Practical work 4. Experimental problems on the nitrogen subgroup
Four kinds of problems are considered in this practical work. 1) Making copper(II) nitrate Cu(NO₃)₂ from CuO, Cu(OH)₂ or (CuOH)₂CO₃ with HNO₃: CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O; Cu(OH)₂ + 2HNO₃ → Cu(NO₃)₂ + 2H₂O; (CuOH)₂CO₃ + 4HNO₃ → 2Cu(NO₃)₂ + CO₂ + 3H₂O. The solution is evaporated and the salt crystallizes. 2) Ammonium fertilizers must not be put on limed (alkaline) soil: NH₄⁺ + OH⁻ → NH₃↑ + H₂O, the nitrogen escapes into the air as ammonia and the fertilizer is wasted. 3) Telling Na₂SO₄, NH₄NO₃ and NaCl apart: BaCl₂ gives white BaSO₄ only with Na₂SO₄; AgNO₃ gives white AgCl only with NaCl; NH₄NO₃ releases ammonia when warmed with NaOH (moist red litmus turns blue). 4) Telling H₃PO₄, H₂SO₄ and HCl apart: BaCl₂ gives a precipitate only with H₂SO₄; AgNO₃ gives yellow Ag₃PO₄ with H₃PO₄ neutralized by an alkali, and white AgCl with HCl. The experiments are done under the teacher’s supervision, in goggles and gloves, in small amounts: acids must not touch the skin, gases are not smelled, barium and nitrate salts are not swallowed, and heating only with permission. In this lesson you prepare the plan, a table and the equations.
“Identification tree”: for each problem draw a tree diagram: 3 unknown substances → 1st reagent (which one separates) → 2nd reagent → result. Work on paper or cards. Tasting or smelling substances is strictly forbidden; the real experiment is only in class with the teacher.