MARKING SCHEME - Homework
MARKING SCHEME
for
HYDROLYSIS : THE REACTION OF IONS WITH WATER
DATA (-2 if not included)
Table 1: Predict whether a solution of the salt is expected to be acidic, basic or neutral.
| Salt |Prediction | Salt |Prediction | Salt |Prediction |
|AlCl3 |acidic |Ca(NO3)2 |neutral |NaCH3COO |basic |
| Al2(SO4)3 |acidic |FeSO4 |acidic |Na2CO3 |basic |
|NH4CH3COO |neutral |KBr |neutral |NaCl |neutral |
| (NH4)2CO3 |basic |K2HPO4 |basic |NaHCO3 |basic |
|NH4Cl |acidic |KH2PO4 |acidic |Na3PO4 |basic |
|(NH4)2C2O4 |neutral |KHSO4 |acidic |NaHSO3 |acidic |
| (NH4)2SO4 |acidic |K2SO4 |neutral | Na2SO3 |basic |
Table I: The pH of distilled water and various aqueous salts
pH of distilled water = ___ 6.75___
|SALT |pH |SALT |pH |SALT |pH |SALT |pH |
|AlCl3 |2 |(NH4)2SO4 |5-6 |KH2PO4 |4-5 |NaCl |7 |
| Al2(SO4)3 |2 | Ca(NO3)2 |7 |KHSO4 |3 |NaHCO3 |11 |
|NH4CH3COO |7 |FeSO4 |2 |K2SO4 |7 |Na3PO4 |11 |
|NH4Cl |5 |KBr |7 |NaCH3COO |8-9 |NaHSO3 |3 |
|(NH4)2CO3 |8-9 |K2HPO4 |8-9 |Na2CO3 |11-12 |Na2SO3 |7 |
|(NH4)2C2O4 |7 | | | | | | |
CLASSIFICATION OF RESULTS
|Group A: No Hydrolysis |Group B: Acidic Hydrolysis |Group C: Basic Hydrolysis |
|NaCl, Ca(NO3)2, KBr |(Cationic) |(Anionic) |
| |NH4Cl, AlCl3 |NaCH3COO, Na2CO3, Na3PO4, |
| | |K2SO4, KHSO4, Na2SO3 |
|Group D: Amphiprotic Hydrolysis |Group E: Cationic and Anionic |
|K2HPO4, KH2PO4, NaHCO3, NaHSO3 |Hydrolysis |
| |Al2(SO4)3 , FeSO4 |
| |(NH4)2SO4, (NH4)2C2O4, NH4CH3COO, (NH4)2CO3 |
ANALYSIS OF RESULTS
(1) 1. The sample of distilled water appears to contain some dissolved carbon dioxide since the pH of water
is less than 7.
2. Group A: Salts in which neither the positive nor negative ions react with water
(1) NaCl
i) NaCl [pic] Na+ + Cl–
ii) Na+ and Cl–are both spectators
iii) Should be NEUTRAL. The observed pH of 6.5 agrees with the observed pH of distilled water.
(1/2) Ca(NO3)2
i) Ca(NO3)2 [pic] Ca2+ + 2 [pic]
ii) [pic] and Ca2+ are both spectators
iii) Should be NEUTRAL. The observed pH of 6.5 agrees with the observed pH of distilled water.
(1/2) KBr
i) KBr [pic] K+ + Br–
ii) K+ and Br– are both spectators
iii) Should be NEUTRAL. The observed pH of 6.5 agrees with the observed pH of distilled water.
Page 2
Group B: Salts in which only the positive ion reacts with water (“cationic hydrolysis”)
(1) NH4Cl
i) NH4Cl [pic] [pic] + Cl–
ii) Cl– is a spectator
iii) [pic] + H2O (NH3 + H3O+ Hence ACIDIC. The observed pH of 5 agrees with the
ACIDIC prediction
(1) AlCl3
i) AlCl3 [pic] Al3+ + 3 Cl–
ii) Cl– is a spectator
iii) [pic] + H2O [pic] Al(H2O)5(OH)2+ + H3O+ Hence ACIDIC. The observed pH of 2
agrees with the ACIDIC prediction
Group C: Salts in which only the negative ion reacts with water AND the negative ion IS NOT amphiprotic (“anionic hydrolysis”)
(1) NaCH3COO
i) NaCH3COO [pic] Na+ + CH3COO–
ii) Na+ is a spectator ion
iii) CH3COO– + H2O [pic] CH3COOH + OH– Hence, BASIC. The observed pH of 8 agrees with the BASIC prediction.
