Intermolecular Forces: Liquids and Solids Lecture 22
Bonding Forces Chemical bonds between atoms in a molecule are strong.
Covalent bonds electrons are shared H2, CO2
Ionic bonds electrons are transferred NaCl, NaF
Intermolecular Forces • Attractions between molecules – NOT chemical bonds – Much weaker than chemical bonds • Create properties of liquids – Vapor pressure, boiling point, viscosity, surface tension
• Three types – Dipole interactions – Hydrogen Bonding (a type of dipole) – Dispersion forces http://www.teachmetuition.co.uk/Chemistry/Intermolecular/hydrogen_bonding.htm
Dipoles Polar covalent molecules：Unequal sharing of electrons Electrons spend more time around more the electronegative atoms
Partial positive charge on less electronegative atom
Partial negative charge at more electronegative atom
Dipole moment http://faculty.njcu.edu/tpamer/gilbert-lessons/lesson5/gilbert6-4.htm
Dipole-Dipole Interactions Polar molecules orient themselves to take advantage of their charge distributions Positive ends are close to negative ends
The greater the dipole, the stronger the attraction
Hydrogen Bonding • Special type of dipole-dipole attraction • H is bound to a highly electronegative atom – H-O, H-N, H-F
• Stronger than other dipoledipole interactions
Two factors – High polarity of bond – Molecules (dipoles) can get closer together because H atom is so small
• Important in water and living organisms
Hydrogen Bonding in Water Hydrogen bonding causes low vapor pressure and high heat of vaporization – Allows water to act as a coolant
Climate regulation – Water in oceans, lakes and rivers help regulate the Earth's temperature – Absorbs heat energy from the sun in warmer seasons – Releases heat into the atmosphere in cooler seasons
Body temperature control – Evaporation of perspiration cools the body
Dispersion Forces Dispersion forces, or London forces, are the result of a temporary dipole. Constant shifting of electrons may make a region temporarily electron poor and slightly positive while another region becomes slightly negative. This creates a temporary dipole and two molecules with temporary dipoles are attracted to each other.
Dispersion Forces • Weakest intermolecular forces
• Present in all molecules • Electrons are not always evenly distributed around atoms or molecules – Temporary dipoles
• Dispersion forces increase with the number of electrons atoms and molecules • Although individual LD forces are small, in large molecules the effect can be large
Identify the Forces DD Dipole-dipole, HB Hydrogen bonding, LD London Dispersion Subst.
H2 O HCl CaF2 H2 S
F2 C4H10
Intra Molecular F
Polar?
Inter Molecular F DD
Inter Molecular F HB
Inter Molecular F LD
Liquids and Solids • How do the intermolecular forces of liquids and solids determine their properties?
Vapor Pressure • Liquid molecules at the surface can break free from the others – Vaporization – Requires Energy!
Greater intermolecular forces result in low vapor pressure and higher energy required to evaporate the liquid
Physical Properties of Liquids There are 4 physical properties of liquids related to the intermolecular attractions: As intermolecular forces increase: Vapor Pressure
decreases
Why?
Boiling Point
increases
Why?
Viscosity
increases
Why?
Surface tension
increases
Why?
Types of Crystalline Solids
Ionic Solids • Made from ionic compounds – ie. NaCl
• • • •
Held together by opposing charges Break into ions when dissolved High melting points Visualized by packing spheres
Molecular Solids • Fundamental unit is the molecule – Sugar C6H12O6
• Individual molecules spread apart but do not break bonds when dissolved • Lower melting than ionic solids because forces are weaker – Dipole-dipole – London dispersion forces http://www.food-info.net/uk/products/sugar/types.htm
Atomic Solids • Properties vary depending on which atoms and how they interact • Group 8 elements (noble gases) – Very low melting – Only London dispersion forces
• Diamond (carbon) – Very high melting – Very hard – Strong covalent C-C bonds giant molecule – Network molecule http://www.1diamondsource.com/designring/your_ring.htm http://cwx.prenhall.com/petrucci/medialib/media_portfolio/13.html
Bonding in Metals
Atomic Solids
Properties – Ductile, malleable, conduct heat and electricity – Changes shape, but durable and high melting – Bonding is strong, but atoms can move around each other easily
Review There are three types of intermolecular bonds: Dipole forces
Hydrogen bonds (A type of dipole force) Dispersion forces
Dispersion forces are the weakest and hydrogen bonds are the strongest.
These intermolecular attractions affect the physical properties of substances.
