Chapter 7 Salt Preparation
Salts
A salt is an ionic compound formed when a metallic ion or an ammonium ion (NH₄⁺) replaces one or more hydrogen ions of an acid.
2HCl + MgO → MgCl ₂ + H₂O
In the example above, H⁺ of hydrochloric acid is replaced by Mg²⁺ ion.
Before determining the method of salt preparation, we need to first determine the solubility of the salt. Below is the solubility table.
| Salt | Soluble | Insoluble |
|---|---|---|
| Group I salt | All are soluble | NIL |
| Ammonium (NH₄⁺) salt | All are soluble | NIL |
| Nitrates (NO₃⁻) | All nitrates are soluble | NIL |
| Chlorides (Cl ⁻) and Iodides (I⁻) | Most are soluble | AgCl , AgI PbCl ₂, PbI₂ |
| Carbonates (CO₃²⁻) | Group I Carbonates, (NH₄)₂CO₃ | Most carbonates are insoluble |
| Sulfates (SO₄²⁻) | Most sulfates are soluble | BaSO₄, CaSO₄, PbSO₄ |
| Hydroxides (OH⁻) and Oxides (O²⁻) | Group I Hydroxides, NH₄OH, Ca(OH)₂, Ba(OH)₂, Sr(OH)₂ | Most hydroxides and oxides are insoluble. |
| Salt |
|---|
| Group I salt |
| Soluble |
| All are soluble |
| Insoluble |
| NIL |
| Salt |
|---|
| Ammonium (NH₄⁺) salt |
| Soluble |
| All are soluble |
| Insoluble |
| NIL |
| Salt |
|---|
| Nitrates (NO₃⁻) |
| Soluble |
| All nitrates are soluble |
| Insoluble |
| NIL |
| Salt |
|---|
| Chlorides (Cl ⁻) and Iodides (I⁻) |
| Soluble |
| Most are soluble |
| Insoluble |
| AgCl , AgI PbCl ₂, PbI₂ |
| Salt |
|---|
| Carbonates (CO₃²⁻) |
| Soluble |
| Group I Carbonates, (NH₄)₂CO₃ |
| Insoluble |
| Most carbonates are insoluble |
| Salt |
|---|
| Sulfates (SO₄²⁻) |
| Soluble |
| Most sulfates are soluble |
| Insoluble |
| BaSO₄, CaSO₄, PbSO₄ |
| Salt |
|---|
| Hydroxides (OH⁻) and Oxides (O²⁻) |
| Soluble |
| Group I Hydroxides, NH₄OH, Ca(OH)₂, Ba(OH)₂, Sr(OH)₂ |
| Insoluble |
| Most hydroxides and oxides are insoluble. |
After determining the type of salt and its corresponding method of preparation, we
can then find suitable reagents (starting materials) to prepare the salt.
| Type of salt | Method | Reagents to be used |
|---|---|---|
| Insoluble | Precipitation | 1) Salt solution (soluble salt dissolved in water) containing the cation 2) Salt solution (soluble salt dissolved in water) containing the anion |
| Soluble, non-A1 salt | Insoluble compound + Acid (ICA or Excess method) | 1) Acid (containing the anion) 2) Excess insoluble substance (containing the cation) such as
|
| A1 salt | Titration | 1) Acid (containing the anion) 2) Alkali (containing the cation) |
| Type of salt |
|---|
| Insoluble |
| Method |
| Precipitation |
| Reagents to be used |
| 1) Salt solution (soluble salt dissolved in water) containing the cation 2) Salt solution (soluble salt dissolved in water) containing the anion |
| Type of salt |
|---|
| Soluble, non-A1 salt |
| Method |
| Insoluble compound + Acid (ICA or Excess method) |
| Reagents to be used |
| 1) Acid (containing the anion) 2) Excess insoluble substance (containing the cation) such as
|
| Type of salt |
|---|
| A1 salt |
| Method |
| Titration |
| Reagents to be used |
| 1) Acid (containing the anion) 2) Alkali (containing the cation) |
* A1 stands for Ammonium or Group I
1) Precipitation method to prepare INSOLUBLE salts
For this method, two aqueous solutions of SOLUBLE salts are required where one
has to contain the cation of the salt to be prepared, and the other has to contain the
anion of the desired salt.
Example 1.1: To prepare AgCl which is insoluble, the following salts can be used:
| SOLUBLE salts containing Ag⁺ | SOLUBLE salts containing Cl ⁻ |
|---|---|
|
|
| SOLUBLE salts containing Ag+ |
|---|
|
| Method |
|
As a guideline, we should use an aqueous metal nitrate containing the metal cation of the salt to be prepared, and an aqueous group I salt containing the anion of the salt to be prepared.
Precipitation reaction:
Balanced chemical equation: AgNO₃(aq) + NaCl (aq) → AgCl (s) + NaNO₃(aq)
Ionic equation: Ag⁺(aq) + Cl ⁻ (aq) → AgCl (s)
Precipitation method is the simplest method of preparation. Here are the steps:
- Mix the two aqueous solutions together.
***[MUST SPECIFY NAMES OF SALTS IN EXAMS] - Stir to ensure homogeneity of reaction mixture.
- Filter the reaction mixture to obtain insoluble salt as residue.
- Wash with distilled water and dry between sheets of filter paper.
2) Excess method to prepare soluble, non-A1 salts
Recap the three acid reactions below:
- a) Acid + reactive metal
- b) Acid + metal hydroxide/oxide
- c) Acid + metal carbonate
For this method, the anion of the salt to be prepared can only come from an acid, while the cation of the desired salt will come from the excess, insoluble metal OR metal hydroxide/oxide OR metal carbonate.
