Supply and release of energy in chemical reactions

The conditions governing how a chemical reaction takes place are decided by the physical state of the substances. There are three different states: the solid state which is characterized by solid form, defined size and strong molecular cohesion; the gaseous state which has no form and very weak molecular cohesion; and the liquid state, which is somewhere between the two other states.

When heated, most substances change from the solid state through the liquid state and to the gaseous state. In a few cases there is no transitional liquid state, and the substance goes directly from the solid to the gaseous state. As molecular cohesion is weakened in the higher states, the majority of chemical reactions need a supply of heat. The amount of heat energy supplied is dependent upon the temperature needed to make the substances transform into the higher state, i.e. the substance's boiling point.

However, there are also chemical reactions which emit energy (Table 4.1). When water is mixed with unslaked lime (CaO), slaked lime (Ca(OH)2) is formed by the release of a great deal of heat. If slaked lime is then burned, unslaked lime will form and water will be given off in the form of steam. The energy supply in this reaction is exactly the same as the amount of energy released in the first reaction.

Each substance has a given energy content, known as the element's cohesive energy. If the energy content in the original substances of a chemical reaction is greater than the energy content of the resultant substances, then energy is released as heat. This is called an exothermic reaction. In an endothermic reaction, energy must be supplied to the reaction. Exothermic reactions usually occur in nature; endothermic reactions are usual in industrial processes.

It is not only energy in the form of heat that can stimulate chemical reactions: radioactivity, electricity and light can also do this. This includes sunlight which can initiate a number of chemical processes in many materials.

One of the most important rules in chemistry is: 'Within a chemical reaction the sum of the mass energy is constant'.

Table 4.1 Classification of volatility for organic substances

Type

Boiling point

VOC Volatile Organic Compounds

Below 250 °C

SVOC Semi-Volatile Organic Compounds

250-380 °C

POM Particle-bound Organic Compounds

Above 380 °C

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