EXP 4: WHAT IS THE OXIDATION STATE OF NITROGEN?

INTRODUCTION:

What is the oxidation state of the nitrogen in the product when a solution of hydroxylamine hydrochloride is oxidized by acidic iron(III) ion ?

(a)Fe³⁺_(aq) + (b) NH₃0H¹⁺ ---> (a)Fe²⁺_(aq) + (b)N?

A known amount of hydroxylamine hydrochloride is reacted with ferric ion which is reduced to ferrous ion. Potassium permanganate is used to re-oxidize the ferrous ion back to ferric ion. If the concentration and amount of the permanganate solution are known then the amount of ferrous ion titrated can be calculated. This amount is equal to the amount of ferric ion that originally reacted with the hydroxylamine hydrochloride. The number of electrons transferred as each ferric ion is reduced to ferrous is one, therefore, the total number of electrons lost per hydroxylamine hydroxide can be calculated and the new oxidation state of nitrogen determined.

Caution: All of the solutions in this experiment are skin and eye irritants. The potassium permanganate solution will oxidize skin and clothing. The iron (III) has been acidified with sulfuric acid and the hydroxylamine hydrochloride solution is irritating to skin. Keep them off your skin! Be sure to rinse any spill with water.

PROCEDURE:

• Obtain three Beralª pipets. Label one Fe³⁺, the second Mn0₄^1- and the third Hydrox (for hydroxylamine hydrochloride).
• Fill each Beral pipet with the proper solution. (Fe³⁺, Mn0₄^1-, and hydroxylamine hydrochloride). Record the concentration of the Hydrox and the permanganate solutions in mole/ g of solution.(These numbers will be found on the reagent bottles.) Mass all three pipettes separately on the analytical balance by placing the pipet in a tared 25 mL beaker.
• Prepare a boiling water bath using a 100 mL beaker about half full of water. Add about 1 mL of the hydroxylamine hydrochloride to a small test tube. (This is about 30 drops) Add about an equal volume of iron(III) to the test tube with the hydroxylamine hydrochloride solution.
• Heat the mixture in the boiling water bath for about two minutes. (Your solution should have a slight yellow color after heating from the excess iron (III). If the solution is clear add a few more drops of the iron (III) solution and heat.)
• Cool the solution with running water for about two minutes. Add Mn0₄^1- dropwise from the Beral pipet to the cooled solution until a faint pink color persists. Mass all three Beral pipets and record the values.
• You may dispose of all solutions by washing them down the drain.

ANALYSIS AND RESULTS:

Hydroxylamine hydrochloride, NH₂0H-HCl, dissolves in water to form hydroxylammonium ion, NH₃0H¹⁺. Assuming that hydrogen and oxygen have their usual oxidation states, what is the oxidation state of N in NH₃0H¹⁺?

The NH₃0H¹⁺ ion is oxidized by the iron(III) ion. You can assume that Fe³⁺ goes to Fe²⁺.

To find the oxidation state of nitrogen the key calculation depends on the fact that the total decrease in oxidation state for the iron must equal the total increase for the nitrogen.

moles of electrons gained by iron(III) = moles of electrons lost by nitrogen

Since iron gains one mole of electrons for every mole of iron(III) reduced: (Fe³⁺ + e^- ---> Fe²⁺), the ratio of moles of NH₃OH¹⁺ to moles of iron will be the change in nitrogen's oxidation state.

In this experiment you added an excess of iron (III) solution. You used potassium permanganate solution to determine the moles of Fe²⁺ present and thus the moles of Fe,³⁺that reacted. (For every mole of Fe³⁺ reacted you get one mole of Fe²⁺.)

The concentrations of your solutions are given in moles/g solution. Multiply the mass of the solution times its concentration to calculate the moles of any reagent used

Be sure to carry out the following calculations and report the answers in your Results and Discussion section (in narrative form): Number each separate calculation.

1. How many moles of hydroxylamine hydrochloride did you put in the test tube?

2. How many moles of potassium permanganate ion did you use?

3. Write a balanced equation for the reaction of Fe²⁺ and MnO₄^1-. (also on the left-hand page)

4. How many moles of iron reacted with the potassium permanganate? (This is the amount of iron that reacted with the NH₃OH¹⁺.)

5. What is the ratio of moles iron to moles of NH₃OH¹⁺? (This is also the change in oxidation state of nitrogen -- show how this is so)

6. What is the probable formula for the nitrogen containing product? (Look for the compound with the correct oxidation state.)

7. Write a balanced equation for the reaction between NH₃0H¹⁺ and Fe³⁺ in acid solution.

Adapted from an experiment submitted to Dreyfus 87 by:
Rob Lewis, Downers Grove North H.S., Downers Grove, IL 60615