2.2 IMF and Potential Energy

Intramolecular Force & Potential Energy

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TOPIC 2.2: INTRAMOLECULAR FORCE &

POTENTIAL ENERGY

Enduring Understanding

SAP-3

Atoms or ions bond due to interactions between
them, forming molecules.

Learning Objective

SAP-3B

Represent the relationship between potential energy
and distance between atoms, based on factors that
influence the interaction strength.

Covalent Bonds

Covalent bonds are the bonds
between two nonmetals when
they share valence electrons.

Covalent bonds can be polar
(electrons shared unequally)
or nonpolar (electrons shared
equally).

Covalent bonds

Covalent bonds can be single, double, triple bonds or an
average of those if there are resonance structures.

Graphs!

●Y-Axis - potential energy

●X-Axis - distance between
nuclei

●+ Energy = repulsion

●- Energy = attraction

●Coulomb's Law

●Goldilocks zone

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Graphs!

●The “lowest” part of the
curve shows the energy
(y-axis) that would be
necessary to break the bond
between atoms as well as
the ideal bond length
(x-axis)

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Covalent bonds and Energy

Covalent bonds occur at the lowest
energy state.

This happens when the attraction
between the nuclei is greatest for
the shared electrons, and the
repulsions between electrons and
between the nuclei is the least.

If the atoms are too close together
the nuclei will repel each other.

If the atoms are too far apart the
attraction will not be enough to
hold them together.

Bond Enthalpy

Bond enthalpy is the energy
required to break a bond, or
the energy released when a
bond is formed. The magnitude
is the same, but the sign is
different.

*Larger atomic radii increase
the bond length. Longer bond
length decreases the bond
energy.

Bond Enthalpy kJ/mol

Bond Enthalpy

*Increasing the bond order (single, double, triple)
increases the bond energy: more electrons and shorter bond
length = greater coulombic attraction.

Multiple Choice

The potential energy of a system of two atoms as a function of their internuclear distance is shown in the diagram above. Which of the following is true regarding the forces between the atoms when their internuclear distance is x?

• 1. The attractive and repulsive forces are balanced, so the atoms will maintain an average internuclear distance x.

• 1. There is a net repulsive force pushing the atoms apart, so the atoms will move further apart.

• 1. There is a net attractive force pulling the atoms together, so the atoms will move closer together.

• 1. It cannot be determined whether the forces between atoms are balanced, attractive, or repulsive, because the diagram shows only the potential energy

Draw

C. A graph of potential energy versus internuclear distance for two  Cl  atoms is given below. On the same graph, carefully sketch a curve that corresponds to potential energy versus internuclear distance for two  Br  atoms.

Bond Order

●Bond order is the number of bonding pairs of
electrons between two atoms. In a covalent bond
between two atoms, a single bond has a bond order
of one, a double bond has a bond order of two, a
triple bond has a bond order of three

●Higher bond order = more stable

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Bond Type

Single

Double

Triple

# of e-

2

4

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Notation

Bond Order

1

2

3

Bond Strength

Weakest

Middle

Strongest

Bond Length

Longest

Middle

Shortest

Ionic Compounds and Lattice Energy

The energy to separate ions in ionic compounds is their
lattice energy. It is defined as the change in energy that
takes place when gaseous ions are combined to form an
ionic solid. Combining ions will release energy.

Lattice energy can be represented using a modified version
of Coulomb’s law:

*Larger charges - higher lattice energy

*Smaller radii - higher lattice energy

Multiple Choice

• The melting point of MgO is higher than that of NaF. Explanations for this observation include which of the following?

I. Mg2+ is more positively charged than Na+.

II. O2- is more negatively charged than F-.

III. The O2- ion is smaller than the F- ion.

• 3. II only

• 1. I and II only

• 1. I and III only

• 1. II and III only

• 1. I, II, and III

Example 1

●The bond energy for a C=C bond is 614 kJ/mol with a
bond length of 1.34 Å. It is shown on the graph
below. Sketch a C-C bond and a C=C bond on the
graph.

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YOu Do, we Review

One way to identify an unknown
substance is to measure the bond
energy. HX, HY and HZ were
analyzed and the following graph
was created.

It was known that HI, HCl and HBr
were tested. Use the graph to
match the unknowns with the
knowns.

Practice Problems

​Video

​https://youtu.be/uSUBEdg3Pa0 

https://youtu.be/uSUBEdg3Pa0 

https://youtu.be/-mZAsdJnnMM