Intermolecular Forces and Properties of Liquids

Intramolecular Forces: the force of attraction within a molecule that occurs between atoms.
Intermolecular Force:  the force of attraction and repulsion between molecules

Dr. LeRoy’s interest is intermolecular forces. He uses quantum mechanics and computer models to define and analyze the basic forces between atoms and molecules. Early in his career, Dr. LeRoy developed a technique for mathematically defining a radius of a small molecule, now known as the LeRoy radius. This established a boundary. Within the boundary, intramolecular bonding is important, and beyond the boundary, intermolecular forces predominate. 

Cotton is very good at absorbing water and easily gets wet. The molecules that make up cotton can form many intermolecular attractions with water molecules. On the other hand, rubber or plastic materials do not absorb water because there is little intermolecular attraction between water molecules and the molecules of the rubber or plastic. A simple property like “wetting” depends to a large extent on intermolecular forces

Intermolecular forces are much weaker than covalent bonds. As an approximate comparison, if covalent bonds are assigned a strength of about 100, then intermolecular forces are generally 0.001 to 15.

Dipole–Dipole Interactions
dipole–dipole force a force of attraction between polar molecules
•	The dipole_dipole force is due to the simultaneous attraction of one dipole by its surrounding dipoles.
•	The strength of the dipole_dipole force is dependent on the polarity of the molecule
•	The dipole_dipole force is due to the simultaneous attraction of one dipole by its surrounding dipoles.
•	The strength of the dipole_dipole force is dependent on the polarity of the molecule

London force Interactions
London force the simultaneous attraction of an electron by nuclei within a molecule and by nuclei in adjacent molecules
•	The London force is due to the simultaneous attraction of the electrons of one molecule by the positive nuclei in the surrounding molecules.
•	The strength of the London force is directly related to the number of electrons in the molecule

Hydrogen Bonding
Hydrogen bonding the attraction of hydrogen atoms bonded to N, O, or F atoms to a lone pair of electrons of N, O, or F atoms in adjacent molecules

Summary
•	Intermolecular forces, like all bonds, are electrostatic—they involve the attraction of positive and negative charges.
•	In this section we considered three intermolecular forces—London, dipole_dipole, and hydrogen bonding.
•	All molecules attract each other through the London force—the simultaneous attraction of electrons and nuclei in adjacent molecules.
•	Dipole_dipole force exists between polar molecules—the simultaneous attraction of a dipole of one molecule for adjacent dipoles.
•	Hydrogen bonding exists when hydrogen atoms are bonded to highly electronegative atoms like N, O, and F—the hydrogen is simultaneously attracted to a pair of electrons on the N, O, or F atom of an adjacent molecule.
•	Intermolecular forces affect the melting point, boiling point, capillary action, surface tension, volatility, and solubility of substances.

Properties of Liquids affected by Intermolecular Forces
Cohesion: is the action or property of like molecules sticking together, being mutually attractive. This is an intrinsic property of a substance that is caused by the shape and structure of its molecules which makes the distribution of orbiting electrons irregular when molecules get close to one another, creating electrical attraction that can maintain a macroscopic structure such as a water drop.
Adhesion: is any attraction process between dissimilar molecular species that can potentially bring them in "direct contact".  The forces that cause adhesion and cohesion can be divided into several different types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion. In addition to the cumulative magnitudes of these intermolecular forces, there are certain emergent mechanical effects that will also be discussed at the end of the article
Surface Tension: is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) and even reptiles (basilisk) to run on the water surface. This property is caused by cohesion of like molecules, and is responsible for many of the behaviors of liquids.

The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area.

Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer

Surface tension is visible in other common phenomena, especially when surfactants are used to decrease it:
•	Soap bubbles have very large surface areas with very little mass. Bubbles in pure water are unstable. The addition of surfactants, however, can have a stabilizing effect on the bubbles. Notice that surfactants actually reduce the surface tension of water by a factor of three or more.
•	Emulsions are a type of solution in which surface tension plays a role. Tiny fragments of oil suspended in pure water will spontaneously assemble themselves into much larger masses. But the presence of a surfactant provides a decrease in surface tension, which permits stability of minute droplets of oil in the bulk of water (or vice versa). 

