Question

Identify the reducing agent in the following reaction? `2NO_(2(g)) + 7H_(2(g)) -> 2 NH_(3(g)) + 4 H_2O_((g))`

Answer 1

Hydrogen gas, `"H"_2`.

Explanation:

Assuming that you are indeed dealing with a redox reaction, start by assigning oxidation numbers to the elements that take part in the reaction

`2stackrel(color(blue)(+4))("N")stackrel(color(blue)(-2))("O")_text(2(g]) + 7stackrel(color(blue)(0))("H")_text(2(g]) -> 2stackrel(color(blue)(-3))("N")stackrel(color(blue)(+1))("H")_text(3(g]) + 4stackrel(color(blue)(+1))("H")_2stackrel(color(blue)(-2))("O")_text(2(g])`

In order for a redox reaction to take place, an element must be oxidized by losing electrons and an element must be reduced by gaining electrons.

When an element loses electrons, its oxidation number Increases. On the other hand, when an element gains electrons, its oxidation number decreases.

So, look for elements that match that description.

Notice that nitrogen's oxidation state goes from `color(blue)(+4)` on the reactants' side, when it's paired with oxygen in nitrogen dioxide, `"NO"_2`, to an oxidation state of `color(blue)(-3)` on the products' side, when it's paired with hydrogen in ammonia, `"NH"_3`.

Since nitrogen's oxidation number decreased, you can say that it has been reduced.

Hydrogen's oxidation number went from `color(blue)(0)` on the reactants' side, when it forms the hydrogen molecule, to an oxidation state of `color(blue)(+1)` on the products' side, when paired with nitrogen in ammonia.

Since hydrogen's oxidation number increased, you can say that is has been oxidized.

So, a redox reaction did take place.

Now, a reducing agent is simply a compound that contains an element that donates electrons, hence it reduces another element by giving it electrons.

By comparison, an oxidizing agent is a compound that contains an element that receives electrons, hence it oxidizes another element by taking its electrons.

So, in your case, you know that hydrogen is being oxidized in this reaction, which means that it's donating electrons. This implies that it is acting as a reducing agent, since it reduces nitrogen.

On the other hand, nitrogen is being reduced here, which means that it takes in electrons. This implies that it is acting as an oxidizing agent, since it oxidizes hydrogen.

Therefore, hydrogen gas, `"H"_2`, is the reducing agent and nitrogen dioxide is the oxidizing agent.