Unit 7.5 Comparing Intermolecular Forces Teacher: Dr. Van Der Sluys

Objectives To determine –Relative melting and boiling points –Relative solubility of various combinations of compounds

Vocabulary Solution - a homogeneous mixture of two or more compounds. Solvent - The compound that comprises the majority of a solution. Solute - A minor component of a solution. Aqueous solution - A mixture of liquid water and one or more solutes.

Types of Bonding IntramolecularIntermolecular Nonpolar CovalentLondon Dispersion Forces Polar Covalent London Dispersion when symmetric Dipole-Dipole when asymmetric Extremely Polar Covalent: H-N, H-O or H-F bonds Hydrogen Bonding Weakly Ionic: NaCl Soluble in water due to ion-dipole interactions Extremely Ionic: Fe 2 O 3 Insoluble in water due to high crystal lattice energies

Like Dissolves Like In order for a mixture to combine and become homogeneous on the molecular level, the compounds must have similar intermolecular bonding properties, i.e. ethanol and water. Two compounds that have very different intermolecular forces will not mix and will produce a heterogeneous mixture, i.e. Italian salad dressing.

Comparing Intermolecular Forces CompoundLondon Dispersion Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O)

Comparing Intermolecular Forces CompoundLondon Dispersion Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) CH 4 methane H 2 O water

Comparing Intermolecular Forces CompoundLondon Dispersion Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) NH 3 ammonia H 2 O water

Comparing Intermolecular Forces CompoundLondon Dispersion Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) Fe 2 O 3 Iron(III) oxide C 8 H 18 octane

Comparing Intermolecular Forces CompoundLondon Dispersion Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) CO 2 Carbon dioxide H 2 O water

Melting and Boiling Points The relative melting and boiling of various compounds are due to the strength of intermolecular forces. If two compounds have the same types of intermolecular forces, the total number of electrons can usually be used to predict the degree of London dispersion forces. The compound with more electrons usually has the higher melting and boiling points. For compounds with hydrogen bonding, the ratio of hydrogen atoms to lone pairs can be important.

Comparing Intermolecular Forces Compound London Dispersion (Total number of electrons?) Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) CH 4 methane C 2 H 6 ethane

Comparing Intermolecular Forces Compound London Dispersion (Total number of electrons?) Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) CO 2 Carbon dioxide H 2 O water

Comparing Intermolecular Forces Compound London Dispersion (Total number of electrons?) Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) NaCl Sodium chloride C 6 H 14 Hexane

Comparing Intermolecular Forces Compound London Dispersion (Total number of electrons?) Dipole- Dipole (AXE?) Hydrogen Bonding (H-F, H-N, H-O) Weakly Ionic (Ions with low charges) Strongly ionic (Both ions have charges >2 Usually Insoluble in H 2 O) CO 2 Carbon dioxide H 2 O water

Summary If compounds have similar intermolecular forces they tend to form homogeneous solutions, “like dissolves like.” The strength of intermolecular forces can be used to predict relative melting and boiling points