# Question 83a9e

Aug 24, 2015

For part (c)

Oxidizing agent: phosphorus
Reducing agent: carbon

#### Explanation:

I'll show you how to do this for part (c), since it's the most complex reaction of the group.

So, you're basically dealing with three redox reactions.

The reaction in part (c) is used to produce white phosphorus, ${\text{P}}_{4}$, by reacting calcium phosphate, "Ca"""_3("PO"_4)_2# with sand an coke at high temperatures.

Now, I assume that you're familiar with the rules you need to use when assigning oxidation numbers, so I won't go into that here.

Once you assign oxidation numbers to all the atoms that take part in the reaction, you need to looks for atoms that have a smaller oxidation number on the products' side than they had on the reactants' side - these atoms are being reduced.

Likewise, atoms that have a bigger oxidation number on the product's side than they had on the reactants' side are being oxidized.

Assign oxidation numbers to all the atoms that take part in the reaction - I didn't include the states of the compounds

$2 \stackrel{\textcolor{b l u e}{+ 2}}{\text{Ca")_3(stackrel(color(blue)(+5))("P") stackrel(color(blue)(-2))"O"_4)_2 + 6stackrel(color(blue)(+4))("Si") stackrel(color(blue)(-2))("O")_2 + 10stackrel(color(blue)(0))("C") -> stackrel(color(blue)(0))("P")_4 + 6stackrel(color(blue)(+2))("Ca") stackrel(color(blue)(+4))("Si") stackrel(color(blue)(-2))("O")_3 + 10stackrel(color(blue)(+2))("C") stackrel(color(blue)(-2))("O}}$

So, the oxidation state of calcium remains unchanged at +2. The same can be said for the oxidation states of silicon and oxygen, which remain constant on both sides of the equation at +4 and -2, respectively.

However, the oxidation states of phosphorus and carbon do change. Phosphorus goes from +5 on the reactants' side, to 0 on the products' side.

This means that phosphorus has been reduced, which implies that it gained electrons.

Carbon's oxidation state goes from 0 on the reactants' side, to +2 on the products' side, which means that carbon has been oxidized, i.e. it lost electrons.

So, if phosphorus gained electrons, that can only mean that it took those electrons from carbon. This means that phosphorus acted as an oxidizing agent for carbon.

If you look at this from carbon's perspective, the electrons it lost went to phosphorus, so you can say that carbon acted as a reducing agent for phosphorus.

So remember, the atom that gets oxidized acts as a reducing agent, and the atom that gets reduced acts as an oxidizing agent.