MARKING SCHEME - Homework



MARKING SCHEME

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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.

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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

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(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.

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(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

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(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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