Chemistry Warm Up - Intermolecular Forces and
Melting/Boiling Points
For the following intermolecular forces:
a. Describe how they work on the submicroscopic
level and tell what substances have these forces
present.
b. Rank all of them in order from weakest to strongest.
1. Ionic Bonds
2. Covalent Bonds
3. Dispersion Forces
4. Hydrogen Bonds
5. Dipole Interactions
6. Rank the following substances from least to
greatest boiling point. Explain your ranking. Identify
the most important forces holding the particles together
in the substances.
CO2
CH4
HCl
H2O CH2O
NaCl Graphite
7. Explain how the submicroscopic structure of metals
and ionic compounds lead to their different properties
of malleability and electrical conductivity.
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Answers:
1. Ionic bonds occur when a less electronegative atom, usually a metal
loses one or more valence electrons to a much more electronegative atom.
The result from the loss/gain of electrons is an ionic bond which is the
attractive force between the negatively and positively charged ions. This is
the force that holds together the particles in an ionic compound.
2. When 2 atoms share their valence electrons to attain a stable octet of
electrons, the attractive force between an atom and the other atom's
electrons is called a covalent bond. Covalent bonds occur between non
metallic atoms. When all of the atoms in a substance are covalently bonded
together, rather than forming individual molecules, the substance is known
as a network solid. Network solids are very hard and have very high
boiling points because to break them or vaporize them requires breaking the
covalent bonds.
3. Dispersion forces occur when the electrons in the outer electron clouds
shift to one end of the molecule, resulting in a temporary dipole. These
temporary dipoles are weakly attracted to each
other and the force is known as dispersion force. This force is present in
all molecules, but is much weaker than dipole forces or hydrogen bonding.
Because it is so weak, it is only important in molecules that are not dipoles.
4. Hydrogen bonding occurs in dipole molecules that have hydrogen
bonded directly to N, O , F or Cl. These very electronegative atoms attract
the electrons around the hydrogen so strongly that the hydrogen atom
becomes temporarily positively charged and can then form a bond by
sharing the electron pair from one of the electronegative atoms on a
neighboring molecule. This results in a bond that is considerably stronger
than an ordinary dipole, but is weaker than a full covalent bond.
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5. Dipole interactions are the forces between dipoles that are the
attraction between the positively charged end of one dipole with the
negatively charged end of another dipole. These forces are found in any
molecules that form dipoles.
The weakest IM forces are dispersion forces followed by dipole interactions,
hydrogen bonds, ionic bonds and covalent bonds.
6. In order from least to greatest melting point:
CH4 < CO2 < CH2O < HCl < H2O < NaCl < Graphite
The boiling point of a substance is affected by the strength of the
intermolecular attractions, with stronger attractions leading to higher boiling
points.
CO2 and CH4 both have low boiling points due to the dispersion forces,
which are the weakest of all IM forces, but CH4 has weaker dispersion forces
because it has fewer electrons. CH2O has dipole interactions which are
stronger than dispersion forces but weaker than hydrogen bonding. HCl
and H2O both have hydrogen bonding, but the hydrogen bonding in H2O is
stronger because of the greater electronegativity difference between H and
O compared with the difference between H and Cl. NaCl is held together by
ionic bonding which is stronger than any of the Van der Waals forces and
graphite is a network solid held together by covalent bonds which are
stronger than any of the other intermolecular forces.
7. Metals conduct electricity as solids because their metallic bonds consist
of + ions surrounded by the free moving valence electrons that attract the
ions. Because the force from the electrons is not completely dependent on
the position of the metal atoms, metallic substances can bend without
breaking. Ionic solids consist of + and - ions that are held together by
attraction to each other. Since the like charged atoms repel each other,
they must be in a specific position to be bonded together. This keeps them
from conducting since the charges can't move and also makes ionic
substances brittle because any movement of individual ions causes the
whole structure to fall apart from the repulsion of like charged ions.
Lecturenotes
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