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Modes of Exchange of Energy

As pointed out earlier the chemical reactions are accompanied by the energy changes. The exchange of energy between the system and surroundings can occur in several ways. Two of the important modes of transference of energy are heat and work which have been described briefly as follows:


HEAT (q)

The transference of energy takes place as heat if the system (reaction mixture) and surroundings are at different temperatures. The exchange of heat occurs through the thermally conducting walls which constitute boundary between system and surrounding. If the system is at higher temperature, the energy is lost to the surrounding as heat, causing a fall in the temperature of the system. The energy transfer continues till the system and the surroundings attain the same temperature. If the system is at lower temperature than the surroundings, the energy is gained by the system from the surroundings causing a rise in the temperature of the system. The amount of beat gained or lost by the system is represented by q. According to the international conventions,


WORK (w)

Exchange of energy between system and surrounding can occur in the form of work. Which may be mechanical, work, pressure volume work or electrical work. For example, let us take a system containing some quantity of water in a thermos flask which does not allow any exchange of beat between system and surrounding. Let temperature of water be . We now insert a set of small paddles in water and rotate them causing churning of water for sometimes. This will result in the increase in the temperature of water. This indicates increase in the internal energy of water by adding mechanical work to the system. The same increase in the temperature of water can be produced by adding equal amount of electrical work with the help of an immersion rod. Let us now become familiar with pressure volume work. The exchange of energy as pressure-volume work can occur if system consists of gaseous substance and there is a difference of pressure between system and surrounding. The amount of work done by the system or on the system is denoted by W. According to international conventions,  The sign conventions of heat and work have been diagrammatically represented in Fig. 16.2.

Fig 16.2.Sign conventions of heat (q) and work (w)

 Units of work. In CGS units, the work is expressed in ergs but in Sf units , work is expressed in joules. The equivalence between joules and other units of work is

1 J = 107 ergs = 1 Nm = 1 kg m2 s-2.



In order to understand the difference between heat and work, .let us compare the effect of adding heat and work to a gaseous system.


(i) When heat is added to a gas, its molecules start moving faster in different directions causing increase in disorder or randomness. Thus heat can be regarded as a mode which stimulates the random motion.


(ii) When work is done on a system by forcing the piston down, the initial effect is to force the molecules to move in the direction of movement of piston. Thus, work can be regarded as a mode that stimulates the organised motion.


To conclude, HEAT refers to random form of energy whereas WORK refers to organised form of energy.