Just a quick question.

A reducing agent is the substance which decreases the oxidation state of another reagent or gives electrons to the other reagent.

Let's see... P has oxidation state (OS) 0, Br2 has OS 0 at first.

In PBr3, P has OS 3+, and Br has OS 1-.

Can you now find which reagent is the reducing agent?

please provide steps to arrive at answer. I already know that Br is the reducing agent. I found it on wikipedia. I need steps. Please provide redox equation.

Hm... Br2 is in fact the oxidising agent...

Recall your definition of a reducing agent.
"A reducing agent is a species which reduces another species, while itself being oxidised."

Oxidation involves the raise in oxidation state, while reduction involves a decrease in the oxidation state.

I already gave you the steps:


Let's see... P has oxidation state (OS) 0, Br2 has OS 0 at first.

In PBr3, P has OS 3+, and Br has OS 1-.

The OS of phosphorous increased from 0 to 3+, meaning, phosphorous was oxidised.
From the definition of a reducing agent, if phosphorous was oxidised, it means it has reduced something, and this means that phosphorous is the reducing agent.
It has reduced Br2 to Br- (from OS 0 to 1-).

I know it's quite confusing, because a reducing agent gets oxidised, and an oxidising agent gets reduced, but you'll have to get used to it.

Is it clear enough? Or should I try to explain it in another way?