Class notes for Chapter 11: Liquids, Solids, and Intramolecular Forces in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include intermolecular forces (dispersion, dipole-dipole, hydrogen, and ion-dipole), int...
Solids, Liquids, and Gasses: A Molecular Comparison
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●Gas state has much lower density than solid or liquid
●Solid and liquid states are similar to each other, and gas is much different
○Note that solid water is slightly less dense than liquid water
■This is atypical behavior – most solids are more dense than their corresponding liquids
Properties of the States of Matter
●Difference in liquid and solid is freedom of movement of the molecules within them
○Particles in a liquid are in close contact but thermal energy partially overcomes those attractions, allowing them to move around one another
○Not the case in solids; particles are locked into position ●Liquid assumes the shape of their containers because the particles are free to flow and move
●Liquids and solids generally cannot be compressed because the particles are already in close contact
○Gas molecules have a lot of space in between them and can be compressed easily
●Crystalline: describes a solid in which particles are arranged in patterns with long range, repeating order
●Amorphous: describes a solid in which particles do not have any long-range order
Changes Between States
●We can transform one state of matter to another by changing the temperature or pressure
●In general, pressure increase favors denser state
○Increased pressure of gas sample → liquid state
Intermolecular Forces
●Structure of the particles in a substance determine the strength of the intermolecular forces → determine whether it’s a solid, liquid, or gas
●Moderate to strong forces → liquids and solids, weak forces → gasses
●Liquids and solids have high melting and boiling points, gasses have low melting and boiling points
●Intermolecular forces originate from the interactions between charges, partial charges, and temporary charges on particles
●Recall that according to Coulomb’s law, the potential energy (E) of two oppositely charged particles (q 1 and q2) decreases with increasing magnitude of charge and decreasing separation (r) ○
●Protons and electrons are attracted to each other because their potential energy decreases as they get closer together ○Molecules with partial or temporary charges are attracted to each other because their potential energy decreases as they get closer together
●Even the strongest intermolecular forces are generally much weaker than bonding forces
○Bonding forces are the result of large charges interacting at close distances
○Intermolecular forces are result of smaller charges interacting at greater distances
○
Dispersion Force
●London (Dispersion) Force: intermolecular force exhibited by all atoms and molecules that results from fluctuations in the electron distribution
○Happens in any particle with electrons (all of them)
●Electrons in a particle may at any one instant be unevenly distributed
○EX: picture a frame-by-frame video of a helium atom
■
■In frame 3, the two electrons are on the same side of the atom → left side has slightly negative charge, right side has slightly positive charge
■Instantaneous/Temporary Dipole: temporary charge separation in a particle when electrons are unevenly distributed
●The positive end of an instantaneous dipole on one helium atom attracts the negatively charged electrons of the neighboring atom → induces instantaneous dipole on neighboring atom
○This attraction is the dispersion force
●Magnitude of dispersion force depends on how easily the electrons in the particle polarize (move around) in response to instantaneous dipole
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