Question

The wavelength of maximum absorption of \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\right]\) is \(580 \mathrm{nm}\) (orange-yellow). What color is a solution of \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\right] ?\)

Short answer

Answer: The color of the solution is blue, as it reflects or transmits the complementary color of the absorbed orange-yellow light.

Step-by-step solution

Complementary Colors

When a substance absorbs a particular wavelength of light (color), it will reflect or transmit the complementary color. Complementary colors are the colors that are opposite to each other on the color wheel.

Identifying the Complementary Color

In this case, the wavelength of maximum absorption is \(580 \mathrm{nm}\) which corresponds to the orange-yellow region of the spectrum. The complementary color of orange-yellow is blue. Therefore, when a solution of the complex \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\right]\) absorbs orange-yellow light, it will reflect or transmit blue color.

Color of the Solution

The color of the solution containing \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\right]\) is blue, as the solution reflects or transmits the complementary color of the absorbed orange-yellow light.

Key concepts

Wavelength of Light Absorption

Understanding the ways substances interact with light is crucial in chemistry, particularly when interpreting the wavelength of light absorption. Light is composed of electromagnetic waves that differ in wavelength. The visible spectrum that we can see with our eyes ranges from about 400 nm to 700 nm, with violet on the lower end and red on the higher end.

When a compound absorbs light at a specific wavelength, it's actually absorbing energy. This energy causes an electronic transition within the compound's molecules, typically an electron moving from a lower to a higher energy level. Notably, not all wavelengths of light are absorbed; the substance will only absorb wavelengths that match the energy needed for these transitions.

Determining Compound Color

In chemistry, Beer's Law is an important principle used to relate the absorption of light to the concentration of the compound. However, what's more visually apparent is the color that the compound exhibits. The compound will distinctly absorb light in the visible region corresponding to a particular color while the remaining light will be reflected or transmitted, giving the substance its characteristic color. For instance, if a substance absorbs light at 580 nm, primarily corresponding to an orange-yellow color, according to the concept of complementary colors, the reflected or transmitted light would appear blue, its complementary hue on the color wheel.

Solution Color Determination

The color we perceive an object, including a chemical solution, to have is directly related to the wavelengths of visible light it does not absorb. Instead, these are the wavelengths that are reflected or transmitted to our eyes. For a solution of a given compound, we can predict its color by knowing its wavelength of maximal absorption. This concept extends from the range of visible light, which is crucial for color determination.

As highlighted in the original exercise, the use of a color wheel allows us to match an absorbed color (or wavelength) with its complementary color. This is what determines the apparent color of solutions containing complex ions or other light-absorbing substances. The practical application of this is widespread in chemistry, from identifying the presence of specific ions in solution to monitoring reaction progress or concentration changes using spectroscopy.

Complex Ions Spectroscopy

Spectroscopy is an analytical technique used to measure the interaction between matter and electromagnetic radiation. Complex ions, which are ions composed of a metal ion at its center with surrounding molecules or ions called ligands, often have unique and diverse spectroscopic characteristics. These can be analyzed to provide information about their structure, environment, and concentration.

In the context of the exercise provided, the spectroscopy of complex ions like can reveal their absorption spectra. For instance, the complex ion exhibits maximum absorption at 580 nm, providing a clue about its chemical environment due to the specific ligands attached to the metal ion. Understanding these absorption patterns is crucial for deducing not only the color of solutions but also for applications such as the creation of coordination compounds in materials science, the analysis of metal ions in environmental samples, or the study of biological systems where metal ions play a role.