Mada za sehemu hiiCompounds Of MetalsMada 6
- Oxides
- Hydroxides
- Carbonates and hydrogen carbonates
- Nitrates
- Chlorides
- Sulphates
- Carbonates occur in many natural forms, for example chalk, limestone and marble. Examples of carbonates that occur naturally are the carbonates of zinc, iron, lead and manganese.
- Sea animals have carbonates in their shells in the form of calcium carbonate. The remains of these animals sink to the bottom of the sea leading to chalk formation after thousands of years; through pressure, the chalk hardens to form limestone.
- All carbonates are insoluble in water except those of sodium, potassium and ammonium.
- All carbonates give carbon dioxide on heating except those of sodium and potassium
- All carbonates give carbon dioxide with dilute acids
a) A solution of sodium hydroxide (caustic soda) is saturated with carbon dioxide, giving a solution of sodium bicarbonate
If a solution of potassium hydroxide (caustic potash) is saturated with carbon dioxide, a solution of potassium bicarbonate is formed
i. This solution is divided into two equal portions; one portion is allowed to evaporate at room temperature, when crystals of sodium hydrogen carbonate (sodium bicarbonate) or potassium hydrogen carbonate (potassium bicarbonate) are obtained which are filtered and dried.
ii. An equal volume of the original sodium hydroxide or potassium hydroxide solution is added to the second portion. Sodium carbonate or potassium carbonate is formed
The solution is evaporated down and allowed to cool; crystals of sodium carbonate are formed.
| Na₂CO₃ | NaHCO₃ | |
|---|---|---|
| 1 | Has two crystalline forms Na₂CO₃·10H₂O and Na₂CO₃, and anhydrous form sodium carbonate efflorescence. | Can only be made in the hydrous form; sodium hydrogen carbonate does not efflorescence (NaHCO₃) |
| 2 | Is soluble in water | Is much more soluble in water |
| 3 | Does not decompose on heating | It decomposes on heating to form carbonate, carbon dioxide and water |
| 4 | Is known as washing soda, used in softening water and removing grease | Sodium hydrogen carbonate is used as a baking powder to make bread; CO₂ given off causes the bread or cake to rise |
All carbonates and hydrogen carbonates react with acids to form carbon dioxide, water and salt.
Dilute sulphuric acid reacts very little with calcium carbonate because the product, calcium sulphate, is insoluble and forms a layer on the outside of the carbonates which stops further reaction.
Also, dilute sulphuric acid and hydrochloric acid react little with lead(II) carbonate because the products, lead sulphate and lead(II) chloride, are insoluble; they form a layer on the outside of the carbonates which causes the reaction to stop further reaction.
a) Add dilute nitric acid to a sample of a substance. Carbon dioxide gas is evolved; water and salts are formed.
b) i) Add magnesium sulphate to a substance; if carbonates are present, white precipitates are formed.
ii) If hydrogen carbonates are present, no precipitates are formed.
c) Action of heat
Potassium carbonate and sodium carbonate do not decompose when heated. Other carbonates decompose to form an oxide and carbon dioxide
- Calcium carbonate is heated in a limekiln
- The calcium oxide is dissolved in water
- The carbon dioxide is reacted with ammonia brine
As the solution cools, sodium hydrogen carbonate, the least soluble combination of ions, precipitates out and is removed.
- The sodium hydrogen carbonate is heated
- The ammonium chloride and calcium hydroxide are reacted
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