Capillary Action: is the ability of liquid to flow against gravity where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials such as paper or in some non-porous materials such as liquified carbon fibre. This effect can cause liquids to flow against the force of gravity or the magnetic field induction. It occurs because of inter-molecular attractive forces between the liquid and solid surrounding surfaces; If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and forces of adhesion between the liquid and container act to lift the liquid

Meniscus: is the curve in the upper surface of a standing body of liquid, produced in response to the surface of the container or another object. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other (cohesion) than to the material of the container (adhesion). This may be seen between mercury and glass in barometers  and thermometers. Conversely, a concave meniscus occurs when the molecules of the liquid attract those of the container's, causing the surface of the liquid to cave downwards

Boiling Point: of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid. 
Volatility: is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure. 
The term is primarily applied to liquids; however, it may be used to describe the process of sublimation which is associated with solid substances, such as dry ice (solid carbon dioxide) and ammonium chloride, which can change directly from the solid state to a vapor without becoming liquid
Viscosity: is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. 

Intermolecular Forces: Understanding Concepts

1.	All molecular compounds may have London, dipole-dipole, and hydrogen-bonding intermolecular forces affecting their physical and chemical properties. Indicate which intermolecular forces contribute to the attraction between molecules in each of the following classes of organic compounds:
(a)	hydrocarbon; e.g., pentane, C5H12 (l) (in gasoline)
(b)	alcohol; e.g., 2-propanol, CH3CHOHCH3 (l) (rubbing alcohol)
(c)	ether; e.g., dimethylether, CH3OCH3(g) (polymerization catalyst)
(d)	carboxylic acid; e.g., acetic acid, CH3COOH (l) (in vinegar)
(e)	ester; e.g., ethylbenzoate, C6H5COOC2H5 (l) (cherry flavour)
(f)	amine; e.g., dimethylamine, CH3NHCH3 (g) (depilatory agent)
(g)	amide; e.g., ethanamide, CH3CONH2 (s) (lacquers)
(h)	aldehyde; e.g., methanal, HCHO (g) (corrosion inhibitor)
(i)	ketone; e.g., acetone, (CH3)2CO (l) (varnish solvent)

2.	Use Lewis structures and hydrogen bonds to explain the very high solubility of ammonia in water.

3.	Predict the solubility of the following organic compounds in ater as low (negligible), medium, or high. Provide your reasoning.
(a)	2-chloropropane, C3H7Cl (l) (solvent)
(b)	1-propanol, C3H7OH (l) (brake fluids)
(c)	propanone, (CH3)2CO (l) (cleaning precision equipment)
(d)	propane, C3H8 (g) (gas barbecue fuel)

4.	For each of the following pairs of chemicals, which one is predicted to have the stronger intermolecular attraction? Provide your reasoning.
()	
(a)	chlorine or bromine
(b)	fluorine or hydrogen chloride
(c)	methane or ammonia
(d)	water or hydrogen sulfide
(e)	silicon tetrahydride or methane
(f)	chloromethane or ethanol
()	

5.	Which liquid, propane (C3H8) or ethanol (C2H5OH), would have the greater surface tension? Justify your answer.

6.	A glass can be filled slightly above the brim with water without the water running down the outside. Explain why the water does not overflow even though some of it is above the glass rim.

7.	Explain briefly what the “LeRoy radius” of a molecule represents. Applying Inquiry Skills

8.	Design an experiment to determine whether or not hydrogen bonding has an effect on the surface tension of a liquid. Clearly indicate the variables in this experiment.

9.	Critique the following experimental design. The relative strength of intermolecular forces in a variety of liquids is determined by measuring the height to which the liquids rise in a variety of capillary tubes.

10.	(a) Draw a bar graph with the temperature in kelvin on the vertical axis and the three isoelectronic compounds listed below on the horizontal axis. For each compound draw a vertical bar from 0 K to its boiling point: propane (-42°C), fluoroethane (-38°C), and ethanol (78°C).
(b) Divide each of the three bar graphs into the approximate component for the intermolecular force involved. (Assume that the London force is the same for each chemical and that the dipole-dipole force is the same for the two polar molecules.)
(c) Based upon the proportional components for the three possible intermolecular forces, order the relative strength of these forces.