Transcript General
Flame Tests for Ions Flame Tests Using an Improvised Alcohol Burner Purpose When a metal salt solution is burned, the metal provides a colored flame. Each metal ion gives a different colored flame. Flame tests, therefore, can be used to test for the absence or presence of a metal ion From the Demonstration Procedure taken from: Dragojlovic, V. Flame tests using improvised alcohol burners. Journal of Chemical Education (76)7, 929. Metal salt solutions in a methanol solvent were burned in 5mL vials with paper wicks. Two unknowns were also tested. Each solution provided a different colored flame. Li K Ca Na Ba Red Violet Orange Yellow Yellowgreen Unknown A Red Unknown B Yellow Important Concepts Atomic Orbitals Energy States of Atoms Ions Atomic Emission Atomic Orbitals Electrons of atoms reside in concentric spheres known as energy “shells” in which they orbit the nucleus of an atom. Each shell is assigned a principal quantum number, n. The value of n is an integer, 1, 2, 3, etc. This number determines the relative energy of the orbital and relates the distance from the shell to the nucleus--the lower the number, the lower the energy of the electron and the closer it is to the nucleus Electrons can be further distinguished according to their location in atomic orbitals, specified regions in space that depend on their energies Energy States of Electrons A ground state refers to the lowest energy of system, in this case an atom. All electrons are in shells with the least energy. As energy is applied to the atom, an electron is promoted from its ground state (residence in an energy shell) to a higher energy shell or an excited state. The excited state has a higher n value, has more energy, and is less stable. Atomic Emission An emission spectra is obtained by adding energy to a material. The addition of energy promotes electrons of that material from the ground state to the excited state. As the electrons “fall” from the excited state to the ground state, they emit the energy they absorbed in the form of electromagnetic radiation (heat, light, etc.) Ions • Ions are charged atoms; we use metal salts in this demonstration which contain metal cations and some non-metal anion • For example, sodium chloride has neither sodium atoms nor chlorine atoms in it…rather, it is made of sodium ions Na+ and chloride ions, ClThe sodium ion is the cation and the chloride ion is the anion • Ions undergo atomic emission and give characteristic colors • This is a way to identify what ion might be present Conclusions Each metal provides a different colored flame by which it may be distinguished from other metals. The color of the flame is caused by the demotion of electrons from an excited energy state to a ground state. Electrons fall from higher energy states to lower energy states which have smaller n values and are more stable. Comments Atomic emission is used in street lamps, fluorescent lights, and neon signs. Two common street lamps using this are the mercury lamp and the sodium lamp. “Neon” signs frequently implement the emission spectra of other gases such as argon and krypton. Very sophisticated instrumental techniques such as “flame photometry” and “atomic absorption” are based on the principles in this demonstration Miniquiz on Flame Tests True or False 1 The color of the first unknown was red, therefore it was calcium. 2 The color of the second unknown was yellow, therefore it was sodium. 3 When an electron “falls” it emits energy in the form of electromagnetic radiation. 4 The greater the n value, the higher the energy, the more stable the orbital. 5 The n value corresponds to the principle quantum number. Answers False. A red flame is indicative of Li. True. True. False. The higher the n value, the less stable the orbital. True.