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7.THERMOCHEMISTRY eLearn.Punjab

There are two fundamental ways of transferring energy to or from a system. These are heat
and work. Heat is not a property of a system. It is therefore not a state function. It is defined as
the quantity of energy that flows across the boundary of a system during a change in its state
due to the difference in temperature between the system and the surroundings. Heat evolved
or absorbed by the system is represented by a symbol q. Work is also a form in which energy is
transfered from one system to another. It is defined as the product of force and distance i.e. W = F
x S. Work is measured in Joules in SI units. There are different kinds of work. The type of work we
most commonly encounter in thermochemistry is pressure-volume work. For example, expansion
can occur when a gas is evolved during a chemical reaction Fig (7.3).
In such cases, the work W done by the system is given by
W = -P D V (In pressure volume work, force becomes pressure and distance becomes volume
change where P is the external pressure and DV is the change in volume. Work is not a state
function. The sign of W is positive when work is done on the system and it is negative when work is
done by the system.
Similarly the sign of q is positive when heat is absorbed by the system from surroundings,
and it is negative when heat is absorbed by the surroundings from the system.

Fig (7.3) Pressure-volume work during expansion of a gas.

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