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Intramolecular and
Intermolecular Forces
Intramolecular Forces – Molecular Forces found within molecules
1) Ionic Bond (electrons transferred between metals and non-metals creates an electrostatic force)
2) Covalent Bond (electrons shared between non-metals creates a bonding force)
3) Polar Covalent Bond (electrons shared between non-metals creates a bonding force)
Intermolecular Forces - Molecular Forces found between molecules
Definitions
Intermolecular Forces - Molecular Forces found between molecules
Intermolecular
http://www.youtube.com/watch?v=wYZg1j7o2x4
A molecule that is slightly positively charged at one end and slightly negatively charged at the other because of electronegativity differences
Polar Molecules
Polar Molecules
Cl H
δ-
δ+
Hydrochloric acid molecule is POLAR
Polar Molecules
Cl C
δ-
δ+
Carbon tetrachloride molecule is NON POLAR because it lacks oppositely charged ends
Cl
Cl
Cl
δ-
δ-
δ-
Some Rules for Determining Polarity of Polyatomic Molecules
General Chemical Formula
Polarity
Examples
Diatomic: 2 different elements
Polar HCl, CO
Nitrogen and 3 other atoms of the same element
Polar NH3, NF3
Oxygen and 2 other atoms of the same element
Polar H2O, OCl2
Carbon and other atoms of two or more elements
Polar CHCl3, C2H5OH
Some Rules for Determining Polarity of Polyatomic Molecules
General Chemical Formula
Polarity
Examples
Diatomic: 2 identical atoms
Non-polar
N2, O2
Carbon and 2 or more atoms of the same element
Non-polar
CH4, CO2
Polarity Practice!(a) HBr (b) NH3 (c) C8H18 (d) CH4 (e) CH3OH
Intramolecular and
Intermolecular Forces
A force that holds atoms or ions together Forces exerted within a compound (e.g.
ionic and covalent bonds) Intramolecular forces influence chemical
properties of a substance Chemical changes involve overcoming
these forces in order for bonds to break and new substances to be synthesized
Intramolecular Forces
A force that exists between molecules or between ions and molecules to influence the physical properties of substances
These are forces of attraction and repulsion that act between molecules
Intermolecular forces include: dipole-dipole, hydrogen bonding, ion-dipole, and dispersion forces (a.k.a. London forces)
Intermolecular Forces(a.ka. van der Waals Forces)
Attraction between opposite partial charges of polar molecules
A polar molecule has a partial negative charge and partial positive charge – these are permanent dipoles
Polar molecules are more attracted to each other than similarly-sized non-polar molecules
Main reason for differences in melting points and boiling points for molecules
Dipole-Dipole Forces
Dipole-Dipole Forces
A strong intermolecular attraction between molecules with a hydrogen atom that is covalently bonded to a highly electronegative atom, often oxygen and nitrogen
When a hydrogen atom is covalently bonded to a highly electronegative atom, the electronegative atom draws the electron away from the hydrogen
Special Dipole-Dipole Force* Hydrogen Bonding *
Because hydrogen has no electrons other than the bonding electrons, this leaves the positively charged nucleus nearly bare
Special Dipole-Dipole Force* Hydrogen Bonding *
H2O(g)H2O(l)H2O(s)
An intermolecular attraction between partial charges of polar molecules and ions
Can be either between cation and polar molecular or anion and polar molecule
Ion-Dipole
Hydration involves an ion or molecule being surrounded by water molecules that are arranged in a specific manner
Hydration
Consider the following diatomic molecules under standard temperature and pressure
Dispersion Forces
Gas Liquid
Solid
A weak intermolecular attraction between ALL molecules, including non-polar molecules, due to temporary dipoles
Often referred to as London forces (Fritz London, 1900-1954)
Caused by non-polar molecules forming spontaneous temporary dipoles
Dispersion Forces
Shared electrons in covalent bonds are in constant motion
This motion, causes momentary, uneven distribution of charge
In other words, a non-polar molecule becomes slightly polar for an instant, and continues on a random basis
Dispersion Forces
This process “disperses” through the substance, creating fleeting dipoles that attract one another
Extremely weak and last only an instant, BUT when many interactions occur at the same time, the overall effect is significant
Dispersion Forces
Factors affecting magnitude of dispersion As SIZE of a molecule
increases, so do the dispersion forces, due to an increase in electrons and therefore temporary dipole formation
SHAPE of a molecule, as the area of contact increases the stronger the dispersion forces
Dispersion Forces
Dispersion Forces
