Question

The chemical reactions that are used in the anion spot tests in this experiment are for the most part precipitation or acid-base reactions.Given the information in each test procedure, try to write the net ionic equation for the key reaction in each test.

NO₃- (Take Al and NO₃^- to be the reactants, and NH₃ and Al(OH)₄^-to be the products; the final reaction also contains OH^- and H₂O as reactants): To 1 mL 0.5 m NaNO₃ add 1 mL 6M NaOH. Then add a few granules of Al metal, using your spatula, and put the test tube in the hot water bath. In a few seconds the Al-NaOH reaction will produce H₂ gas, which will reduce the NO₃^- ion to NH₃, which will come off as a gas. To detect the NH₃, hold a piece of moistened red litmus paper just above the end of the test tube. If the sample contains nitrate ion, the litmus paper will gradually turn blue, within a minute or two. Blue spots caused by effervescence are not to be confused with the blue color over all the litmus exposed to NH₃ vapors. Cautiously sniff the vapors at the top of the tube; you may be able to detect the odor of ammonia.

Answer 1

Devarda's alloy (Cu/Al/Zn) is a reducing agent. When reacted with nitrate in sodium hydroxide solution, ammonia is liberated. The ammonia formed may be detected by its characteristic odor, and by moist blue litmus, signalling that it is an alkali very few gases other than ammonia evolved from wet chemistry are alkaline.

So taking that the reactants are NO3, Al, OH and water, and the products NH3 and AlOH the net equation should be:

3NO₃^- + 8Al + 5OH^- + 18H₂O --------> 3NH₃ + 8[Al(OH)₄]^-

Try this and tell me in a comment if there's something wrong.

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