Page 51 - Instant notes

The first law 37

Energy is transferred as either heat or work, which, whilst familiar, are not always
easily defined. One of the most useful definitions is derived from the mechanical fashion
in which energy is transferred as either heat or work. Heat is the transfer of energy as
disorderly motion as the result of a temperature difference between the system and its
surroundings. Work is the transfer of energy as orderly motion. In mechanical terms,
work is due to energy being expended

Fig. 1. Examples of an open system
(left), a closed system (center) and an
isolated system (right).

against an opposing force. The total work is equal to the product of the force and the
distance moved against it. Work in chemical or biological systems generally manifests
itself in only a limited number of forms. Those most commonly encountered are pressure-
volume (PV) work and electrical work (see section E).

Internal energy

A fundamental parameter in thermodynamics is the internal energy denoted as U. This is
the total amount of energy in a system, irrespective of how that energy is stored. Internal
energy is the sum total of all kinetic and potential energy within the system. U is a state
function, as a specific system has a specific value at any given temperature and pressure.
In all practical systems, the value of U itself cannot be measured, however, as it involves
all energy terms including nuclear binding energies and the mass itself. Thermodynamics
therefore only deals with changes in U, denoted as ∆U. The sign of ∆U is crucially
important. When a system loses energy to the surroundings, ∆U has a negative value, for
example, −100 kJ. When the internal energy of a system is increased by gain of energy,
∆U has a positive value, for example +100 kJ. The ‘+’ or ‘−’ sign should always be
explicitly written in any thermodynamic calculation, and not simply implied.

State functions and path functions

The physical properties of a substance may be classified as extensive or intensive
properties. An extensive property is one in which the value of the property changes
according to the amount of material which is present. The mass of a material is one
example, as it changes according to the amount of material present. Doubling the amount