edited by
J. DUDLEY HERRON Purdue University Wed LafmyeHe, Indiono
47907
Nitrogen Oxldes Experiments C h a r l e s E. O p h a r d t Elmhurst College Elmhurst, lllinois~60126
In an effort t o provide students with interesting experiments in which the results are not known in advance, this series of experiments involving four different nitrogen oxygen compounds has been developed. The properties of nitric oxide and nitrogen dioxide gases and their interaction with air and water to produce nitrate and nitrite ions are studied in these experiments. While doing these experiments, the student uses descriptive chemistry, equilibrium principles, oxidationreduction principles, and gas laws. Facts and properties learned are immediatelv. a .. ~ ~ l i into e d new situations. In the first laboratory period, nitrogen dioxide is prepared and its properties (color and solubility in water) are studied. T h e redox properties of the aqueous solution of nitrogen dioxide containing nitrate and nitrite ions is then investigated. Nitric oxide is then prepared and its properties are compared to nitroeen dioxide. In the second laboratow Deri~d,a classical determynation of the gram-molecular voiime of hydrogen using hydrochloric acid and magnesium is performed. Then the student is asked to predict what will happen when nitric acid and magnesium are reacted together. Preparation of Nltrogen Oxldes
T h e gas generation and collection apparatus which is used in both preparations and the determination of molecular volume is shown in the figure. Nitrogen Dioxide
Nitric Oxide
Three 2 x 15 em test tubes are filled with water and immersed upside down in the 1-1beaker. Iron filings (1-2 g) and 3 ml of water are out =~ into the eeneratine tube. Fifteen milliliters of 6 M nitric acid ir poured into the zepnratory funnel. If opening the stupper end stopcock does nor release the nitric arid. the stopper should he used like a "plunger" to force the acid ~ n t othe generating t u h ~ . ~
cooled test tube. If it is given that the lightest colored gas is dinitrogen tetraoxide, the student is expected to answer questions and apply the following concepts to the equilibrium: exothermic and endothermic reactions, LeChatelier's Principle, and entropy. 2) Action of water. The fact that nitrogen dioxide is soluble in water may be easily observed by the student. The stopper on one test tuhe of ess is redaced with a one-hole stonner and an eve droooer mntainr'ns into the Lie.After the water Gkx~ .~. 1 ml n of -water ~ inserted ~ pelled from thedropper into the rest tuhe, the tube is inverted under water and the dropper is removed. Two to four mrlliliters of wawr will be sucked into the tube. (The contentsshould be saved for testingin the next part.) The student is expected to give reasons for the increme in water volume and the proofs for the solubility of NOz in water. ~
The small U tube is removed from the end of the gas generating tube for thir preparation. Copper shot 11gl is put intn the 2.5 X 20 cm test tube. Five milliliters of 16 A1 nitric acid is poured into the oep. aratory funnel. The nitrogen dioxide is collected by the upward disnlacement of air so the beaker is not needed. Three 2 X 15 cm test lubes are filled with NO1. The gas delivery tuhe is immersed in a test tube of water to trap excess gas at the end of the preparation.
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metal Apparatus for experiments wlth nltrogen oxldes.
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Nitric Oxide
Action of Air. The mouth of a test tube containing NO gas ie quickly placed tightly to the mouth of s dry test tube containing only air. A yellowbrown color should soon be apparent. After havinp: seen the color of nitrogen dioxide, the student has little trouhle postulating the oresence of SO2 resulting from the reaction oi NO and air. The oroof of this mav be obtained bv testine the solubilit~in water as above. The student is asked tu postulate rhe presence d a l l gases alter the additionof air and wnter,andexplain the wnouscalorchanges. Air pollution type reactions may be intrduced and considered at thir time. Propertles of Nllrate and Nltrlte Ions ~
Propertles of Nltrogen Oxldes
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Nitrogen Dioxide 1) Equilibrium. Merely by placing a test tube of NO2 gas in an
ice-bath, the following equilibrium may be studied 2 NOnt N204
+ heat
After 5-10 min a very noticably lighter color may be observed in the 374 / Journal of Chemical Education
Oualitative Tests Tests for both nitrate and nitrite ions in the presence of each other were previously difficult to perform. T h e methods of
' Grotz, L. C., J. CHEM. EDUC., 50,63(1973).
Grotz' have simplified both tests and are used in this experiment. Ngrrare Ion. Three dropsof test solutron (either 0.1 M NaNO:, or unknown) arc placed inn smnll test tuhe. Then 3 drops of diphcnylamine reagent and 10dro~a of I8 M H2S04areadded After mirma. a deep blue color slowly &ging to d&p &let indicates the presenfe of nitrate ion. Nitrite Ion. Five droos of test solution (either 0.1 M NaNO? or unknown) areadded to the purple solution resulting from apo~itive nitrate ion test. The presence uf "mite ton IS indicated hy a rnpad cuhrr change from violet to yellow. Application of Oxidation-Reduction Principles
Oxidation-reduction definitions and principles should be presented a t this time, if not previously presented in lecture or prior experiments. Reducing agents have a n increase in oxidation state and oxidizing agents have a decrease in oxidation state. If a redox reaction occurs. both a n oxidizine and a reducing agent must be present. If one agent is identified, the other agent must b e the opposite of the first. T h e student must be given sufficient information in order t o identify one agent and thus figure out the other a s well. T h e amount of information given would depend on their prior experience. A h e l ~ f uexercise l a t this ~ o i nist to have the students determine the-oxidation states of hitrogen in the four compounds of this experiment.
