AP®ChemistryCollege BoardStudy GuidesUnit 2: Molecular & Ionic Compound Structure & PropertiesChemical BondingCoulomb's Law & Attractive Forces

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

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Written by: Fallon

Reviewed by: Stewart Hird

Coulomb's Law & Attractive Forces

  • Coulomb's law describes the electrostatic force between two charged particles

  • The formula for Coulomb's law is straight F subscript coulombic equals space straight k fraction numerator straight q subscript 1 straight q subscript 2 over denominator straight r squared end fraction, where:

    • Fcoulombic 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, Fcoulombic

    • 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

    • Fcoulombic 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

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    Fallon

    Author: Fallon

    Expertise: Chemistry Content Creator

    Fallon obtained a double major in chemistry and secondary education, and after graduating she taught Chemistry and Organic Chemistry for 7 years. Fallon’s passion for creating engaging classroom materials led her to pursue a career in content development. For over 3 years, Fallon has created videos, review materials, and practice questions for AP Chemistry, IGCSE, and other international exam boards.

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.