Balancing Redox Reactions

Assign oxidation numbers for the species below. Determine which species is the oxidizing agent and which is the reducing agent. Which species is being oxidized? Which species is being reduced?

Cr$^{3+}$ + Pb $\longrightarrow$ Cr + Pb$^{2+}$

The following reaction takes place under acidic conditions. Assign oxidation numbers and balance the oxidation reduction reaction using the half reaction method.

HNO$_3$ + Fe$^{2+}$ $\longrightarrow$ Fe$^{3+}$ + NO$_2$

Balance the following redox reaction given acidic conditions, using the half reaction method.

$S (s) + NO_2^+ (aq) \longrightarrow SO_2^- (aq) + NO (g)$ .

A voltaic cell is constructed using a Cd/Cd$^{2+}$ half cell and a Sn$^{2+}$/Sn half cell. Write out the balanced equation, determine E$^\circ$, and calculate the cell potential at 25$^\circ$C if [Cd$^{2+}$] = 0.50 M and [Sn$^{2+}$] = 0.30 M.