Mission B6 - Intermolecular Bonding “Aim B6 – How do IMFs Change the Boiling Points”

Bonding in non-Molecules Ionic solids held by ionic bonds between all the ions Metallic solids are atoms of a metal bound to each other in a sea of mobile electrons Network solids diamond and sand have all their atoms bound together All have high melting points due to these bonds

Intermolecular Forces Individual molecules in a molecular solid are held together by covalent bonds BUT – molecular solids are not! There are three types of molecule - to - molecule attractions in molecular solids called intermolecular forces or IMFs London forces (aka Van der Waals forces) – electrons of one atom can attract the protons of another in a very weak bond Dipole-dipole interactions – the “mini magnet effect” H-bonds – a special case of the “mini magnet effect”

The Weakest - London Forces London Dispersion Forces Protons have attractions for electrons both their own and other atoms A very weak bond occurs between atoms due to these attractions Example helium molecules monoatomic molecules

The Weakest - London Forces London Dispersion Forces The larger the molecules The more electrons / protons The greater the attraction between the two molecules The greater the boiling point Examples BP of Neon = -246oC BP of Argon = -186oC BP of Krypton = -152oC BP of Xenon = -107oC Increasing molar masses

Which of these two molecules has stronger London forces? Tend to raise melting and boiling points Due in part to masses of molecules RELATIONSHIP The larger the molecules the greater the London forces, and the higher the MP and BP Which of these two molecules has stronger London forces?

A little Bit Stronger - Dipole-Dipole Interactions A polar covalent bond occurs between each H and O in water H2O But not an even sharing Due to electronegativity differences Electrons spend more time around oxygen Oxygen becomes slightly (-), H becomes slightly (+) A “mini magnet” or dipole is formed

Dipole-Dipole Interactions Positive ends of dipole attracted to the negative ends of other dipoles Again, electrostatic effect or “mini magnet effect”

Polarity and Molecular Symmetry Polar molecules are dipoles – they are asymmetrical molecules Shape affects polarity! Linear shapes with different atoms attached Ex: hydrogen chloride (HCl) Angular shapes bent Ex: water (H2O) Pyramidal shapes like a tripod Ex: ammonia (NH3) Note – all have polar bonds too!

Polarity and Molecular Symmetry Nonpolar molecules are symmetrical They DO NOT form dipoles! Linear shapes with same atoms on each end Ex: carbon dioxide (CO2) Tetrahedral shapes Four sided pyramid shape All corners have same elements Ex: methane (CH4) Note – even though they have polar bonds, the whole molecule is nonpolar due to symmetry!

Dipole-Dipole Interactions Keep in mind! London Forces occur in all molecules Many molecules also have dipole-dipoles Question: what will happen to the boiling point of similar sized particles, one that is nonpolar with only London Forces, and the other with polar molecules with dipole-dipole interactions? HIGHER MELTING / BOILING POINTS

The ultimate force – HYDROGEN BONDING Special form of dipole-dipole interaction Occurs in molecules with: Hydrogen atoms And small atoms Much stronger form of dipole bonding Reason for the boiling point of water being only 100oC Hydrogen sulfide molecule Water molecule

Boiling points of hydrogen compounds

When Ions and Molecules Collide! Dissolving - Molecule-Ion Attractions NaCl dissolves in water (a given) This is due to the attraction of the (+) and (-) ends of the polar water molecules to the Na+ ions and Cl- ions dissolved in the water Hydration - water molecules surround the ions, separating them and thus dissolving the solid

Molecule-Ion Attractions – Dissolving Sodium chloride and other ionic substances don’t dissolve in oil! No hydration occurs Oil is nonpolar Main reason for oil and water not mixing Always remember: “Like dissolves like” Polar dissolves polar Salt, sugar, in water Nonpolar dissolves nonpolar Oil paint in turpentine, styrofoam in acetone