): K b (SO32−) =
−14
Kw
= 1.0 x 10 −7 = 1.0 x 10−7 = K b (SO32−) = 1.0 x 10−7
−
1.0 x 10
K a ( HSO3 )
Ka(NH4+) < Kb(SO32−)
5.6 x 10−10 < 1.0 x 10−7
Since Kb(SO32−) is larger then Ka(NH4+) the solution will be basic.
compare Ka(cation)to Kb(anion):
2. Predict whether (NH4)2C2O4 will be acidic, basic or neutral.
1: Write the SALT’S dissociation equation to identify its CATION and ANION.
(NH4)2C2O4(aq) → 2NH4+(aq) + C2O42−(aq)
2: Use the above flow chart to predict which ion will hydrolyze.
a: If the cation does hydrolyze, write its hydrolysis equilibrium.
b: If the anion does hydrolyze, write its hydrolysis equilibrium.
CATION:
ANION:
NH4+ is NOT A GROUP 1 OR 2 ion, ∴ IT WILL HYDROLYZE AS AN ACID.
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
C2O42− is the conjugate base of a WEAK ACID, ∴ IT WILL HYDROLYZE AS A BASE.
C2O42−(aq) + H2O(l)  HC2O4−(aq) + OH−(aq)
Continued on the next page.
© Rob Ashby 2014. Duplication by permission only.
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3: Compare the results of the flow chart.
a: If the cation hydrolyzes, the salt will be acidic.
b: If the anion hydrolyzes, the salt will be basic.
c: If BOTH the cation and the anion hydrolyze, compare the Ka(cation) to the Kb(anion).
i: If the Ka is greater than the Kb, the salt will be acidic.
ii: If the Kb is greater than the Ka, the salt will be basic.
iii: If the Kb is equal to the Ka, the salt will be neutral.
Ka(cation): Ka(NH4+) = 5.6 x 10−10
calculate Kb(anion):
(
)
K b C2O42− =
−14
Kw
= 1.0 x 10 −5 = 1.56 x 10−10 = K b (C2O42−) = 1.6 x 10−10
−
6.4 x 10
K a ( HC2O4 )
Ka(NH4+) > Kb(C2O42−)
5.6 x 10−10 > 1.6 x 10−10
Since Ka(NH4+) is larger then Kb(C2O42−) the solution will be acidic.
compare Ka(cation)to Kb(anion):
F) REQUIRED PRACTICE 3: Predict whether or not these salts will be acidic, basic, or neutral. Write
hydrolysis equations for each ion that undergoes hydrolysis. {Answers are on page 8}
1. (NH4)3PO4
2. (NH4)2CO3
3. (NH4)2SO4
IV) COMPLEX IONS
A) A COMPLEX ION is as any ion that becomes surrounded by water molecules to form a COMPLEX looking ion
formula. There are three metals that form complex ions when they dissociate in water.
Aluminum: The Al3+ ion forms a complex ion having the formula Al(H2O)63+.
Chromium: The Cr3+ ion forms a complex ion having the formula Cr(H2O)63+.
Iron: The Fe3+ ion forms a complex ion having the formula Fe(H2O)63+.
NOTE: THESE THREE IONS ARE FOUND ON THE RELATIVE STRENGTH OF ACIDS AND BASES TABLE. LEARN
WHERE THEY ARE AND DO NOT FORGET TO RECOGNIZE THEM.
B) HYDROLYSIS of COMPLEX IONS
1) Complex ions tend to hydrolyze as acids according to this example, where X is Al, Cr or Fe.
X(H2O)63+(aq) + H2O(l)  X(H2O)5OH2+(aq) + H3O+(aq)
a) One water molecule surrounding the Al3+, Cr3+ or Fe3+ ion donates a proton to a free floating a water
molecule creating a hydronium ion and a complex ion having a positive 2 charge.
C) SAMPLE PROBLEMS 4: Examples 20-13 & 14 on pages 600-602.
1. Predict whether Fe(NO3)3 will be acidic, basic or neutral.
1: Write the SALT’S dissociation equation to identify its CATION and ANION.
Fe(NO3)3(aq) → Fe3+(aq) + 3NO3−(aq)
2: Use the above flow chart to predict which ion will hydrolyze.
a: If the cation does hydrolyze, write its hydrolysis equilibrium.
b: If the anion does hydrolyze, write its hydrolysis equilibrium.
CATION:
ANION:
Since Fe3+ is NOT a GROUP 1 OR 2 ion and it is a complex ion having the
formula Fe(H2O)63+ when dissolved in water and IT WILL HYDROLYZE AS AN ACID.
Fe(H2O)63+(aq) + H2O(l)  Fe(H2O)5OH2+(aq) + H3O+(aq)
NO3− is the conjugate anion of a STRONG ACID, ∴ IT WILL NOT HYDROLYZE.
Continued on the next page.
© Rob Ashby 2014. Duplication by permission only.
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8
3. Compare the results of the flow chart.
a: If the cation hydrolyzes, the salt will be acidic.
b: If the anion hydrolyzes, the salt will be basic.
c: If BOTH the cation and the anion hydrolyze, compare the Ka(cation) to the Kb(anion).
i. If the Ka is greater than the Kb, the salt will be acidic.
ii. If the Kb is greater than the Ka, the salt will be basic.
iii. If the Kb is equal to the Ka, the salt will be neutral.
Only the cation hydrolyzes, so the solution is acidic.
D) REQUIRED PRACTICE 4: Predict whether or not these salts will be acidic, basic or neutral. Write
hydrolysis equations for each ion that undergoes hydrolysis. {Answers are on page 8}
1. FeCl3
2. Al(NO3)3
3. Cr2(SO3)3
ANSWERS TO THE REQUIRED PRACTICE
Required Practice 1 from page 2
1. Kb > Ka thus solution is basic 2. Ka > Kb thus solution is acidic
4. Ka > Kb thus solution is acidic
Required Practice 2 from page 6
1. acidic 2. neutral 3. neutral
4. basic
3. Ka > Kb thus solution is acidic
5. basic
Required Practice 3 from page 7
1. basic 2. basic 3. acidic
Required Practice 4 from page 8
1. acidic 2. acidic 3. acidic
IT IS NOW TIME FOR YOU TO COMPLETE THE U4TE ASSIGNMENT!!
© Rob Ashby 2014. Duplication by permission only.