Which balanced equation represents a redox reaction?

(1) AgNO3 + NaCl = = >AgCl + NaNO3

(2) BaCl2 + K2CO3 = = > BaCO3 + 2KCl

(3) CuO + CO==> Cu + CO2

(4) HCl + KOH = = > KCl + H2O

In order to determine which reaction is redox, you need to compare oxidation the oxidation numbers of each element on both sides of the equation (a change in the oxidation number for any of the elements indicates oxidation-reduction). In order to do that, you first need to assign oxidation numbers to each element on both sides of the equation (if you don't know how to do this, you should learn this first). The answer is (3). Cu goes from + 2 to 0 and C goes from + 2 to + 4.

Answer: option (3) CuO + CO ⇄ Cu + CO₂

Explanation:

1) A redox reaction is an oxidation-reduction reaction.

2) An oxidation-reduction reaction is one in which at least one species is oxidized and other (or others) is reduced.

3) Oxidation is the increase on the oxidation number, which occurs by releasing electrons.

4) Reduction is the decrease of the oxidation number, which occurs by gaining electrons.

5) By looking at the oxidation numbers of the reactants and the products you can tell whether a reaction is a redox one.

6) There are some rules to calculate the oxidation states which you need to handle to determine the oxidation states of the atoms in any chemical formula.

7) The main rule that you can use at a glance is if an element is alone in one side of the equation and is bonded to other kind of atom in the other side.

That is because the rule states that for any element that is not combined with other element its oxidation state is 0.

In this case, you can apply that rule to the equation of the choice number (3). In that, you can see that Cu is pure which as per the mentioned rule means that its oxidation state is 0.

As Cu is combined with O, CuO, in the reactant side of the chemical equation, then its oxidation number is not 0. Indeed, the rule is that in the formula CuO the oxidation states of Cu and O add up 0, and since the oxidation number of O is 2-, the oxidation number of Cu is 2+.

Therefore, having Cu changed its oxidation state from 2 + to 0, it was reduced, and this is a redox reaction.

Of course other atoms had to be oxidized. It was C whose oxidation state went from 2 + in CO to 4 + in CO₂.