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Why does the potential energy of molecules of system increases when the distance between them increases?

Why does the potential energy of molecules of system increases when the distance between them increases?


1 Answers

Chetan Mandayam Nayakar
312 Points
10 years ago

Electrical Forces and Bonding. Chemists are fascinated by the manner in which atoms aggregate to form molecules and lattices; and molecules aggregate to form condensed phases of matter. Aggregation of atoms, oppositely charged ions, and molecules is a consequence of electrical forces exerted on the electrons of one particle by the nucleus (or nuclei) of the other. In this chapter, we learn that all physical and chemical bonding is the result of molecular forces that are electrical in origin.

  • Forces between atoms and molecules are electrical, and are described by Coulomb's Law.
  • There are only two types of energy, kinetic and potential. All energy possessed by atoms and molecules is one of these two types.
  • Potential energy arises from forces between bodies. Potential energy of molecules arises from electrical forces between them.
  • Intramolecular forces (ionic bonds, covalent bonds, and metallic bonds) and intermolecular forces (dipole-dipole, dispersion, ion dipole, hydrogen bonds) are fundamentally very similar, described essentially by Coulomb's Law.
  • Real gases deviate from the ideal gas law primarily because of intermolecular forces.

Intermolecular forces include the forces that operate between one atom or molecule and another, or between the ion of one substance and the molecules of another. Although there are differences among the various types of forces, they are all fundamentally similar: they give rise to potential energy between the interacting particles, described by the potential well. We shall see that an interplay between the depth of this well and the thermal kinetic energies of particles is responsible for many of the physical properties of substances.

Intermolecular Forces

  • Attractions exerted by one molecule of a molecular substance on another, such as the force of attraction between water molecules in ice.
  • Attractions between atoms of the noble gas elements, helium through radon.
  • Attractions between molecules of one substance and molecules of another, as when two liquids are mixed, or a molecular solid such as sugar is dissolved in a liquid.
  • Attraction between molecules of one substance and ions of another, as when an ionic compound dissolves in a liquid.

In succeeding sections, we will discuss each of these molecular forces, and their relative magnitudes. We will begin with intramolecular forces.

Intermolecular Forces . Having described the forces that operate within molecules (including ionic crystals and metals), we are ready to discuss the interactions that occur between species (atoms, molecules, and ions) that are not covalently bonded to each other. These are called intermolecular forces; they can originate in a number of ways.

Dipole-Dipole Forces. Intermolecular forces that operate between neutral molecules having molecular dipole moments are called dipole-dipole forces. We saw in Chapter 3 how the structure of a molecule may cause its centers of positive and negative charge to be displaced from one another, giving the molecule a dipole moment. The dipole moments of two neighboring molecules tend to align with the + end of one dipole near the - end of the other, so that forces of attraction between them are maximized. The alignment of two dipoles is shown in Figure 6-9.

The maximum force of attraction between two dipoles, m1 and m2, separated by a distance r is given by equation 6-5-1.

(6-5-1): F = m1*m2/W*r4

Equation 6-5-1 is Coulomb's Law, in disguised form. It is necessary to raise the distance of separation to the 4th power in the denominator to cancel the distance units that appear in both dipole moments in the numerator. Physically, the 4th power dependence means that the attraction between dipoles falls off much more rapidly with distance than would the attraction between isolated charges. This is because the + and - ends of dipole 2 are almost equidistant from dipole 1 when the dipoles are separated by a distance that is much greater than the dipole length. The force of attraction between the + end of dipole 1 and the - end of dipole 2 is almost cancelled by the force of repulsion between the two + ends. Dipole-dipole forces operate between molecules of water, and between molecules of the substances pictured in Figure 6-10.

Such forces are obviously much weaker than those operating in ionic or covalent network solids, and give rise to potential wells having depths in the approximate range 5-20 kJ/mole. Many molecular substances with dipolar molecules exist as liquids at ambient temperature, and have relatively low boiling points. In particular, many organic (carbon-containing) compounds are of this type.

Some Consequences of Intermolecular Forces . The operation of the types of forces discussed in this chapter between molecules, atoms, and ions has profound implications for the properties, function, and reactivity of chemical substances. By way of illustration, we discuss intermolecular forces in that most fundamental and essential of all molecules, water. The water molecule is bent, with a bond angle of about 104 o. This shape is understandable in terms of the VSEPR theory developed in Chapter 3. A consequence of this shape is that the molecule is polar (i.e., has a molecular dipole, Chapter 3). The molecular dipole moment and the fact that water contains hydrogen bonded to the very electronegative oxygen atom leads to strong intermolecular forces of the hydrogen bonding (dipole-dipole) type. A consequence of the existence of these forces is that the melting and boiling points of water are much higher than would be expected on the basis of molar mass (e.g., the boiling point of methane, with molar mass 16, is -162 oC). Thus water is a liquid or solid under temperature conditions found on the earth's surface. As a liquid, it provides a medium for the genesis and sustenance of life. Another consequence of polarity is that water has excellent solvating properties: it dissolves substances as diverse in structure as salts, sugars, and huge protein molecules that function as enzymes. Due to the shape of the water molecule and the intermolecular hydrogen bonding, water adopts an open lattice structure when it freezes, in which each oxygen atom is covalently bonded to two hydrogen atoms and hydrogen bonded to two more via its two lone pairs of electrons. A consequence of this open lattice is that ice is less dense than liquid water, and floats. Lakes therefore freeze top down, the layer of ice on top serving to insulate the unfrozen liquid below and thereby to protect aquatic life. If, like most substances, water were more dense as a solid, ice would sink, entire lakes would freeze, and life as we know it would not have arisen. (Of course, the density inversion of water has negative consequences, too, with which we are all familiar: potholes in roads, erosion of rock formations, and the like.) Finally, the structure of the water molecule and the resulting intermolecular interactions are at the root of the ability of water to function as both an acid and a base. This aspect of the reactivity of water is crucial in many biochemical and geological processes.

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