Unit 5: Laboratory Preparation of Salts
Learning the practical methods to synthesise soluble salts through neutralisation.
5.14 Preparation from Acid & Insoluble Base
This is one of the most straightforward methods for preparing a soluble salt that does not contain a Group 1 or ammonium cation. It is suitable for preparing salts like copper(II) sulphate or zinc chloride. The base used (e.g., copper(II) oxide, zinc carbonate) must be insoluble in water.
The Procedure:
- React: Gently heat the required acid in a beaker. Add the insoluble base in small amounts, stirring continuously, until no more base dissolves. This means the base is in excess and all the acid has been neutralised.
- Filter: Filter the warm mixture to remove the excess, unreacted insoluble base. The filtrate collected is a pure solution of the desired salt.
- Crystallise: Gently heat the filtrate in an evaporating dish to evaporate about half of the water, creating a saturated solution.
- Dry: Leave the saturated solution to cool slowly. Crystals of the salt will form. Once formed, the crystals can be collected by filtration and dried between sheets of filter paper.
Using excess base is crucial to ensure that the final salt solution is not contaminated with any unreacted acid.
Solved Examples:
- Which two substances would you react to make copper(II) sulphate using this method?
Solution: Sulphuric acid and an insoluble copper base, such as copper(II) oxide ($CuO$) or copper(II) carbonate ($CuCO_3$). - Why is the insoluble base added in excess?
Solution: To ensure all the acid reacts completely, so the final salt solution is pure and not contaminated with leftover acid. - How is the unreacted excess base removed from the mixture?
Solution: By filtration. The insoluble base is trapped by the filter paper, while the soluble salt solution passes through as the filtrate. - What is the name of the salt solution that passes through the filter paper?
Solution: The filtrate. - Why is it important not to evaporate all the water when crystallising the salt?
Solution: Evaporating to dryness can lead to the formation of a powder instead of well-defined crystals. For hydrated salts, overheating can also drive off the water of crystallisation. - Suggest a suitable insoluble base to prepare zinc nitrate.
Solution: Zinc oxide ($ZnO$) or zinc carbonate ($ZnCO_3$). - Write a balanced equation for the preparation of magnesium chloride from magnesium oxide.
Solution: $MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$. - After filtering, a student has a clear blue solution. What salt is likely present?
Solution: A soluble copper(II) salt, such as copper(II) sulphate or copper(II) nitrate. - Could you use this method to prepare sodium sulphate? Why or why not?
Solution: No. All sodium bases (like $NaOH$ or $Na_2CO_3$) are soluble in water. There would be no way to separate the excess base from the salt product by filtration. - What is the purpose of gently heating the acid at the start of the reaction?
Solution: To increase the rate of reaction between the acid and the insoluble base.
5.15 Preparation from Acid & Soluble Base (Alkali)
When preparing a soluble salt from a soluble base (an alkali), you cannot simply add an excess of one reactant because both are dissolved. There would be no solid to filter off, and the final product would be contaminated.
The correct technique is titration. This method allows you to determine the exact volume of acid needed to completely neutralise a specific volume of alkali.
The Procedure:
- Titrate with Indicator: Use a pipette to measure a precise volume (e.g., 25.0 cm³) of the alkali into a conical flask. Add a few drops of a suitable indicator (like methyl orange or phenolphthalein). Add the acid from a burette slowly until the indicator just changes colour (the end point). Record the volume of acid used. Repeat for accuracy.
- React without Indicator: Now that you know the exact volumes required, repeat the experiment. Add the recorded volume of acid from the burette to the same volume of alkali in a clean flask, but this time, do not add the indicator.
- Crystallise and Dry: The resulting solution is a pure sample of the salt dissolved in water. It can be crystallised and dried using the same method as described in section 5.14 (gentle heating to saturate, followed by slow cooling).
This method is essential for preparing all Group 1 and ammonium salts (e.g., $NaCl$, $KNO_3$, $(NH_4)_2SO_4$).
Solved Examples:
- Why is titration necessary when both the acid and base are soluble?
Solution: To find the exact volumes needed for complete neutralisation, ensuring the final product is a pure salt solution without any excess acid or base. - What is the purpose of the indicator in the first part of the experiment?
