I) Write an equation for the reaction that takes place in the gas jar.

Here are the solutions to the questions:

i) Write an equation for the reaction that takes place in the gas jar.
The diagram shows the catalytic oxidation of ammonia. Dry ammonia ($\text{NH}_3$) reacts with oxygen (from the air in the gas jar) in the presence of a hot catalyst.

$$4\text{NH}_3(g) + 5\text{O}_2(g) \xrightarrow{\text{Hot nichrome wire}} 4\text{NO}(g) + 6\text{H}_2\text{O}(g)$$

ii) Why is it necessary to have a hot nichrome wire in the gas jar?
The hot nichrome wire acts as a catalyst for the reaction. It provides the necessary activation energy to initiate the reaction between ammonia and oxygen, allowing it to proceed at a suitable rate.