Oxidation reactions can be used to cause explosions due to the fact that these reactions can produce large volumes of products from a relatively small volume of reactants. The products mainly consist of hot gasses

Mark Drake

Candidate Number: 0174

Chemistry Open Book Paper

An oxidation reaction is where an element or compound burns in air thus reacting with oxygen. For example in a combustion reaction with carbon in air, carbon gains an oxygen molecule:

C(s) + O2(g) CO2(g)

The carbon has now gained oxygen and become carbon dioxide; also the oxidation state has risen from 0 to +4.

Oxidation reactions can be used to cause explosions due to the fact that these reactions can produce large volumes of products from a relatively small volume of reactants. The products mainly consist of hot gasses including CO2 and SO2. The increase of gaseous products leads to an increase in pressure. In an open space the gasses can freely escape, however in a confined container, the increase in pressure leads to an explosion.

The rate of oxidation depends on the amount of oxygen is present in the reaction. With the reaction to form CO2 the oxygen comes from the surrounding air, which is only 1/5th oxygen. To speed up an oxidation reaction the oxygen must be present within the reactants, in the form of an oxidiser such as Potassium Nitrate (KNO3). The oxidiser contains the oxygen needed for the reaction, instead of taking oxygen from the air, which means the reaction would be slower; this means the reaction can take place within a fraction of a second.

During the First World War British shells had to be rationed, this is because of a lack propanone used in making cordite, the main propellant in artillery shells. Before the war the British had to import propanone, as they had no means of producing their own. At first Britain relied on imports from the US, with German U-boats patrolling the Atlantic it was difficult to get sufficient amounts of propanone. An alternative had to be found.

Chaim Weizmann, a Russian immigrant living in England came up with a process of making propanone from bacterial fermentation of the starch in maize, bacteria break down the starch in the maize to produce Propanone and Butanol:

(C6H10O5)n CH3COCH3 + CH3(CH2)2CH2OH

n = refers to number of glucose monomer units.

Shipping maize from the US eventually became too difficult and also the Americans needed it for the war effort, to feed soldiers and the public. A replacement was needed, Weizmann was requested to modify his technique to use conkers instead as ...

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(C6H10O5)n CH3COCH3 + CH3(CH2)2CH2OH

n = refers to number of glucose monomer units.

Shells that contain the propellant cordite

Over the centuries chemical compounds used in explosives have changed dramatically, with each new development providing an advantage over it’s predecessor.

The earliest known explosive was potassium nitrate (KNO3) this was mixed with sulphur and charcoal. The black powder, as it was known, was the main propellant in military artillery and weapons from the 14th to 19th century. The problem with black powder was the fact masses of white smoke were given off when a weapon was discharged, this meant positions were given away or the amount of smoke meant that generals couldn’t see their armies.

The next development was cellulose nitrate, accidentally formed by mixing sulphuric and nitric acid with cellulose:

The fact that cellulose nitrate has it’s own oxygen supply means that it can be triggered by a sudden impact rather than lighting fuse, also this meant that the reaction was faster therefore less smoke was given off.

Nitro-glycerine was a natural by-product of soap making, it differed from nitro cellulose in that instead of being glucose based it was fat based. It is formed by reacting Glycerine with Nitric acid:

C3H5(OH)3 + 3HNO3 C3H5(NO3)3 + 3H2O

Nitro-glycerine was a high explosive; however it was very volatile and could detonate with little reason. Alfred Nobel was the first man to make a safe high explosive by mixing nitro-glycerine with a clay like substance to make a paste, which could be moulded in to shapes. This became known as dynamite; Nobel perfected the formula with different mixtures until it was perfected.

Production of Nitrocellulose

During the Second World War trinitroluene (TNT) was developed unlike nitro-glycerine TNT doesn’t react with metals so it can be used in metal containers to make bombs.

An explosive reaction must take place very quickly, be exothermic and as many of the products formed must be gasses. Low explosives explode rapidly, these include smokeless powders. Reactions with High explosives take place much faster than low explosives, give out more heat and have higher increases in pressure:

C3H5N3O9(l) 3CO2(g) + 2.5H2O(g) + 1.5N2(g) + 0.25O2(g)

1 mole of liquid 7.25 moles of gas

Pressure is increased because there are more molecules in the same area, the expansion of the hot gasses created cause an explosion so the more hot gasses created by a reaction the bigger the explosion. High explosives manage to create more hot gasses in a quicker time, this means they are more explosive than Low explosives. High explosives react faster as they have more oxygen present in the reaction so oxygen doesn’t have to be used from the air surrounding.

A molecule of TNT an example of a high explosive.

Comparisons between a high and low explosive:

During the mid 1800s the Italian scientist, Sobero was experimenting with nitroglycerine. In his experiments Sobero tasted the substance to see if it was dangerous. Apart from a little nausea he was fine, however this was not very safe as nitro-glycerine could have also quite easily have killed him.

When Nobel was experimenting with nitro-glycerine there was an explosion at his factory killing some chemists, after that the Swedish government prohibited the manufacture of nitro-glycerine near homes. Nobel was to carry out all his experiments on a barge in the middle of a lake.

Sources

Box 1: Why does gunpowder explode?
Cordite from Conkers – “Conkering Cordite” by Wilson Flood, Chemistry Review, Volume 10, Number 2, November 2000
information on making cordite. information on making cordite
information on nitrocellulose
information on high an low explosives
Box 2: Cellulose Nitrate
“The Big Bang – a History of Explosives” by GI Brown, which appeared in chemistry review, Volume 9, Number 4, November 1999.
The chemistry of explosives pg 11 – Table of comparisons for high and low explosives.