(1) Na2CO3
i) Na2CO3 [pic] 2 Na+ + [pic]
ii) Na+ is a spectator
iii) [pic] + H2O [pic] [pic] + OH– Hence BASIC. The observed pH of 11 agrees with the BASIC prediction
(1) Na3PO4
i) Na3PO4 [pic] 3 Na+ + [pic]
ii) Na+ is a spectator
iii) [pic] + H2O [pic] [pic] + OH– Hence BASIC. The observed pH of 10 agrees
with the BASIC prediction
(1) K2SO4
i) K2SO4 [pic] 2 K+ + [pic]
ii) K+ is a spectator
iii) [pic] + H2O [pic] [pic] + OH– Hence BASIC. The observed pH of 7 does not
agree with the BASIC prediction
Page 3
(1) KHSO4
i) KHSO4 [pic] K+ + [pic]
ii) K+ is a spectator
iii) [pic] + H2O [pic] [pic] + H3O+ Hence ACIDIC. The observed pH of 4 agrees
with the ACIDIC prediction
Group D: Salts in which only the negative ion reacts with water AND the negative ion IS amphiprotic
(2) K2HPO4
i) K2HPO4 [pic] 2 K+ + [pic]
ii) K+ is a spectator
iii) [pic] + H2O [pic] [pic] + H3O+ ; Ka = 2.2 x 10–13
[pic] + H2O [pic] [pic] + OH– ; Kb = [pic] = 1.6 x 10–7
Since Kb > Ka the solution should be BASIC. The observed pH of 8.5 agrees with the BASIC prediction
(2) KH2PO4
i) KH2PO4 [pic] K+ + [pic]
ii) K+ is a spectator
iii) [pic] + H2O [pic] [pic] + H3O+ ; Ka = 6.2 x 10–8
[pic] + H2O [pic] H3PO4 + OH– ; Kb = [pic] = 1.3 x 10–12
Since Ka > Kb the solution is ACIDIC. The observed pH of 4.5 agrees with the ACIDIC prediction.
(2) NaHCO3
i) NaHCO3 [pic] Na+ + [pic]
ii) Na+ is a spectator
iii) [pic] + H2O [pic] [pic] + H3O+ ; Ka = 5.6 x 10-11
[pic] + H2O [pic] H2CO3 + OH– ; Kb = [pic] = 2.3 x 10–8
Since Kb > Ka the solution is BASIC. The observed pH of 10 agrees with the BASIC prediction.
Page 4
(2) NaHSO3
i) NaHSO3 [pic] Na+ + [pic]
ii) Na+ is a spectator
iii) [pic] + H2O [pic] [pic] + H3O+ ; Ka = 1.0 x 10–7
[pic] + H2O [pic] H2SO3 + OH– ; Kb = [pic] = 6.7 x 10–13
Since Ka > Kb the solution is ACIDIC. The observed pH of 4 agrees with the ACIDIC prediction
Group E: Salts in which both the positive and negative ions react with water
(2) (NH4)2SO4
i) (NH4)2SO4 [pic] 2 [pic] + [pic]
ii) No spectators
iii) [pic] + H2O [pic] NH3 + H3O+ ; Ka = 5.6 x 10–10
[pic] + H2O [pic] [pic] + OH– ; Kb = [pic] = 8.3 x 10–13
Since Ka > Kb the solution should be acidic. The observed pH of 6.5 agrees with ACIDIC prediction
(2) Fe2(SO4)3
i) Fe2(SO4)3 [pic] 2 Fe3+ + 3 [pic]
ii) no spectator ions
iii) [pic] + H2O [pic] Fe(H2O)5(OH)2+ + H3O+ ; Ka = 6.0 x 10–3
[pic] + H2O [pic] [pic] + OH– ; Kb = [pic] = 8.3 x 10–13
Since Ka> Kb the solution should be acidic. The observed pH of 2 agrees with the ACIDIC prediction
(2) (NH4)2C2O4
i) (NH4)2C2O4 [pic] 2 [pic] + [pic]
ii) no spectators
iii) [pic] + H2O [pic] NH3 + H3O+ ; Ka = 5.6 x 10–10
[pic] + H2O [pic] [pic] + OH– ; Kb = [pic] = 1.6 x 10–10
Since Ka > Kb the solution should be acidic. The observed pH of 8.5 does not agree with the ACIDIC prediction
Page 5
(2) NH4CH3COO
i) NH4CH3COO [pic] [pic] + CH3COO–
ii) no spectators
iii) [pic] + H2O [pic] NH3 + H3O+ ; Ka = 5.6 x 10–10
CH3COO– + H2O [pic] CH3COOH + OH– ; Kb = [pic] = 5.6 x 10-10
Since Ka = Kb the solution should be neutral. The observed pH of 7 agrees with the NEUTRAL prediction
(2) (NH4)2CO3
i) (NH4)2CO3 [pic] 2 [pic] + [pic]
ii) no spectators
iii) [pic] + H2O [pic] NH3 + H3O+ ; Ka = 5.6 x 10–10
[pic] + H2O [pic] [pic] + OH– ; Kb = [pic] = 1.8 x 10–4
Since Kb > Ka the solution should be BASIC. The observed pH of 8.5 agrees with the BASIC prediction.
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