Problem 71 - Chemistry and Chemical Reactivity

Question

A large pickle is attached to two electrodes, which are then attached to a \(110-\mathrm{V}\) power supply. As the voltage is increased across the pickle, it begins to glow with a yellow color. Knowing that pickles are made by soaking the vegetable in a concentrated salt solution, describe why the pickle might emit light when electrical energy is added. (IMAGE CAN'T COPY)

Step-by-Step Solution

Answer

The yellow glow is due to sodium ions in the pickle emitting light when excited by electrical energy.

Step 1: Understand the Components Involved

The setup includes a large pickle, which is essentially a cucumber soaked in brine (salt solution), placed between two electrodes attached to a 110-V power supply. The solution within the pickle mainly contains ions due to the high concentration of salt.

Step 2: Consider the Role of Electricity

When the voltage is applied, the electrical energy causes the movement of ions within the pickle. The applied voltage provides energy that enables the flow of these ions, particularly sodium ions from the salt, within the pickle.

Step 3: Identify the Light Emission Mechanism

The movement of sodium ions is crucial. When electrons within these ions are excited to higher energy levels by the electrical energy and subsequently return to their lower energy state, they emit photons. The emitted photons correspond to the energy gap that is specific to sodium, resulting in the characteristic yellow glow.

Step 4: Relate Sodium to the Emitted Color

The yellow color observed is typical of sodium emission. This occurs because the electrons in sodium return to a lower energy level after excitation, releasing energy in the form of light at a sodium-specific wavelength, which is in the yellow spectrum of visible light.

Key Concepts

Ionic conduction

In the context of electroluminescence, ionic conduction plays a crucial role. When a voltage is applied to a pickle, it creates an electric field. This field causes ions within the pickle to move, which is a process known as ionic conduction.

The pickle acts like an electrolyte due to the presence of a high concentration of ions from the salt solution in which it was soaked. These ions are predominantly sodium ( ext{Na}^+) and chloride ( ext{Cl}^−) ions. In a salt solution:

• Ions move toward electrodes: Sodium ions move towards the negative electrode, while chloride ions move towards the positive electrode.
• This movement of ions constitutes an electrical current through the pickle.

This current is essential for the following steps leading to photon emission. Thus, ionic conduction not only facilitates the flow of electricity through the pickle but also sets the stage for the sodium emission spectrum.

Sodium emission spectrum

When energy is supplied to the pickle through the applied voltage, sodium ions particularly play a significant role. The sodium emission spectrum is central to understanding why this energy results in the emission of light.

Sodium's unique property enables it to emit light, specifically in the yellow region of the visible spectrum. Here is how it works:

• As the electrical energy excites these sodium ions, their electrons absorb energy and jump to higher energy levels.
• Electrons do not remain at higher levels for long and revert to their lower energy states, emitting energy as they do so.
• The corresponding release of energy is observed as light at a specific wavelength, characteristic of sodium, which is approximately 589 nm (nanometers).

This specific emission results in the typical yellow color associated with sodium lighting, such as streetlights.

Photon emission process

The photon emission process involves an understanding of how light, in general, creates a visible glow when electrical energy is applied, as observed in the pickle experiment.

Firstly, when sodium ions become energetically excited due to the applied voltage:

• The electrons within these ions gain energy and move to higher energy levels. This process can be likened to climbing a hill, where energy (provided by the voltage) allows the electrons to "climb" to higher energy points.
• Once at these elevated levels, electrons do not remain stable and will "fall back down" to their ground state (stable low energy level).
• This transition back to the lower energy level releases energy in the form of photons—particles of light.

The emitted photons correspond to the energy gaps between these levels. In the case of sodium ions, these gaps emit photons at precisely the right wavelength to produce the characteristic yellow light.