4 Answers
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4 NH3 + 3 O2 → 2 N2 + 6 H2O
In NH3, the oxidation number of nitrogen is -3. In N2, the oxidation number of nitrogen is 0
N -3 → N0 + 3 e-
In O2, the oxidation number of oxygen is 0. In H2O, it is -2.
O 0 + 2 e- → O -2
During reduction, the oxidation number is reduced. So the oxygen is reduced. This means the nitrogen in the ammonia is oxidized. Below is a sentence that can be helpful.
Leo the lion goes ger.
Leo means loss of electrons is oxidation. This happened to the nitrogen atom in the ammonia. ger means gain of electrons is reduction. This happened to the oxygen. To determine if the oxidation number is positive or negative, you need to look at an electronegativity table. In ammonia, the electronegativity of nitrogen is 3.0. The electronegativity of hydrogen is 2.1. Since nitrogen has a higher electronegativity than hydrogen, nitrogen has a negative oxidation number of nitrogen. I hope some of this is helpful for you.
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Look at the half reactions:
O2 + 4 e- --> 2 O -2 which shows oxygen gains 2 electrons per atom or 4 electrons per mole
2 NH3 --> 6H+ + N2 + 6e- which shows N loses 3 electrons per atom or 6 electrons per mole
Oxidation is the LOSS of electrons - Reduction is the gaining of electrons. Clearly oxygen gains electrons and is reduced while nitrogen (ammonia) loses electrons
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check the oxidation numbers which change in the reaction
-3 . . . . . . . . . . . 0
. | . . . . . . . . . . . |
4NH3 +3O2 → 2N2 + 6H2O
. . . . . . . .| . . . . . . . . . . . . . |
. . . . . . . 0 . . . . . . . . . . . . -2
O goes from 0 to -2 → it is reduced
N goes from -3 to 0 → it is oxidized
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Reduced ... oxidized.....
Oxidation is the increase in oxidation state. Reduction is the decrease in oxidation state.
.-3+1......0..........0.........+1-2 ........... oxidation states
4NH3 +3O2--> 2N2 + 6H2O
As you can see, nitrogen is oxidized, and oxygen is reduced.
The oxidizing agent "causes" the oxididation and is the reactant which includes the element reduced. The oxidizing agent is O2 gas. The reducing agent "causes" the reduction and is the reactant which includes the element oxidized. The reducing agent is ammonia, NH3.
Also, keep in mind that oxidation states are not actual charges. For instance, in ammonia, nitrogen atoms do not carry a charge of -3. Nitrogen is estimated to have a charge of about -1.1, while each hydrogen has a charge of about +0.36. There is no complete transfer of 3 electrons to nitrogen. The transfer of electrons, as used in the description of oxidation and reduction, and in half-reactions, is hypothetical. It is a convenient way to balanced redox reactions, but it does not reflect the reality of oxidation and reduction.
Hypothetical half-reactions.....
2(2NH3 --> N2 + 6H+ + 6e-) ............ oxidation
3(O2(g) + 4H+ + 4e- --> 2H2O) ....... reduction
4NH3 + 3O2 + 12H+ --> 2N2 + 12H+ + 6H2O
simplify
4NH3(g) + 3O2(g) --> 2N2(g) + 6H2O(g)
The hydrogen ions are strictly hypothetical since the reaction takes place in the gaseous state and are used only as an aid in balancing the half-reactions.