Foundations of Chemistry. Philippa B. Cranwell

bond 20–45

      Worked Example 2.11

      Determine the types of intermolecular forces present in the following molecules: (a) silane (SiH₄); (b) methanol (H₃COH); and (c) dichloromethane (CH₂Cl₂).

       Solution

      1 Silane is a tetrahedral molecule with four Si—H bonds. The electronegativities of silicon and hydrogen are very similar and the molecule is symmetrical, so there is no overall dipole present. This means that the most likely type of interaction between two silane molecules is instantaneous dipole–induced dipole or London dispersion forces.

      2 Methanol contains an O—H bond, so there is a large dipole between the carbon and the oxygen and an even bigger dipole between the hydrogen and the oxygen atoms. This can create permanent dipole to permanent dipole interactions. More specifically, because the hydrogen of the O—H bond is bonded to oxygen, methanol can undergo hydrogen bonding with other molecules of methanol, CH3OH.

      3 Dichloromethane contains two carbon‐chlorine bonds, which both have a permanent dipole due to the different electronegativities of carbon and chlorine; carbon is δ+ and chlorine δ−. This is a permanent dipole; therefore, the dominant intermolecular force in dichloromethane is permanent dipole–permanent dipole.

      2.4.5 A special case: ion‐dipole intermolecular forces

      One final intermolecular force should be considered, although it follows the same rules and conventions as the intermolecular forces already discussed: the ion–dipole force. The ion can be either positively charged (a cation) or negatively charged (an anion). The dipole that it interacts with can be induced, instantaneous, or permanent. It really depends upon the molecules involved.

For example, a sodium ion (Na⁺) could interact with ethane (a non‐polar molecule) through either of the following:

       An instantaneous dipole, where the ethane itself could develop an instantaneous dipole due to random electron movement within the bonds in the ethane molecule

       An induced dipole, where the ethane itself could develop an induced dipole due to the close proximity of the sodium ion to it

      Another example could be the sodium ion (Na⁺) ion interacting with methanol (a polar molecule). The methanol itself has a permanent dipole, so the interaction would be between the δ− of the oxygen on methanol and the Na⁺ ion itself.

Quick‐check summary

       There are three main types of chemical bonding: metallic, covalent, and ionic. Metallic bonding can be described by a lattice of positive metal ions held together by a sea of negative electrons.

       Ions are formed when atoms lose or gains electrons.

       Ionic bonding occurs between positive and negative ions held together in a regular lattice by electrostatic forces.

       Covalent bonding occurs when atoms share one or more pair of electrons.

       Dative covalent bonding occurs when one atom provides both electrons to another atom capable of accepting them.

       Covalent bonding results in simple molecular and giant molecular structures.

       VSEPR is a theory that allows the prediction of shapes of molecules based upon the arrangement of pairs of electrons around the central atom.

       The type of bonding in a compound determines its physical properties.

       A polar bond is one that has a permanent dipole moment due to a difference in electronegativities between the atoms in the bond.

       A polar molecule possesses an overall dipole moment.

       Intramolecular forces are the strong forces within a molecule that hold the atoms together.

       Intermolecular forces are weak forces between molecules that hold them together.

       There are three main types of intermolecular forces: instantaneous dipole to induced dipole, permanent dipole to permanent dipole, and hydrogen bonding.

End‐of‐chapter questions

      1 Draw a dot‐and‐cross diagram to show the bonding present in a molecule of nitrogen, N2.

      2 Explain the difference between ionic, covalent, and metallic bonding, giving an example of each type of bonding.

      3 State the most likely type of bonding present in the following species:The term ‘species’ is used to represent a compound such as a molecular substance or an ionic material or a metal.AluminiumNaClFormaldehyde, CH2OMgBr2Bronze (an alloy of copper and tin)Ethyne, C2H2Borane, BH3

      4 Draw the bonding present in the following species:StrontiumCaCl2Formaldehyde, CH2OAlCl3Ethyne, C2H2Borane, BH3NiCl2

      5 Determine whether the following species contain a permanent dipole:Ethyne, C2H2Ethane, C2H6AlCl3CO2CH3OH1,3‐difluorobenzene1,4‐difluorobenzene

      6 List the most likely intermolecular forces that exist in each of the following compounds. There may be more than one type of intermolecular force in each molecule.Ammonia, NH3Iodine, I2Formaldehyde, CH2OEthanoic acid, CH3COOHMethane, CH4Silane, SiH4

      7 Draw the hydrogen bonding between the following molecules:Ammonia, NH3 and methanol (CH3OH)Ethanoic acid (CH3COOH) and waterTwo hydrogen fluoride, HF, molecules

      8 Suggest the shapes of the following molecules:BF3CHCl3PCl3H2SBeCl2