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Describe the Laboratory preparation of Nitric (V) acid.

      

Describe the Laboratory preparation of Nitric (V) acid.

  

Answers


Kavungya
(i). Apparatus

fig3532019.png

(ii). Reagents
- Nitric acid is prepared in the laboratory by action of concentrated sulphuric acid on solid nitrates e.g. potassium nitrate (KNO3) and sodium nitrate (NaNO3)

(iii). Procedure
- 30-40 grams of small crystal of KNO3 are put in a retort flask.
- Concentrated sulphuric acid is added just enough to cover the nitrate; and then heated (warmed) gently.
- The apparatus is all glass.

Reason:
- Nitric (V) acid would attack rubber connections.
- The neck of the retort flask is inserted into a flask that is kept cool continually under running water; this is where nitric acid is collected.

Note:
The cold water running over the collection flask is meant to cool (condense) the hot fumes of nitric (V) acid.

(iv). Observations and explanations
- Fumes of nitric are observed in the retort;

Equation:
eq2532019200.png

- If Lead (II) nitrate was used;
eq3532019202.png

Note: with lead (II) nitrate the reaction soon stops because the insoluble lead (II) sulphate coats the surface of the nitrate preventing further reaction; yield of nitric (V) acid is thus lower;
-These fumes of nitric acid appear brown.

Reason:
- Due to the presence of nitrogen (iv) oxide gas formed by thermal decomposition of nitric acid.

Equation:
eq4532019205.png

- Pure nitric (V) acid is colourless but may appear yellow (brown) due to the presence of Nitrogen (IV) oxide.
- The brown colour can be removed by blowing air through the acid.
- Fuming nitric acid boils at 83oC and is 99% pure; while concentrated nitric acid is only 70% acid and 30% water.

Note: Nitric acid is usually stored in dark bottles.

Reason:
- To avoid its decomposition by light to nitrogen (IV) oxide, oxygen and water.
- The reaction in the retort flask is a typical displacement reaction; in which the more volatile nitric (V) acid is displaced from nitrates by the less volatile sulphuric acid.
- The nitric acid distills over because it is more volatile than sulphuric acid.
Kavungya answered the question on March 5, 2019 at 11:07


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