Question

By exposing potassium chloride to an excess of potassium vapour, a compound forms which appears violet in colour. Explain the source of the colour. What sort of defect is formed?

Short answer

The source of the violet colour when potassium chloride is exposed to excess potassium vapor is due to the formation of a Frenkel defect. This defect occurs as excess potassium atoms incorporate into the KCl crystal lattice, replacing some of the potassium cations (K+). This leads to a structure with both K+ ions and neutral K atoms, with some K+ vacancies. These vacancies create additional energy levels within the crystal, leading to electron transitions and the emission of light, perceived as the violet color when observed.

Step-by-step solution

1. Understand the reaction

When potassium chloride (KCl) is exposed to an excess of potassium vapor, the potassium atoms are incorporated into the KCl crystal lattice structure due to the excess potassium. Consequently, the KCl crystal structure experiences some changes, resulting in the formation of an ionic defect.

2. Determine the type of defect

In the KCl crystal structure, potassium cations (K+) and chloride anions (Cl-) are present. When excess potassium is added, the extra potassium atoms will replace some of the potassium cations in the lattice structure. This results in the formation of a crystal containing K+ ions and un-ionized K atoms, while some of the K+ sites remain vacant. This type of defect is called a Frenkel defect, which occurs when a smaller ion (in this case, K+) gets displaced from its usual position and moves to an adjacent interstitial site. In the case of KCl, this creates a vacancy or "hole" in the lattice structure.

3. Explain the source of the color

The presence of the Frenkel defect in the KCl crystal lattice leads to the formation of the violet color when it is exposed to excess potassium vapor. The excess potassium atoms in the lattice create extra energy levels within the KCl crystal, enforced by an electron transfer from K atoms to K+ vacancies. When the KCl crystal absorbs light, electrons are promoted from their ground state to these additional energy levels. The absorbed light corresponds to the energy difference between the ground and higher energy states. When the electrons relax back down to their original energy levels, they emit light at a certain wavelength, which is perceived as the violet color.

4. Recap the findings

Upon exposing potassium chloride to an excess of potassium vapor, a Frenkel defect is formed in the KCl crystal lattice, which is caused by the displacement of K+ ions by excess potassium atoms. The presence of these defects leads to the creation of the violet color when observed.