Why is the insoluble metal/metal oxide/hydroxide/carbonate added in excess?
This is to ensure that all acid is reacted away to form the salt, allowing the excess insoluble substance to be easily removed via filtration. The soluble salt is collected as the filtrate.
Example 2.1: To prepare copper(II) sulfate crystals
| Acid containing sulfate (SO₄²⁻) ions | Substance that will provide Cu²⁺ | Does it work? |
|---|---|---|
| H₂SO₄ | Cu metal | NO. Take note that this method can only work with reactive metals except for Group I metals (too reactive, reaction will be explosive). |
| Cu(OH)₂ or CuO | Copper hydroxide and copper oxide are insoluble salts. Therefore, both substances can be used to react with sulfuric acid to prepare CuSO₄ crystals. | |
| CuCO₃ | Copper carbonate is an insoluble salt. Hence, it can be used to react with sulfuric acid to prepare CuSO₄ crystals. | |
| Cu(NO₃)₂ CuCl₂ | NO. Thus, it is not feasible to use soluble copper(II) salts. |
| Acid containing sulfate (SO₄²⁻) ions |
|---|
| H2SO4 |
| Substance that will provide Cu 2+ |
| Cu metal |
| Does it work? |
| NO. Although copper metal is insoluble, it is an UNREACTIVE METAL. Thus, it will not react with sulfuric acid to form copper sulfate. Take note that this method can only work with reactive metals except for Group I metals (too reactive, reaction will be explosive). |
| Acid containing sulfate (SO₄²⁻) ions |
|---|
| H2SO4 |
| Substance that will provide Cu 2+ |
| Cu(OH)2 or CuO |
| Does it work? |
| Copper hydroxide and copper oxide are insoluble salts. Therefore, both substances can be used to react with sulfuric acid to prepare CuSO4 crystals. |
| Acid containing sulfate (SO₄²⁻) ions |
|---|
| H2SO4 |
| Substance that will provide Cu 2+ |
| CuCO3 |
| Does it work? |
| Copper carbonate is an insoluble salt. Hence, it can be used to react with sulfuric acid to prepare CuSO4 crystals. |
| Acid containing sulfate (SO₄²⁻) ions |
|---|
| H2SO4 |
| Substance that will provide Cu 2+ |
| Cu(NO3)2 CuCl2 |
| Does it work? |
| NO. These are soluble salts. Although CuSO4 is formed due to the exchange of ions, it is impossible to isolate pure CuSO4. Thus, it is not feasible to use soluble copper(II) salts. |
Example 2.2: To prepare magnesium nitrate crystals
We have talked about the reason behind adding the insoluble compound in excess, but how do we know it is in excess? It is when no more of the insoluble compound can dissolve into the acid solution. Hence, a suspension is formed, containing the solution of soluble salt and insoluble excess, unreacted reagent.
To obtain the hydrated crystals of salt of interest, the following steps are taken:
- Filter to remove the excess, unreacted reagent and obtain the filtrate.
- Heat the filtrate to reduce the volume until a saturated solution is obtained.
- Allow hot, saturated solution to cool for crystallization to take place.
- Filter the mixture to obtain the crystals as residue.
- Wash with little, cold distilled water and dry between sheets of filter paper.
3) Titration method to prepare A1 salts
Once you determine the salt to be prepared is an ammonium (NH4⁺) salt or a Group I salt, titration is the only way to prepare the salt.
The reagents will be an acid (that contains the anion of the salt) and an alkali (containing the cation of the salt to be formed).
Example 3.1: To prepare potassium sulfate crystals
This titration method requires the use of a pipette, burette and an indicator. Why?
This is because to prepare an A1 salt, exact volumes of both reagents are required to
have a complete reaction. If any of the reagents are in excess, the crystals obtained
will be contaminated with the excess reagent.
Thus, the first titration is done with an indicator to help determine the exact volume of acid or alkali required for complete neutralisation. Once the value has been determined, a second titration is carried out without addition of an indicator. At the end of the titration, a solution containing only the salt of interest is obtained.
To obtain the hydrated crystals of salt of interest, the following steps are taken:
- Heat the solution to reduce the volume until a saturated solution is obtained.
- Allow hot, saturated solution to cool for crystallization to take place.
- Filter the mixture to obtain the crystals as residue.
- Wash with little, cold distilled water and dry between sheets of filter paper.
Question 1
When excess pieces of calcium carbonate is added to dilute hydrochloric acid, the reactions gradually become slower and finally stops. Which statement best explains why this happens?
A. An insoluble layer of calcium chloride is formed on the calcium carbonate.
B. The pieces of calcium carbonate gradually become smaller.
C. The calcium carbonate is covered by bubbles of carbon dioxide.
D. The concentration of hydrochloric acid gradually reduces to zero.
Answer:
D. The concentration of hydrochloric acid gradually reduces to zero.
Question 2
A student wishes to prepare four different kinds of salts and the suggested
reactants are listed as follows.
Which pair of reactants cannot be used to prepare the salt?
| salt | suggested reactants | |
|---|---|---|
| A B C D | Cu(NO3)2 Fe(NO3)3 (NH4)2SO4 PbCl2 | nitric acid and copper(II) carbonate nitric acid and iron(III) oxide sulfuric acid and ammonium carbonate silver chloride and lead(II) nitrate |
| salt | suggested reactants | |
|---|---|---|
| A B C D | Cu(NO3)2 Fe(NO3)3 (NH4)2SO4 PbCl2 | nitric acid and copper(II) carbonate nitric acid and iron(III) oxide sulfuric acid and ammonium carbonate silver chloride and lead(II) nitrate |
Answer:
D. Pbcl 2 , silver chloride and lead (II) nitrate
Continue Reading Chapter 8: Qualitative analysis