Introductory Chemical Engineering Thermodynamics (Prentice Hall International Series in the Physical and Chemi)

by J. Richard Elliott

Attractive forces are quantified by negative numerical values, and repulsive forces will be characterized by positive numerical values.

The potential, u(r), is the work (integral of force over distance) of bringing two molecules from infinite distance to a specific distance, r.

A key feature of dipole-dipole forces is the temperature dependence.

A similar consideration of the spherical averaging described in relation to electrostatic forces results again in a dependence of r–6 as approximately

Disperse Attraction Forces (Dispersion Forces) When two nonpolar molecules approach, they may also induce dipoles into one another owing to fluctuating distributions of electrons. Their dependence on radial distance may be analyzed and gives the form for the attractive forces:

Repulsive Forces The forces become repulsive rapidly as radial distance decreases, and quickly outweighs the attractive force as the atoms are forced into the same space.

size of the molecule by the parameter σ and the attractive strength (i.e. “stickiness”) by the parameter ε.

a large molecule like buckminsterfullerene (solid at room temperature) would have a larger value for σ and ε than would methane (gas at room temperature).

Potentials in Mixtures

The size parameter for interaction between different molecules is reasonably well represented by

For historical reasons, this constant is typically referred to as “kij” or the binary interaction parameter, and defined through the following rule: The default value is k12 = 0.

Furthermore, the concept of favorable energetic interactions between acids and bases can lend broad insights into the mysteries of chemical formulations. As an engineering approach, we can make large adjustments to the spherical nature of these forces such that we can often approximate them with a single characteristic constant to obtain a workable engineering model.

microscopic energies are combined and we consider kinetic and configurational energy collectively as the internal energy of the system, which is given the symbol U.