Coulomb Force

Coulomb Force Definition and Examples

Thinking back to our balloon example, what caused the balloon to stick to the wall? It was an electrostatic force, also known as Coulomb Force.

Fig. 1: Ions attracting and repulsing according to Coulomb's law.

Remember, the attraction between negatively charged particles on the balloon and the positively charged wall caused it to "stick"! But repulsive forces were actually at play here too! The wall was initially neutral, but when the negatively charged balloon is brought near, it repels the negatively charged electrons from the area. This leaves the wall more positively charged than the balloon; thus, it sticks! From this one example, we can see the fundamental principles of Coulomb's force: Opposites attract and like repels!

Now, let's think for a second about how and why this physics concept matters to us chemists. What keeps the subatomic particles of an atom together? How are ionic chemical bonds formed? These are different questions with the same answer: the electrostatic attraction between oppositely charged particles!

• Atoms are made up of positively charged protons that are orbited by negatively charged electrons.
• Ionic bonds are made up of positively charged cations and negatively charged ions. These opposite charges are attracted to one another!

We will spend most of our time exploring what the Coulomb Law tells us about the energy involved in ionic bonds. To do that, we first must know Coulomb's Law mathematically and what it means!

Coulomb Force Equation

The equation commonly associated with Coulomb Force is \[ F={k}\frac{q_1 q_2}{r^{2}}\] but since we are interested in the energy associated with ionic bonds we can use the following equation

\[ E={k}\frac{q_1 q_2}{r} \]

Let's break apart these different variables.

E = energy required to break apart ions (force of attractive and repulsive forces)

Q₁ and Q₂ = the charge of the cation and anion

r = refers to the distance between the nuclei of the two ions (bond length)

k = Coulomb's constant and not relevant to the chemistry side of things

This equation is the mathematical expression that backs up Coulomb's Law. Now that we know some foundational aspects of Coulomb's force, we can redefine Coulomb's Law according to this equation. First, what does this equation tell us about ionic bonds and charged particles in general?

1. It shows a proportional relationship between the magnitude of charges and the strength of the ionic bond
2. It shows an inversely proportional relationship between the distance between the ions and the strength of the ionic bond

The Magnitude of Coulomb Force

So what are we calculating when we figure out the magnitude of a force or, in this case, the magnitude of energy in an ionic bond? We are looking at the strength of the interactions between cations and anions.

Based on what we've already covered this means the strength of the ionic bond depends on two factors:

1. charge of ions: as the charges increase, the ions are more strongly attracted to one another which increases the ionic strength

2. size of ions: as the size increases, the internuclear distance between ions increases which decreases ionic strength

To test your grasp on all this knowledge, let's try and apply it when comparing the strength of two ionic bonds!

We can also use Coulomb's law to estimate the lattice energy of ionic compounds.

Let's continue looking at different applications of Coulomb's Law in chemistry!

Applications of Coulomb Force

We already know one of the most important applications of Coulomb's forces is the interaction strength of ionic bonds and lattice energy, but Coulomb forces are everywhere, so there are a couple more applications worth mentioning!

1. Lattice Energy and Solubility

• Lattice Energy directly impacts the solubility of an ionic solid. The stronger the ionic bond, the less soluble it is. According to Coulomb's Law, if it requires more energy to break apart ions, the ions have a high charge and are close together. If it's harder for the ions to be separated, doesn't it make sense that it's less likely that ions will attach to water molecules?

2. Ionization Energy

• Coulomb's Forces also apply to the ionization energy of ions.

The same principles of distance and charge apply to elements, except between the negatively charged electrons and a positively charged nucleus. The greater the distance between the nucleus and valence electrons, and the weaker the charge of the nucleus, means it takes less energy to remove the electron.

Coulomb Force of Replusion

Thinking back to Coulomb's Law, it includes attractive forces between opposite charges and repulsive forces between like charges. Arguably, attractive forces between oppositely charged particles are very significant and prevalent throughout chemistry. This is why we've focused most of our time on them. However, repulsive forces exist in situations like electron-electron expulsion and even within the nucleus between positively charged protons.

You should now be familiar with Coulomb's law and how we can use it to understand ionic bonds and strength. For more examples of how to apply the concept of coulomb force to ionic bonds, work through the flashcards!