Chemistry Lab 2 - Flame Tests - Emission Spectroscopy

NAME: Anna .T. Ramgulam I.D #: 00039038 CLASS: CHEM 131 – General Chemistry I CRN: 12560 CLASS LECTURER: Ms. Risha Kalloo LAB LECTURER: Mr. Bisram Ramdatt LAB: TWO (2) TITLE: FLAME TESTS – EMISSION SPECTROSCOPY

 INTRODUCTION AIM: 1. To investigate and predict the identity of metal ions, Li+ , Na+ , K+ , Ca2+ , Ba2+ , Cu2+ with the use of flame tests. 2. To determine the cations in each of the two unknown substance by observing the flame colour of different metal compounds. BACKGROUND: A number of common metal ions, (Li+ , Na+ , K+ , Ca2+, Ba2+ , and Cu2+ ) give a distinct colour in the presence of a flame. Therefore, a flame test is often used as a confirmatory test in identifying an unknown metal. Compounds of these ions provide the beautiful colours in a fireworks display. When glass is melted in a Bunsen burner flame, sodium ions colour the flame bright yellow. A copper wire inserted into the flame often results in a striking deep blue or green colour.

THEORY: According to the Bohr Theory of the atom, electrons may occupy only specific energy levels. When an atom absorbs sufficient energy, an electron can “jump” to a higher energy level. Higher energy levels tend to be less stable, however, and if a lower energy level is available, the electron will “fall” back, giving off energy in the process. The difference in energies between the two levels is emitted in the form of a photon of electromagnetic radiation. The energy of each photon is described by the equation E = hv, where h is Planck‟s constant and v is the frequency of the radiation. If the wavelength of the released photon is between 400 and 700 nm, the energy is emitted as visible light. The colour of the light depends on the specific energy change that is taking place. White light is a continuous spectrum in which all wavelengths of visible light are present. An excited atom, however, produces one or more specific lines in its spectrum, corresponding to the specific changes in energy levels of its electrons. Because each element has a distinct electron configuration, each has a unique line spectrum. Flame tests are a quick method of producing the characteristics colours of metallic ions. The loosely-held electrons of a metal are easily excited in the flame of a lab burner. The emission of energy in the visible portion of the spectrum as those electrons return to lower energy levels produces a coloured flame. The colour is a combination of the wavelengths of each transition, and may be used to determine the identity of the ion. If two metals are present in a mixture, the colour of one flame may obscure or hide that of the other. However, if cobalt glass is used, it is possible to absorb one of the colours and not the other. Therefore we will also look at a flame produced from a mixture of compounds. From the data, we will then be able to identify which metals are in the unknown substance based on what colour flame it produces upon heating.

 MATERIALS AND METHODS APPARATUS & MATERIALS: Bunsen burner

NaCl(s)

0.5M ZnSO2 (aq)

11 beakers

0.5M NaCl (aq)

0.5M Ba(NO3)2 (aq)

10 test tubes

0.5M LiNO3 (aq)

0.5M KNO3 (aq)

Nichrome wire

Na and K mixture

Test tube with 7.0M HCl

Unknown metal 1 & 2

Micro test tube

PbNO3 (aq)

METHOD: PART A: Known Cations Each test tube (not the test tube containing the 7.0M HCl) was filled to a depth of 1cm with their respective stock solutions. The Bunsen burner was then ignited and the flame was adjusted to produce a non-luminous (smokeless blue flame with a pale blue inner core) flame. The nichrome wire was then cleaned by dipping it into the test tube containing the 7.0M HCl and then held in the hottest part of the flame. This was repeated until the wire imparted no colour to the flame. The loop of the clean nichrome wire was then inserted into the test tube containing the solution and held in the hottest part of the flame. The wire was then cleaned as instructed before and the flame test was repeated for each solution. The colour of the flame for each of the cation was then recorded in a Data Table. The flame test was then repeated for Na+ ions using a little dry sodium chloride and the results were recorded as well.

PART B: Unknown Cations The nichrome wire was cleaned and the flame test was done on the two unknown solutions. The solutions were also retested to ensure accuracy when identifying them. The results were then recorded on the Data Table as well. PART C: Solutions containing ≥1 Cation The nichrome wire was cleaned again and the flame test was carried out on the solution containing the mixture of the KNO3 and NaNO3. The results were then recorded in the Data Table. PRECAUTIONS: Safety: 1) Proper care was taken when handling the HCl to prevent injury as it is caustic and corrosive. 2) Proper care was taken when rinsing out the test tubes containing the acid, as adding water to acid will result in an explosion. Efficiency: 1) Each flame colour emitted was observed thoroughly to ensure accuracy 2) Each flame test were repeated to ensure precise and efficient results RESULTS: The following data table shows the results that were obtained in this experiment:

Table 1: Colours Emitted from the Various Cations in the Presence of a Flame Metallic Ion

Colour of Flame

Sodium (from aqueous NaCl)

Bright Yellow

Sodium (from solid Na Cl)

Bright Orange

Lithium Nitrate

Crimson / Bright red

Lead Nitrate

Blue

Barium Nitrate

Yellowish-green

Potassium Nitrate

Lilac

Zinc Sulphate

Bluish-green

Sodium and Potassium mixture

Reddish-orange

Unknown Metal 1 (Sodium Chloride)

Orange

Unknown Metal 2 (Potassium Nitrate)

Pinkish-purple

LIMITATIONS: 1) The test cannot detect low concentrations of most ions. 2) The brightness of the signal varies from one sample to another. For example, the yellow emission from sodium is much brighter than the red emission from the same amount of lithium. 3) Impurities or contaminants affect the test results. Sodium, in particular, is present in most compounds and will colour the flame. 4) The test cannot differentiate between all elements. Several metals produce the same flame colour. Some compounds do not change the colour of the flame at all.