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Water Solutions of Nitrogen Dioxide
1) Woter Alone: The solution resulting from the solubility eaperiment in the section on Action of Water on NO* in the part on Properties of Nitrogen Oxides should be tested for the presence of nitrate and nitrite ions. Both should be present as evidenced by a fleeting blue color quickly changing to yellow. In addition a litmus test should indicate the presence of hydrogen ions. Equations can be written for these reactions. 2) Action of Potassium Permongonote: Two milliliters of water are added to a test tuhe of NO2 gas and shaken to dissolve the gas. Then 2-3 ml of 6 M HzSOa are added and 0.1 M KMnOl is added dropwise until a faint pink color remains or not more than 20 drops have been added. The demlorization reaction involves oxidation-reduetian. Althoueh amazing as it may seem, sume+tudenrsreport no reaction n t this time since they fail tu realire t ha1 a purple color diroppearinp is ewdenre of areadion. In the previous experiment both nitrateand nitrite ions were shown to be present. An obvious question is: Which of the two ions reacts with the permanganate ion to produce the colorless M n W ion? From the information given the student should be able to figure out that MnOac ion is an oxidizine .. ..agent..therefore a reducine.. went is needed. S u m nitrogen in N01- ikm ir nlrsndy nt the maxmum oxidarum smtt ( T ~ J , then NO:'. ~ o would n hen reducing agent s w e nitrbgw is 1-3) and can i n c r r a s in state to ( - 5 , giving S O I - iona* the fuml product of the reaction. A ronfirmntim d the preceding hypothesis may he carried out hy lesrina the nroduct of the reactim firr the uresrnrr oi nitraw ion and the a&ence of nitrite ion. Due to the incompletenessof the reaction, a prior treatment of 6 drops of the solution after the KMnOl addition ~
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with 10 drop. of 0.2 M NaHS03,2 drops of 18M HzSOa, and heating in a b o i i water bath 5 minis necessary. Half of the solution is tested for nitrate ions and the other half is tested for nitrite ions. 3) Action of Starch-Potassium Iodide Paper A moist piece of star&-KI paper is put into a test tube containing NOz gas. The paper turns blue-black. Mvstudents are exoected to know the sienifieanee of this result from previous experimentr. T h e application of uxidation-reduction principles wuuld lead to a prupcrly written equation in the manner of the previous example. n
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Nitric Oxide Gas 1)Action of IronUIl Sulfate. Fiftv millierams of FeSOa is dissolved in 3-4 &I of ;a& and this solution isedded to a tube of NO gas. This makes the Fe(NO)+complex, a classical indicator for NO
gas. 2) Action of Potassium Permonganate. Five drops of 0.1 M KMnO4,5 drops of 6 M H2SO4, and 3-5 ml of water is quickly added to a tuhe of NO gas. The resulting solution is tested for nitrate and nitrite ions as done previously. An equation to express these results
can be written. Determination of Gram Molecular Volume of Hydrogen T h e purpose of this classical experiment is to experimentally determine the molar volume of hydrogen gas. This molar t volume of eas t o b e volume can then be used t o ~ r e d i c the obtained from t h e reaction of nitric acid and magnesium. The aooaratus in the fieure is used for this exoeriment. A 100-ml graduate is filled with waier and inverted in the beaker of water. A 7-8 em (0.01-0.09 g) piece of magnesium ribbon is weighed on the analytical balance and placed in the generating tube. Two milliliters of 6 M HCI and 8 ml of water are poured into the separatory funnel. Various gas laws are used to calculate the molar volume of hydrogen gas. Reaction of Magnesium with Nitric Acid This final experiment gives the student a chance to evaluate and synthesize the results of all the previous experiments. Which gas and how much will be produced from the reaction of magnesium with nitric acid? Three possibilities e x i s t N 0 , NO?. andlor HI. T h e student should write and balance the three equation; with these products. Another piece of magnesium ribbon is then weighed on a n analytical balance. T h e moles of magnesium, balanced equations, and the molar volume of a eas can be used to calculate the volumes of the three gases a t ~ T which P could be obtained. The e x ~ e r i m e nis t carried out in the same manner as above using 2 &I of 6 M H N 0 3 instead of HC1. T h e students are usually surprised t h a t none of t h e predicted volumes of gas are obtained. They are expected t o postulate reasons for their results. Amone- the reasons usuallv cited are t h a t a mixture of gases is produced-one of which may he NO1 which is soluble in water-and t h a t t h e gas measured in the laboratory is not a t STP. A more subtle reason may be t h a t if NO is originally produced, it reacts with air in the generating tube t o make NO2 which then dissolves in water.
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Volume 53. Number 6, June 1976 / 375
