Chemistry Rules!'03

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Middle Part - Nitrogen

Introduction

 

Ammonia production

Ammonium salts

Nitric acid production

 

Nitrates

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Nitrogen - Introduction

Nitrogen is 

 

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Nitrogen - Ammonia production

Ammonia is produced industrially by the Haber process.  This process involves the reaction between hydrogen gas and nitrogen gas at a high temperature ( 400 C ), a very high pressure ( 200 atm ) and with the help of a catalyst.

The nitrogen for this reaction is obtained by the fractional distillation of liquid air.

The hydrogen is produced by the catalytic steam cracking of methane, which gives oxides of carbon as well as hydrogen gas.

The catalyst used for the Haber process is iron.

3H2(g) + N2(g) examb.gif (902 bytes) 2NH3(g)

The reaction only gives 15% conversion of the starting materials into products. The gases coming out at the end of the reaction are cooled down to about -50 oC to condense the ammonia ( which is piped away ) and the remaining hydrogen and nitrogen are recycled back to the beginning of the reaction so as not to be wasted.

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Nitrogen - Ammonium salts

(1) Heating ammonium salts :

Ammonium compounds are easily decomposed by simply heating them in a test tube with the hot Bunsen burner flame.

They release ammonia gas, for which a positive test result is when damp red litmus paper turns blue.   The gas also smells very strongly.

e.g. NH4Cl(s) arrow.gif (1035 bytes) NH3(g) + HCl(g)

 

(2) Reaction with hydroxide compounds :

In a very similar reaction to the above process, when ammonium compounds are heated together with a hydroxide, e.g. sodium hydroxide, they decompose to give ammonia gas, water vapour and a salt.

e.g. NH4Cl(s) + NaOH(s) arrow.gif (1035 bytes) NH3(g) + H2O(g) + NaCl(s)

(3) Tests for ammonia gas :

There are two main ways of testing for ammonia gas :

  1. A piece of damp red litmus paper will turn blue.  This is because ammonia is a basic gas.
  1. When concentrated hydrochloric acid is placed near ammonia gas fumes, a large amount of white smoke is given off.  This is ammonium chloride, produced when the acidic and basic gases react together.

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Nitrogen - Nitric acid production

 

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Nitrogen - Nitrates

(1) Heating nitrates :

When solid metal nitrate salts are subjected to strong heating they decompose. The products of this decomposition vary with the metal nitrate used.

Group I metals form stable compounds since they are at the top of the reactivity series. Their nitrates only decompose to give oxygen gas and a metal nitrite salt ( see below ).

Group II and Transition metals form less stable compounds and their nitrates decompose to give oxygen gas, brown nitrogen dioxide gas and a metal oxide ( see below ).

The table below gives a summary of the effects of heating metal nitrates as well as two examples of balanced chemical equations -

metals examples equation example
Group I Li, Na, K, Rb, Cs 2NaNO3(s)2NaNO2(s) + O2(g)
Group II and Transition Mg, Ca, Fe, Cu, Pb 2Mg(NO3)2(s)2MgO(s) + 4NO2(g)

 + O2(g)

(2) Test for the nitrate ion :

When a small amount of nitrate salt is dissolved in aqueous sodium hydroxide, some powdered aluminium ( or Devarda's alloy which is an alloy of aluminium (45%), zinc(5%) and copper(50%)) added and the whole mixture gently heated, ammonia gas is produced. This ammonia gas can be detected for in the normal manner.

The equation for the reaction is very complex,

3NO3- + 8Al + 5-OH + 18H2O3NH3(g)  +  8[Al(OH)4]-

(source : Vogel's Qualitative Inorganic Analysis 6th edt., pg 183 )

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Bonding and Structure - Metallic bonding

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Written by Dr Richard Clarkson : © Monday, 1 April 1998

Updated : Tuesday, 25 November, 2003

mail to: chemistryrules@rjclarkson.demon.co.uk

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