Coulomb's Law & Attractive Forces

Coulomb's law describes the electrostatic force between two charged particles
The formula for Coulomb's law is $F_{coulombic} = k \frac{q_{1} q_{2}}{r^{2}}$ , where:
- F_coulombic is the force between the charges
- k is Coulomb's constant
- q₁ and q₂ are the value of the charges
- r is the distance between the charges
For a cation and anion in an ionic bond, F_coulombic
- increases with increasing charge of each ion
- decreases with increasing ionic radius

Coulombic force between cations and anions

coulombic-force

Diagram showing that the Coulombic force between a cation and anion decreases with increasing ionic radius and increases with increasing charge

The greater the force between the charges
- the stronger the interaction between the ions
- the greater the bond energy (also known as lattice energy)
- the more negative the minimum potential energy of a stable ionic bond between the two ions

Worked example

potential-energy-diagram-ionic-compound

The graph shows the potential energy of three potassium salts, X, Y, and Z, as a function of internuclear distance. Based on the data, what are the most likely identities of compounds X, Y, and Z?

	X	Y	Z
A.	KI	KBr	KCl
B.	KBr	KCl	KI
C.	KCl	KI	KBr
D.	KCl	KBr	KI

Answer:

Each potassium salt contains a K⁺ ion and a halide ion with a 1- charge
- So, we do not need to consider the effect that charge has on internuclear distance or potential energy
The ionic radius of the halide ions increases down the group, thus Cl^- < Br^- < I^-
As ionic radius increases
- the internuclear distance between the potassium and halide ions increases
- F_coulombic between the ions decreases
- the lattice energy decreases
- the potential energy minimum becomes less negative
So, KCl with the smallest halide ion will have the smallest internuclear distance and the most negative minimum potential energy
KI with the largest halide ion will have the largest internuclear distance and the least negative minimum potential energy
Therefore, the answer is D

Coulomb's Law & Attractive Forces (College Board AP Chemistry)

Revision Note

Coulomb's Law & Attractive Forces

Coulombic force between cations and anions

Worked example

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Author: Fallon