1. Does the half reaction below demonstrate oxidation or reduction?
Br2 + 2e- → 2Br-

2. In the reaction below, what is the oxidation number for silver in the product?
2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)

1) reduction

2) zero

Explanation:

Let us look at the definition of oxidation and reduction in a nutshell.

Oxidation is electron loss(oxidation number is increased)

Reduction is electron gain(oxidation number is decreased)

With the above definitions in mind let us look again at the equation in question 1:

Br2 + 2e- → 2Br-

We can see that the bromine molecule gained two electrons. Oxidation number decreased from zero on the left hand side of the reaction equation to -1 on the right hand side of the reaction equation hence the equation represents a reduction half equation.

For question 2, silver was displaced from its salt. On the left hand side of the reaction equation, silver had an oxidation number of +2. But remember than one of the riles for determining oxidation number of species is that the oxidation number of an uncombined element is zero. Hence, the oxidation number of silver element on the right hand side of the reaction equation is zero.