Assign oxidation states to the elements involved in the following reactions. Indicate which are redox reactions and which are not. (a) $\mathrm{MgCO}_{3}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \longrightarrow$ $\mathrm{Mg}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{CO}_{2}(\mathrm{g})$ (b) $\mathrm{Cl}_{2}(\mathrm{aq})+2 \mathrm{Br}^{-}(\mathrm{aq}) \longrightarrow 2 \mathrm{Cl}^{-}(\mathrm{aq})+\mathrm{Br}_{2}(\mathrm{aq})$ (c) $\mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{NO}_{3}^{-}(\mathrm{aq}) \longrightarrow$ $\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1)+\mathrm{NO}_{2}(\mathrm{g})$ (d) $2 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{CrO}_{4}^{2-}(\mathrm{aq}) \longrightarrow \mathrm{Ag}_{2} \mathrm{CrO}_{4}(\mathrm{s})$

Assigning oxidation states to each species in the reactions before and after the reaction. Remember that for a neutral compound, the sum of the oxidation states is zero, and for an ion, the sum of the oxidation states is equal to the charge of the ion. For reaction (a), \( Mg \) has an oxidation state of +2 both before and after the reaction. This is due to its position in the periodic table. \( C \) goes from +4 in \( MgCO_{3} \) to +4 in \( CO_{2} \). \( O \) is in -2 both in \( MgCO_{3} \) and \( CO_{2} \). The \( H \) atoms go from +1 in \( H^{+} \) to +1 in \( H_{2}O \). For reaction (b), \( Cl \) goes from 0 in \( Cl_{2} \) to -1 in \( Cl^{-} \). \( Br \) goes from -1 in \( Br^{-} \) to 0 in \( Br_{2} \). Reaction (c), \( Ag \) goes from 0 in \( Ag \) to +1 in \( Ag^{+} \). \( N \) goes from +5 in \( NO_{3}^{-} \) to +4 in \( NO_{2} \). \( O \) stays at -2 and \( H \) at +1. For reaction (d), \( Ag \) stays at +1, \( Cr \) stays at +6, and \( O \) stays at -2.

After assigning oxidation states, look for changes. For reaction (a), no atoms change oxidation state, so it is not a redox reaction. For reaction (b), \( Cl \) goes from 0 to -1 (reduction) and \( Br \) goes from -1 to 0 (oxidation) thus it is a redox reaction. For reaction (c), \( Ag \) goes from 0 to +1 (oxidation) and \( N \) goes from +5 to +4 (reduction), so it also is a redox reaction. For reaction (d), no atoms change oxidation state, so it is not a redox reaction.