Chemistry, Third edition

LATTICE ENTHALPY 233

One commercially useful group of materials which produce heat energy

extremely rapidly are explosives. These are discussed in Box 13.2.

BOX 13.2

Explosives

An explosive is a substance which undergoes a very rapid

chemical reaction in which gas is produced. The volume of gas

produced is large compared with the bulk of the explosive, and

the gas escapes so rapidly that a pressure wave is set up in

the surrounding air – this causes the bang. For example, the

explosive nitroglycerine (Fig. 13.7) decomposes according to

the overall equation

4C 3 H 5 (NO 3 ) 3 (l) 12CO 2 (g)10H 2 O(g)6N 2 (g)O 2

Although the earliest explosive was gunpowder (black powder),

consisting of sulfur, potassium nitrate (as oxidizing agent) and

charcoal, it was the discovery of nitroglycerine (in 1847), of

trinitrotoluene (TNT, in 1863) and of dynamite (made in

1866 by Alfred Nobel by absorbing nitroglycerine into finely

powdered earth) that revolutionized the explosives industry.

The early investigations of explosives caused many fatalities,

including Alfred Nobel’s brother and father. Although we

associate explosives with their military applications, most

explosives are used in quarries, mines and for clearing land in

road construction. The commonest explosive is called ANFO

and consists of about 95% by mass of ammoniumnitrate

(NH 4 NO 3 (s), as oxidising agent) and 5% fueloil.

All common explosives contain nitrogen, and nitrogen gas is

always produced as one of the gaseous products. The

explosion reaction is over in a very short time. For example,

although the burning of nitroglycerine produces only 7 kJ g^1

of heat (Table 13.3), this heat energy is produced in less

than 0.001 s. This is equivalent to a power of 7000 kW, and

the temperature of the escaping gas may reach 3000 °C

and speeds of up to 7000 ms^1!

To illustrate the pressures involved, suppose that 5 g of nitro-

glycerine is exploded in a sealed container of volume 10 cm^3

(10^5 m^3 ). About 0.16 mol of gases are produced at 3000 °C.

We can crudely estimate the pressure of the gas at the instant

of the explosion using the ideal gas equation

P nRT 0.168.3145(2733000) 4  108 Pa (4000 atm)

V 10 ^5

This is an enormous pressure (roughly 40 times the gas

pressure in a large commercial gas cylinder) and explains the

ability of explosives to smash rocks and metal.

Fig. 13.7Structures of the explosives TNT, nitroglycerine and

HMX (tetramethylenetetranitramine). Nitroglycerine and HMX

contain sufficient oxygen to burn completely, but TNT is

blended with 80% ammonium nitrate to ensure complete

oxidation during the explosion.

Lattice enthalpy

The standard enthalpy change in which 1 mol of a crystal lattice is broken up into

isolated gaseous particles is called the lattice enthalpy(H^ —L) of the lattice. For

example, 776 kJ of energy is needed to break up 1 mol of pure sodium chloride crystal

lattice into gaseous sodium and chloride ions at 298 K and 1 atm pressure:

Na,Cl(s) Na(g)Cl(g) H—^ L771 kJ mol^1

When 1 mol of gaseous sodium ions condenses with 1 mol of chloride ions, under

the same conditions, 1 mol of sodium chloride crystals are made:

Na(g)Cl(g)Na,Cl(s) H—^  H—^ L 771 kJ mol^1

Generalizing:

Heat needs to be absorbed in order to break down a crystal lattice

Heat is given out when a crystal lattice is formed

13.8