Doc Brown's Advanced A level theoretical chemistry revision notes

8.2.5 Explaining permanent dipole – induced dipole intermolecular bonding forces, examples of Debye or induction forces described

8.2.5 Examples of permanent dipole – induced dipole intermolecular bonding interactions

• Also called Debye forces or induction forces.

• These permanent dipole - induced dipole forces get little mentioned in pre-university courses.

• In terms of Van der Waals forces, they are weaker than permanent dipole - permanent dipole forces (Keesom forces) but stronger than instantaneous dipole - induced dipole forces (London dispersion forces) on a molecule to molecule basis.

• A polar molecule is the result of having at least one polar bond due to a significance difference in electronegativity of the two bonded atoms.

• A list of accurate Pauling electronegativities is given below

• element	H	Si	P	C	S	I	Br	Cl	N	O	F
  electronegativity	2.20	1.90	2.19	2.55	2.58	2.66	2.96	3.16	3.04	3.44	3.98

• A polar molecule i.e. with a permanent dipole, produces an electric field which can distort the electron cloud of a neighbouring molecule.

• So, if for example, a non-polar molecule is brought near a polar molecule, the electric dipole field from the polar molecule will induce a dipole moment in the non-polar molecule causing an attraction between the molecules.

• The permanent dipole - induced dipole attractions are called the Debye forces.

• Note that this applies to any pure polar molecule or in a mixture of a polar and non-polar molecules as illustrated below.

• However, in most cases, the Debye forces tends to be the minor contribution to a molecule's total intermolecular bonding forces.

• The permanently polar bond in one molecule can induce a dipole in a different neighbouring molecule in a mixture, whether the other molecule is polar or non–polar, it makes no difference, induction happens!

  • (•••• represents the intermolecular bond)

• Example 1.

• Hydrogen chloride gas dissolved in a hydrocarbon solvent

  •   non-polarized hydrocarbon molecule

  • •••• polarized hydrocarbon molecule - attraction

  • Remember, here, hydrogen chloride retains its covalent molecular character (unlike the ionisation that occurs in water).

  • -

• Example 2.

• A mixture of propanone and a hydrocarbon

  • •••• polarized hydrocarbon molecule - attraction

  • This intermolecular bonding based on Debye forces allows a mutual solubility, if not complete miscibility.

  • -

• Example 3.

• Why non-polar gas molecules can dissolve in water (albeit with a very low solubility)

• This type of induced dipole force is why of oxygen (a non-polar molecule) is slightly soluble in water.

  • The highly polar water molecule, with its permanent dipole, induces a dipole in the non-polar oxygen molecule creating a weak intermolecular bonding force that enables some oxygen to dissolve.

  • •••• Debye force interaction

  • The oxygen solubility is low, but it isn't zero!

  • This limited solubility of oxygen is extremely important for all aerobic aquatic life.

  • The solubilities of non-polar gases in water are relatively low because the hydrogen bonds of water are disrupted and little compensation attractive forces between the water and gas molecules.

• -

and related pages on intermolecular bonding

comparing 8 organic molecule boiling points

and homologous series comparison

and other case studies of boiling points related to intermolecular forces

WHAT NEXT?

8.2 INDEX for intermolecular bonding (intermolecular attractive forces) and examples explained

Sub-index for this page on intermolecular forces an intermolecular bonding

(1) Three introductory sections important to read (a) to (c) first

(a) Introduction to intermolecular forces – Van der Waals forces

(b) Dipole moments and the polarizability of a molecule

(c) Electronegativity, covalent bond character and polar molecules

Four types of intermolecular attractive force sections (2) to (4) are the most important

(2) Instantaneous dipole – induced dipole interactions (London forces or dispersive forces)

(3) Permanent dipole – permanent dipole interactions (Keesom forces/orientation forces)

(4) Hydrogen bonding intermolecular forces

(5) Permanent dipole – induced dipole interactions (Debye forces or induction forces)

ALL my advanced A level theoretical chemistry revision study notes

INDEX Part 8. Phase equilibria–vapour pressure, boiling/melting points, solubility and intermolecular forces

Part 8 sub–index: 8.1 Vapour pressure, nature, origin and examples explained * 8.2.1 Introduction to the types of intermolecular forces and examples explained (index) * 8.2.2 Detailed comparative discussion of boiling points of 8 organic molecule of similar molecular mass * 8.3 Boiling point plots of six organic homologous series - graphs and explanation * 8.4 Other case studies of boiling points related to intermolecular forces * 8.5 Steam distillation – theory and practice * Evidence and theory for hydrogen bonding in simple covalent hydride * 8.7 Solubility of covalent compounds, miscible and immiscible liquids

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle–rules * Part 2. K_c and K_p equilibrium expressions and calculations * Part 3. Equilibrium and industrial processes * Part 4. Partition, solubility product and ion–exchange * Part 5. pH, weak–strong acid–base theory and calculations * Part 6. Salt hydrolysis, Acid–base titrations–indicators, pH curves and buffers * Part 7. Redox equilibria, half–cell electrode potentials, electrolysis and electrochemical series