Chemistry Rules!^'03

Introduction

Ammonia production

• the Haber Process

Ammonium salts

• heating ammonium salts
• reaction with hydroxide ions
• tests for ammonia gas

Nitric acid production

Nitrates

• heating nitrates 
• test for the nitrate ion

back to top

Nitrogen - Introduction

Nitrogen is 

back to top

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.

3H_2(g) + N_2(g) 2NH_3(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.

back to top

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) NH_3(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) NH_3(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.

2. 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.

back to top

Nitrogen - Nitric acid production

back to top

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 )

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

Updated : Tuesday, 25 November, 2003

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

and ChemWindow®5.1