Salt Preparation Flashcards
What are the 3 reactions that produce salts?
- Acid + Reactive Metal → Salt + H2
- Acid + Metal Carbonate → Salt + CO2 + H2O
- Acid + Base/Alkali → Salt + H2O
What is the general equation for the preparation of salts by precipitation?
Aqueuous Solution AB + Aqueuous Solution XY → Insoluble Salt AY + Aqueuous Solution XB
State the pre-requisites to use the method of precipitation in order to prepare a salt.
- The salt to be prepared is insoluble (solid).
2. The starting reagents are soluble (aq solutions).
What are the general equations for preparing a salt using the excess method?
- Acid + Moderately Reactive Metal → Salt + H2
- Acid + Insoluble Metal Carbonate → Salt + CO2 + H2O
- Acid + Insoluble Base → Salt + H2O
State the pre-requisites to use the excess method in order to prepare a salt.
- The salt to be prepared is soluble (aqueous).
2. One starting reactant must be insoluble (solid), and the other soluble (aqueous).
Why must the metal, base or carbonate be insoluble in water and used in excess?
Insoluble metal, base or carbonate is
- used in excess: to ensure that all the acid is used up. Otherwise, the salt produced will be contaminated with the acid.
- insoluble in water: so that it can be filtered from the salt solution at the end of the reaction.
State the step-by-step procedure to prepare clean and dry crystals of zinc sulfate via excess method.
- Using a measuring cylinder, measure 50 cm3 of H2SO4 into a beaker.
- Add Zn powder into the sulfuric acid in small portions and stir until no more effervescence is observed. (i.e. Zn powder is in excess)
- Filter to remove the excess (unreacted) Zn powder. Collect the filtrate, which is ZnSO4 solution.
- Heat the filtrate to saturate the ZnSO4 solution.
- Test for saturation. The solution is saturated when crystals form on glass rod.
- Leave the solution to cool and crystallise.
- Filter to obtain crystals.
- Rinse with cold, distilled water. Pat dry between pieces of filter paper.
What metals cannot be used for the excess method, and why?
The excess method is suitable for moderately reactive metals such as zinc, magnesium and iron. However, it is not suitable for:
- very reactive metals such as potassium, sodium and calcium. These metals react violently with acids, so the reaction is very dangerous;
- unreactive metals such as copper and silver. These metals do not react with dilute acids.
For metals that are unreactive or too reactive, how then can they be prepared via the excess method?
- Acid (aqueous) + Insoluble Carbonate of Unreactive/ Too Reactive Metal (excess) → Salt + CO2 + H2O
- Dilute Acid (aqueous) + Insoluble Base of Unreactive/ Too Reactive Metal (excess) → Salt + H2O
What are the general equations for preparing a salt using titration?
- Acid + Soluble Metal Carbonate → Salt + CO2 + H2O
2. Acid + Alkali → Salt + H2O
What are the pre-requisites for preparing a salt through titration?
- The salt to be prepared is soluble. (sodium, potassium and ammonium salts)
- All starting reagents must be soluble.
- The volume of each starting material used must be exact — If any of the starting materials is in excess, it cannot be removed from the salt solution by filtration, and the salt produced will be impure.
What are the two parts of essential steps to titration?
Part 1: Determine exact volumes of reactants involved for complete reaction using a suitable indicator.
Part 2: Use the above exact volumes of reactants to prepare salt. At this stage, the indicator is not used as we are preparing uncontaminated salt solution.
State the step-by-step procedure to prepare clean and dry insoluble barium sulfate. (via precipitation)
- Pour about 50cm^3 of barium nitrate solution into a small beaker. Add sodium sulfate solution (in excess) and stir thoroughly. A white precipitate of barium sulfate forms.
- Filter the mixture. The barium sulfate is collected as residue.
- Rinse the precipitate with some cold, distilled water to remove impurities.
- Dry the precipitate by pressing it in between filter paper.
