When excess oxygen gas is mixed with ammonia in the presence of platinum, a catalytic oxidation reaction occurs, resulting in the formation of nitrogen gas (N₂) and water (H₂O). This reaction is highly exothermic.

Here's the balanced chemical equation:

4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g)

Explanation:

* Platinum acts as a catalyst: It speeds up the reaction without being consumed itself. Platinum provides a surface for the ammonia and oxygen molecules to adsorb and react more efficiently.

* Excess oxygen: The presence of excess oxygen ensures that all the ammonia is oxidized.

* Exothermic reaction: The reaction releases a significant amount of heat, which is why it's sometimes used in industrial processes.

Important Note:

This reaction is a crucial step in the Ostwald process, a major industrial process for the production of nitric acid. Nitric acid is an important chemical used in the production of fertilizers, explosives, and other chemicals.

Side Reactions:

It's worth noting that under different conditions or with different catalysts, other reactions can occur. For example, nitrogen oxides (NOx) can be formed, which are pollutants. However, under the conditions described in the prompt, the primary reaction is the formation of nitrogen gas and water.