5) The HCl became saturated but was replaced with fresh HCl. SOURCES OF ERROR: 1) The position of the wire in the flame. If the nichrome wire was held too low of too high, sometimes the right colours were not seen properly. 2) Mixing up of the elements which would result in a different colour flame. 3) Traces of impurities from the last substance tested. 4) How long the wire was kept in the fire and how strong the fire was as well.

 DISCUSSION AND CONCLUSION DISCUSSION: Different salts will produce different colours when placed in a flame, due to the fact that each element has a different atomic structure. The electrons will jump to higher energy levels according to the amount of energy absorbed. It was seen that some elements produced similar flame colour as well. In the experiment, it was noted that impurities can mask the flame colour, for example, sodium, with its intense yellow flame, was capable of masking the colour produced by other elements if it is present as an impurity. It was seen that the flame test is used to visually determine the identity of an unknown metal or metalloid ion based on the characteristic color the salt turns in the presence of a bunsen burner flame. The heat of the flame converted the metal ions into atoms which became excited and emitted visible light. The characteristic emission spectra was also used to differentiate between some unknown elements. Therefore, it can be said that flame tests are an example of a qualitative test, that is, they can detect the presence of certain elements, however, it cannot tell us how of the element is present in the sample.

CONCLUSION: In conclusion, we were capable of observing and evaluating the colours produced by certain metal ions when they are vaporized in a flame. The results of this lab were obvious, other than finding out the unknown element. The obvious part was that every element has a unique spectrum as no two elements have the same number of electrons, or electron configuration. When elements are exposed to energy, their electrons may enter an excited state. The energy added may be in the form of electricity or in this case heat. In this excited state, electrons move from their normal position around the nucleus to higher energy levels. When the excited electrons return to their ground sate or normal position, they give off energy in the form of light. The colour of the light we see when this occurs is really a combination of several colours of light in the spectrum of that element. Each element or compound emits a unique set of wavelengths, only those wavelengths that correspond to the quanta of energy necessary for that element‟s electrons to jump from ground state to the excited state. The uniqueness of each substance‟s spectrum allows scientists to use them as a tool in identifying unknown chemicals. One method used to demonstrate the emission spectra of chemicals is the flame test. Using this method, a small amount of a substance is heated in a Bunsen burner flame and the flame colour is observed.

The arrangement of electrons in an atom determines the sizes of the quantum jumps, and thus the energy and colours of the collection of photons emitted, known as an emission spectrum. In this way the emission spectrum serves as a „fingerprint‟ of the element to which the atoms belong. We can view the emission spectrum of colours all at once with the naked eye. It will appear to be one colour, which we will carefully describe. It is also possible to view the separate colours of the emission spectrum by using a spectroscope, which bends light of different energies

differently. Low energy red light is bent the most and high energy violet the least. This allows us to see the various distinct colours of the emission spectrum of a sample.

POST LAB QUESTIONS: 1) Flame coloration is a test for the Metallic ion because, the metallic ions will enter an excited state and release photons energy, in the form of light, as they return to their ground state. Nitrate contains nitrogen and oxygen, and these atoms do not have energy levels that would give a color to a flame. 2) Dry sodium chloride and the solutions of sodium nitrate and sodium chloride all impart the same colour because: By placing atoms of a metal into a flame, electrons can be induced to absorb energy and jump to an excited energy state, a quantum jump. They then return to their ground state by emitting a photon of light (the law of conservation of energy indicates that the photon emitted will contain the same amount of energy as that absorbed in the quantum jump). The amount of energy in the photon determines its color; red for the lowest energy visible light, increasing energy through the rainbow of orange yellow green blue indigo, and finally violet for the highest energy visible light. Photons outside the visible spectrum may also be emitted, but we cannot see them. 3) The test for sodium and potassium ions when both are present is as follows: First, get a wire and bend it into a ring and put a few crystals of your solid on it. Do a flame test using a bunsen burner or a match. The colour emitted will show that Potassium will give a violet flame and Sodium will give a yellow flame. 4) If recalled correctly from the lab, not much of the solution was needed to identify each compound. It is very sensitive, because you can see light emitted by a "relatively" small number of atoms.

5) Some difficulties that may be encountered in the use of the flame test for identificationare: Some ions may have little to no reaction at all, and the color of the flame may appear no different, therefore making it difficult to identify between different ions. 6) The flame colour of three Group 1, three Group 2 and one Transition Group metals are: Group 1 metals: Lithium – Crimson Sodium – Bright yellow Potassium – Violet Group 2 metals: Strontium – Bright red Magnesium – Bright white Barium – Yellowish-green Transition metal: Copper - Blue 7) (a) Metallic Ion

Observation

Cation

Unknown #1

Yellowish green

Barium Nitrate

Unknown #2

Scarlet

Strontium Nitrate

Unknown #3

Orange-red

Calcium Nitrate

(b) In the laboratory when several of the flame tests were shades of red, to identify one of these ions you can; get samples of known lithium and strontium compounds and repeat the flame test, comparing the colours produced by one of the known compounds and the unknown compound side by side until you have a very good and close match.

8) During a flood, the labels from three bottles of chemicals floated away. The unlabelled bottles of white solids were known to contain SrNO3, (NH4)2CO3 and K2SO4. To easily re-label these three bottles you could mix them with Barium chloride. The one that gives a white precipitate is potassium sulfate. This is the sulfate test. Doing a flame test, would give a red flame for Strontium and a violet colour for potassium. The gas given off for burning the solid containing ammonium carbonate will turn red litmus paper to blue. This is the test for ammonia.