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Chapter 7: Problem 93

The flame tests for sodium and potassium are based on the emissions at \(589 \mathrm{nm}\) and \(404 \mathrm{nm},\) respectively. When both elements are present, the \(\mathrm{Na}^{+}\) emission is so strong that the \(\mathrm{K}^{+}\) emission can be seen only by looking through a cobalt-glass filter. (a) What are the colors of the \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) emissions? (b) What does the cobalt-glass filter do? (c) Why is \(\mathrm{KClO}_{4}\) used as an oxidizing agent in fireworks rather than \(\mathrm{NaClO}_{4} ?\)

Short Answer

Expert verified
a) Na鈦� is yellow, K鈦� is violet. b) Cobalt-glass filter blocks yellow light. c) KClO鈧� provides desirable violet color for fireworks.

Step by step solution

01

Identify the emission color of Na+

The emission wavelength for sodium (Na鈦�) is given as 589 nm. Wavelength in this range typically corresponds to a visible color in the yellow region. Hence, the color of the Na鈦� emission is yellow.
02

Identify the emission color of K+

The emission wavelength for potassium (K鈦�) is given as 404 nm. Wavelength in this range typically corresponds to a visible color in the violet range. Hence, the color of the K鈦� emission is violet.
03

Explain the function of the cobalt-glass filter

The cobalt-glass filter is used to filter out the strong yellow emission from sodium (Na鈦�) so that the weaker violet emission from potassium (K鈦�) can be observed. The cobalt-glass effectively dims the yellow light and allows the violet light to pass through.
04

Understand KClO4's use in fireworks over NaClO4

KClO鈧� (potassium perchlorate) is used in fireworks because the emission of K鈦� ions gives a violet color, which is desirable for displays. In contrast, NaClO鈧� (sodium perchlorate) would emit a strong yellow color due to Na鈦� ions, which can overpower other colors in the firework display.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

sodium flame test
In chemistry, the sodium flame test is used to identify the presence of sodium ions (Na鈦�) in a sample. During this test, a sample is heated in a flame, causing the sodium ions to emit light at a specific wavelength. Sodium emits light at 589 nm, which coincides with the yellow region of the visible spectrum. This bright yellow emission is highly characteristic and can easily be observed with the naked eye, making it a straightforward method to detect sodium in a mixture.
potassium flame test
The potassium flame test is similar to the sodium flame test but is used to identify the presence of potassium ions (K鈦�). When a sample containing potassium is introduced into a flame, the ions emit light at a wavelength of 404 nm, which falls within the violet range of the visible spectrum. The violet color might be fainter than the yellow sodium emission, but it is distinctive under the right conditions. This makes the potassium flame test a useful method for distinguishing potassium from other elements in a sample.
cobalt-glass filter function
A cobalt-glass filter plays an essential role in flame tests, especially when both sodium and potassium are present. Sodium's bright yellow emission can overshadow potassium's violet emission. The cobalt-glass filter selectively absorbs the yellow light of sodium, allowing the violet light of potassium to pass through. By using this filter, chemists can observe the potassium emission more clearly, even in the presence of sodium. This selective absorption is crucial for accurate detection and analysis of multiple elements in a mixture.
fireworks chemistry
Fireworks rely on the principles of chemistry to create stunning visual displays. Different compounds are used to produce various colors when ignited. Potassium perchlorate (KClO鈧�) is a common oxidizing agent in fireworks. Its use ensures a violet emission due to K鈦� ions, adding vibrant hues to the display. In contrast, sodium perchlorate (NaClO鈧�) would produce a dominant yellow emission, often overwhelming other colors. The choice of compounds like KClO鈧� allows for a more diverse and engaging visual effect in fireworks shows, demonstrating the importance of precise chemical selection in pyrotechnics.
emission spectra
Emission spectra are patterns of light emitted by atoms or ions when they are energized. Each element emits light at specific wavelengths, producing a unique spectrum. These spectra serve as 'fingerprints' for identifying elements. In flame tests, the light emitted by ions like Na鈦� and K鈦� at 589 nm and 404 nm, respectively, is observed and measured. Understanding emission spectra is critical in fields like astronomy, chemistry, and even art restoration. It enables scientists to identify the composition and properties of various substances accurately.

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Most popular questions from this chapter

Rank these photons in terms of increasing energy: blue \((\lambda=\) \(453 \mathrm{nm}) ;\) red \((\lambda=660 \mathrm{nm}) ;\) yellow \((\lambda=595 \mathrm{nm})\)

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