Chemical bonding & structure

Transcription

1 Chemical bonding & structure Ionic bonding and structure Covalent bonding Covalent structures Intermolecular forces Metallic bonding Ms. Thompson - SL Chemistry Wooster High School

2 Topic 4.4 Intermolecular forces Intermolecular forces include London (dispersion) forces, dipole-dipole forces, and hydrogen bonding The relative strengths of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonds.

3 Nature of science Obtain evidence for scientific theories by making and testing predictions based on them London (dispersion) forces and hydrogen bonding can be used to explain special interactions. For example, molecular covalent compounds can t exist in the liquid and solid states. To explain this, there must be an attractive forces between their particles that are significantly greater than those that could be attribute it to gravity.

4 Theories on intermolecular forces Intermolecular forces are interactions between molecules within a compound. Weaker than covalent bonds Since the intermolecular forces of attraction a relatively weak, the molecules of a covalent compound are not held strongly together and for this reason many covalent compounds are gases (CO2, N2, O2, and CO) or liquids (H2O). Ionic compounds have very strong electrostatic forces of attraction between ions, meaning that ionic compounds are solid at room temperature and have high melting points. Collisions between one gaseous particle and another are not completely elastic - gaseous species can be converted into liquid at some temperature. It is the existence of intermolecular forces of attraction that enable molecules of a covalent compound to exist in the condensed phase (liquid and solid). In order to understand these physical properties, we will take into account intermolecular forces of attraction in all three phases: Solid, liquid, and gas.

5 Intermolecular forces The three main types of intermolecular forces of attraction are: London forces (also call dispersion forces or instantaneous induced dipole-induced dipole forces) Dipole-dipole forces Hydrogen bonding Collectively the first two intermolecular forces are turned van der Waals forces London (dispersion) forces + dipole-dipole forces + dipole-induced dipole = van der Waals forces The strengths of intermolecular forces of attraction between molecules can vary significantly, however, these forces are considerably weaker than the ionic or covalent bonds.

6 London forces Existing all molecules Named after German- American theoretical physicist, Fritz Wolfgang London Instantaneous induced dipole-induced dipole forces (temporary dipole)

7 London forces What affects the magnitude of London forces? There are three factors: Number of electrons Size (volume) of the electron cloud Shapes of molecules

8 Number of electrons The greater the number of electrons, the larger the distance between the valence electrons and the nucleus. The attraction of the valence electrons to the nucleus will be reduced in the electron cloud can be polarized more easily London forces decrease rapidly with increasing distance, r 6, based on the relationship: V 1 r 6 Where V is the potential energy associated with the interactions

9 Number of electrons The greater the number of electrons, the larger the distance between the valence electrons and the nucleus. The attraction of the valence electrons to the nucleus will be reduced in the electron cloud can be polarized more easily London forces decrease rapidly with increasing distance, r 6, based on the relationship: V 1 r 6 Where V is the potential energy associated with the interactions

10 Size (volume) of the electron cloud In a large electron cloud, the attraction of electrons to the nucleus we ll not be as great as in a smaller electron cloud, so the electrons in a large electron cloud can be polarized more easily Consider the boiling point of two alkanes propane, CH3CH2CH3 and octane, CH3(CH2)6CH3 The number of carbon atoms in octane is greater than in propane, which results in stronger London forces and has a higher boiling point for octane than for propane. Alkane Boiling Point / ºC Space-filling model propane (C3H8) octane (C8H18) 125

11 Shapes of molecules As the contact area increases of the molecule the London forces between the molecules will have a greater magnitude, which results in a higher boiling point.

12 Dipole-dipole forces Exists in all polar molecules with a permanent (not instantaneous) dipole moment, µ. There is an attraction between the positive end of one permanent dipole and the negative end of another permanent dipole on an adjacent molecule. Since ICl is highly polar, in addition to London forces it also has dipole-dipole forces of attraction between the ICl molecules, which leads to a higher boiling point. Isomer Boiling Point / ºC Types of intermolecular forces present ICl (M = g mol -1 ) 97.4 London forces + dipole-dipole forces Br2 (M = g mol -1 ) 58.8 only London forces

13 Hydrogen bonding One of the more important types of intermolecular forces Can occur when there is an H-F, O-H, or N-H bond present Often depicted as: X - H Y - Z Where the hydrogen donor is X - H Acceptor may be an atom or an anion, Y, a fragment or a molecule Y - Z in which Y is bonded to Z. Hydrogen bond is represented by three dots or a dash Hydrogen bonds occur between: water molecules, H2O ammonia molecules, NH3 hydrogen fluoride molecules, HF water molecules and dimethyl ether molecules, (CH3)2O Hydrogen bonding has a large influence on properties and structures of materials

14 Hydrogen bonding water molecules and dimethyl ether molecules, (CH3)2O ammonia molecules, NH3 water molecules, H2O hydrogen fluoride molecules, HF

15 Hydrogen bonding One of the more important types of intermolecular forces Can occur when there is an H-F, O-H, or N-H bond present Often depicted as: X - H Y - Z Where the hydrogen donor is X - H Acceptor may be an atom or an anion, Y, a fragment or a molecule Y - Z in which Y is bonded to Z. Hydrogen bond is represented by three dots or a dash Hydrogen bonds occur between: water molecules, H2O ammonia molecules, NH3 hydrogen fluoride molecules, HF water molecules and dimethyl ether molecules, (CH3)2O Hydrogen bonding has a large influence on properties and structures of materials

16 Comparison of various relative strengths of intermolecular forces Type of intermolecular force Relative strength / kj mol -1 London force weak (1-10) - this can increase with number of electrons, size (volume) of electron cloud, and shape of molecule dipole-dipole force weak to moderate (3-25) hydrogen bonds moderate to strong (10-40)

17 Practice Problem... I Do... Identify the intermolecular forces in the following substances: He CH3(CH2)4CH3 He - London forces only CH3(CH2)4CH3 Non-polar molecule so London only

18 Practice Problem...We Do... Identify the intermolecular forces in the following substances: NF3 (CH3)2O NF3 Since F is more electronegative than N [Xp(F) = 4.0, Xp(N) = 3.0], this is trigonalpyramidal molecule has a net dipole moment and therefore is polar: London + dipole-dipole

19 Practice Problem...We Do... Identify the intermolecular forces in the following substances: NF3 (CH3)2O (CH3)2O Even though the highly electronegative element oxygen is present, there is no O - H bond so therefore no hydrogen bonding is possible. London + dipole-dipole

20 Practice Problem 20 mins... You Do... Work with a partner and answer the following question: Identify the intermolecular forces in the following substances: CH3F CH3CH2OH

21 Topic 4.4 Intermolecular forces Intermolecular forces include London (dispersion) forces, dipole-dipole forces, and hydrogen bonding The relative strengths of these interactions are London (dispersion) forces < dipole-dipole forces < hydrogen bonds.

22 HOMEWORK Answer the following questions in your notebook and show all work for full credit: State and explain which of the following species can form hydrogen bonds with water molecules: ammonia, NH3, propane, CH3CH2CH3, ethanoic acid, CH3COOH. As a general rule the relative strengths of intermolecular forces follow the order: London (dispersion) forces < dipole-dipole forces < hydrogen bonds Comment, basing your answer on the intermolecular forces, on the fact that the boiling point of carbon tetrachloride, CCl4, is 76.2ºC whereas the boiling point of fluoromethane, CH3F, is -78.2ºC.