Salts - preparing soluble salts

There are 4 methods of preparing soluble salts:

1. Acid + metal (MAZIT metals) ---> salt + hydrogen
2. Acid + carbonate                      ---> salt + carbon dioxide + water
3. Acid + alkali (soluble base)      ---> salt + water
4. Acid + base metal (insoluble base)  ---> salt + water

Preparing Soluble Salt using Acid + metal (MAZIT metals) ---> salt + hydrogen

Steps:      dissolve >> stir >>  filter >> evaporate >> crystallize

 

In the reaction, hydrogen ions from the acid gain electrons from the metal atoms to form hydrogen gas.

                2H⁺                          +            2e^-                         -->       H_{2 (g)}

hydrogen ions(from acid) + electrons (from metal) --> hydrogen gas

Metals more reactive than hydrogen will reacts with acid to form a salt and hydrogen gas.




Reactivity Series of Metals

 

Most metals react with a dilute acid to form a salt and hydrogen gas.
However, this method is not suitable for

• calcium and alkali metals (elements of Group I in the periodic table) because they react violently with acid. (Alkali metals react violently with water, halogens, and acids)
• lead because it reacts too slowly with acid.
• copper, silver and gold because they do not react at all with acids.

This method is used with less reactive metals - the MAZIT metals.
Magnesium

Aluminium

Zinc

Iron

Tin

Most commonly used dilute acid are:

dilute hydrochloric acid

dilute nitric acid and

dilute sulphuric acid

Salt produced are:
metal chloride
metal nitrate
metal sulphate (sulfate)

Examples of Metal + Acid (dilute)  Chemical equations:

Magnesium + Hydrochloric acid   --> Magnesium chloride + Hydrogen

    Mg (s)    +        2HCl (aq)          -->         MgCl₂ (aq)           +     H₂ (g)

 

Magnesium +   Sulfuric acid   -->   Magnesium sulphate (sulfate)  + Hydrogen 

   Mg (s)      +     H₂SO₄  (aq)  -->       MgSO₄ (aq)          +      H₂ (g)

Magnesium +    Nitric acid      -->    Magnesium nitrate + Hydrogen  

   Mg (s)      +     2HNO₃ (aq)     -->       Mg(NO₃)₂ (aq)    +   H₂  (g)

Zinc      +   Sulphuric acid   -->   Zinc sulphate (sulfate)  +  Hydrogen

  Zn (s)  +      H₂SO₄ (aq)     -->    ZnSO₄ (aq)   +     H₂ (g)  

Aluminium + Hydrochloric acid  -->  Alumiuim chloride + Hydrogen
    2Al (s)     +      6HCl (aq)         -->     2AlCl₃ (aq)            +     3H₂ (g)  

Iron     + Hyddrochloric acid    --> Iron (II) chloride  +  Hydrogen
  Fe(s)  +         2HCl (aq)           -->        FeCl₂ (aq)                  +     H₂ (g)  

Tin     +  Hydrochloric acid     -->  Tin (II) chloride   + Hydrogen
 Sn(s) +         2HCl (aq)           -->        SnCl₂ (aq)         +     H₂ (g)  

 

Magnesium chloride often used as anti-icing on the roads in winter, nutritional supplement, also as coagulant in the preparation of tofu from soy milk, storage for hydrogen and so-on.

 

 

Magnesium sulphate often used to as bath salt, agricultural nutrient, making tofu and so-on.

 

 

Preparing Soluble Magnesium Chloride salt 

Steps:      dissolve >> stir >>  filter >> evaporate >> crystallize

 

dissolve
Pour some dilute hydrochloric acid into a test tube or beaker.
Add a small amount of magnesium ribbon to dilute hydrochloric acid solution

stir
Keep adding the magnesium ribbon and stir it until it is in excess( Excess magnesium is added to ensure dilute hydrochloric is fully reacted).

Observation: effervescence due to production of hydrogen gas.

 




magnesium strap in
hydrochloric acid







Test: lighted splinter explored with a 'pop' sound, it's hydrogen gas.

 

 



filter
Remove the excess(residue) metal by filtration.

evaporate
Evaporate slowly the solution (filtrate) till crystallisation point or saturated.

Test for saturation: dip a cold stirring rod into the hot concentrated solution. If the solution is ready to crystallize, salt crystals would form at the end of the stirring rod).

Saturation  meaning: solution that contains as much solute as can be dissolved at a particular temperature.

If the salt solution is evaporated in high heat, only powder would form.

crystallize
Leave the saturated solution to cool and crystallize at room temperature.
Dry the crystals on a piece of filter paper or tissue paper.

 Summary of  Preparing Insoluble Salt
Steps:      dissolve >> stir >>  filter >> evaporate >> crystallize

 

 Common IGSE questions:

1. Reason for using excess metal -  to ensure metal is fully reacted with acid, so that all acid is used up of neutralised.
2. How the excess metal oxide can be removed from the solution -  by filtration.
3. What is meant by the term saturated solution - no more solid/solute can dissolved at a particular temperature.
4. What practical method could show the solution to be saturated - by dipping a stirring/glass rod into the solution, solution crystals would form along the rod or crystals on the edge of the solution may be observed.
5. Why are crystals dried using filter paper instead of by heating - to prevent breakdown of the crystals. Heating cause water lost too quickly breaking down the crystals and become powder.