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Oxides of Nitrogen - Form Three Chemistry

Oxides of Nitrogen

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Oxides of Nitrogen

Nitrogen has a position in second period of group V in the modern periodic table. It has molecular formula N2. It has atomic number 7 and atomic weight 14.08 and its electronic configuration of 2,5.

Introduction to Oxides of Nitrogen

Besides combining with hydrogen and forming NH3 , nitrogen combines with oxygen in different ratios and forms five different oxides.
The oxides of nitrogen and the details of the oxygen states of nitrogen and the N:O ratio can be presented in a tabular form as:
Name Formula Oxidation state of N Ratio of N:O
Nitrogen Oxide or
nitrous oxide
NO +2 1:1
Nitrogen dioxide NO2 +4 1:2
Nitrous oxide
(Laughing gas)
N2O +1 2:1

- All of these oxides of nitrogen are gases, excepting NO and N2O the other oxides are brownish gases.
- Except the oxides of NO and N2O all the other oxides are acidic. NO and N2O are neutral.
- The other oxides are prepared in the laboratory using different methods characteristic to each oxide.

Example of Oxides of Nitrogen

Nitric Oxide - NO

Structure

Structure of Nitric Oxide

Laboratory Preparation

The oxide is prepared in the laboratory by treating the metallic copper with a moderately concentrated nitric acid (1:1) at room temperature.
The reaction is given as :

3Cu + 8HNO3 3Cu(NO3)2 + 2NO + 4H2O

Laboratory Preparation of Nitric Oxide

- The gas is collected by downward displacement of water.
- The apparatus used is Wolfe's apparatus.
- The purification is done by absorbing the NO gas in freshly prepared ferrous sulphate solution.
- Ferrous sulphate absorbs all the NO gas and forms Fe(H2O)5NO and the solution becomes brown.
- On heating this solution pure Nitric Oxide is obtained.

Physical Properties

NO is not a combustible gas. At high temperature around 1000°C it decomposes into N2 and O2.

2NO N2 + O2 (at high temperature)

From the equation above we can see that once the decomposition starts 50% O2 gets evolved and this O2 supports combustion thus making the reaction more violent.

Chemical properties

1) NO acts as an oxidising agent, oxidising SO2 in presence of water to give H2SO4.

SO2 + 2NO + H2O H2SO4 + N2O

2) NO acts as a reducing agent
i) It can also reduce aqueous solution of I2 to HI

3I2 + 2NO + 4H2O 2HNO3 + 6HI

3) With halogens NO can form addition compounds as:

2NO + Cl2 2NOCl    (NOCl is nitrosyl chloride)

It reacts in the same way with fluorine and bromine.
4) With ferrous sulphate NO forms an addition compound as

FeSO4 + 5H2O + NO [Fe(H2O)5NO]SO4
penta aqua nitrosyl iron (II) sulphate

This is the famous brown ring test used to identify the nitrate radical or the NO radical.

Uses

Nitric Oxide is used to prepare nitric acid.

Nitrogen Dioxide - NO2

Structure

Structure of Nitrogen Dioxide

Laboratory Preparation

In the laboratory NO2 is prepared by thermal decomposition of Pb(NO3)2. Thus:

2Pb(NO3)2 2PbO + 4NO2 + O2

Please Note: Hydrated nitrate salts on heating react violently and explode.

Physical Properties

NO2 is a poisonous gas, main source being the exhaust of auto mobiles.
  1. At room temperature it is a deep brown gas.
  2. It does not support combustion.
  3. It is not combustible

Chemical properties

1) With cold water NO2 reacts to give a mixture of HNO2 and HNO3 acid.

2NO2 + H2O HNO2 + HNO3

2) With hot water the reaction is

3NO2 + H2O 2HNO3 + NO

3) Being acidic it reacts with bases as

2NO2 + 2KOH KNO3 + H2O + KNO2

4) It is also a strong oxidising agent.

H2S + 3NO2 3NO + H20 + S02

5) With excess oxygen and water NO2 gives HNO3.

4NO2 + O2 + 2H2O 4HNO3

6) It reacts with concentrated H2SO4 to give nitrosyl hydrogen sulphate

2NO2 + H2SO4 SO2(OH)ONO + HNO3

Uses

NO2 is used as a fuel in rockets besides being used to prepare HNO3

Nitrous Oxide - N2O (laughing Gas)

Structure

Structure of Nitrous Oxide

Laboratory Preparation

- N2O can be prepared in the laboratory by heating NH4NO3 below 200°C to avoid explosion.
- Sometimes as a safety measure instead of directly using NH4NO3, a mixture of (NH4)2SO4 and NaNO3 are heated to give NH4NO3 which decomposes further to give N2O.

Structure of Nitrous Oxide

NH4NO3 N2O + 2H2O
(endothermic reaction)

Physical Properties

1) N2O has a faint sweet smell and produces a tickling sensation on the neck when inhaled and makes people laugh hysterically. Excess of inhalation leads to unconsciousness.
2) Unlike other oxides of nitrogen, N2O supports combustion though it does not burn itself.

Chemical properties

1) At very high temperature N2O decomposes to N2 and O2

2N2O 2N2 + O2

If a glowing piece of Mg, Cu, or P is introduced in such an environment, these pieces burn brightly due to the O2 produced from decomposition of N2O.
2) With Sodium and potassium N2O reacts to give the corresponding peroxides liberating N2 in the process.

2N2O + 2Na Na2O + 2N2
Na2O is sodium peroxide

Uses

1) It is used as propellent gas.
2) Used in combination with oxygen in the ratio N2O : O2 = 1:10 as a mild anaesthetic

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