Solution: To signal the end point of the reaction, which is when the acid has exactly neutralised the alkali. - Why is the indicator left out during the final preparation of the salt solution?
Solution: To avoid contaminating the final salt product. The indicator is itself a chemical compound. - Name a salt that must be prepared using titration.
Solution: Any Group 1 or ammonium salt, such as potassium nitrate ($KNO_3$) or sodium chloride ($NaCl$). - A student needs 22.5 cm³ of $HCl$ to neutralise 25.0 cm³ of $NaOH$. What volumes should they mix to get a pure salt solution?
Solution: They should mix exactly 22.5 cm³ of $HCl$ with 25.0 cm³ of $NaOH$. - What piece of equipment is used to accurately measure the volume of alkali into the flask?
Solution: A pipette. - What piece of equipment is used to add the acid during a titration?
Solution: A burette. - Write the balanced equation for the preparation of potassium sulphate from potassium hydroxide and sulphuric acid.
Solution: $2KOH(aq) + H_2SO_4(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)$. - What is the "end point" of a titration?
Solution: The point at which the indicator changes colour, signifying that neutralisation is complete. - Could you use this method to prepare copper(II) sulphate?
Solution: You could, but it's unnecessarily complicated. The method with an insoluble base (copper(II) oxide) is much simpler and more common.
5.16 Preparation from Acid-Salt Reactions
As discussed previously, a stronger or less volatile acid can displace a weaker or more volatile acid from its salt. While this is often used to prepare the displaced acid (like $HCl$ gas), the other product is a new salt. Therefore, this can be considered a method of salt preparation, although it is less common for general synthesis.
This method is particularly useful when the desired salt is insoluble and can be precipitated from the reaction mixture, or when the reactants are readily available solids.
Example: Preparation of Calcium Phosphate
Reacting solid calcium chloride with concentrated phosphoric acid produces solid calcium phosphate and hydrogen chloride gas.
$$3CaCl_2(s) + 2H_3PO_4(l) \rightarrow Ca_3(PO_4)_2(s) + 6HCl(g)$$
The solid calcium phosphate can then be separated from any unreacted starting materials.
Solved Examples:
- What new salt is formed when concentrated sulphuric acid reacts with sodium nitrate?
Solution: Sodium hydrogensulphate ($NaHSO_4$) or sodium sulphate ($Na_2SO_4$). - Write the equation for the preparation of potassium hydrogensulphate from potassium chloride.
Solution: $KCl(s) + H_2SO_4(l) \rightarrow KHSO_4(s) + HCl(g)$. - What type of acid is best suited for displacing other acids from their salts?
Solution: A strong, non-volatile acid like concentrated sulphuric or phosphoric acid. - What salt is produced alongside carbon dioxide when hydrochloric acid reacts with sodium carbonate?
Solution: Sodium chloride ($NaCl$). - Can this method be used to prepare sodium chloride from sodium sulphate?
Solution: No. Hydrochloric acid is weaker than sulphuric acid and cannot displace it. - What is the main driving force for the reaction between concentrated $H_2SO_4$ and $NaCl$?
Solution: The formation and escape of the volatile product, $HCl$ gas, which drives the equilibrium to the right. - Name the two products of the reaction between sulphuric acid and calcium carbonate.
Solution: Calcium sulphate ($CaSO_4$) and carbonic acid ($H_2CO_3$), which decomposes to $H_2O$ and $CO_2$. - A student wants to prepare potassium sulphate. They react sulphuric acid with potassium chloride. Is this a good general method?
Solution: Not typically. It's easier to prepare it by neutralising sulphuric acid with potassium hydroxide or potassium carbonate. This displacement method is more specialized. - What is the state symbol for the hydrochloric acid produced in the reaction $H_2SO_4(l) + NaCl(s) \rightarrow NaHSO_4(s) + HCl$?
Solution: Gas (g). - What salt is formed when phosphoric acid ($H_3PO_4$) reacts with sodium chloride?
Solution: Sodium phosphate ($Na_3PO_4$) or a sodium hydrogen phosphate salt.