PROBLEM: Aqueous Sodium Chromate plus aqueous Hydrochloric Acid yields aqueous Chlorous acid plus aqueous Chromium(III) Chloride Hint: There is another compound that is needed to balance the reaction but it is only made of spectator ions and you need to figure that out as you solve the problem. Step 1: Write the unbalanced formula equations Na 2 CrO 4 (aq) + HCl(aq) > HClO 2 (g) + CrCl 3 (aq) Step 2: Identify the species that are oxidized and reduced. Start by labeling the oxidation #s. Since Cr goes from +6 to +3 it gained e – and is reduced. Since Cl goes from –1 to +3 it lost e – and is oxidized. Do not be distracted by Cl – on the products side since that is a spectator ion. Yes the Cl – is both oxidized and a spectator at the same time Step 3: Write and balance the oxidation half reaction (Note: Chlorous acid is a weak acid) Cl – > HClO 2 2 H 2 O + Cl – > HClO H + Since the Cl is already balanced, balance the O by adding 2 H 2 O 2 H 2 O + Cl – > HClO 2 Next add 2 H + to balance the H mass 2 H 2 O + Cl – > HClO H e – Next add e – to balance the charge – since the left is –1, and the right is +3, add 4e– to the right Na 2 CrO 4 (aq) + HCl(aq) > HClO 2 (g) + CrCl 3 (aq) +1+3–2+6–1+1 –1+3–2

Step 5: Next make the oxidation and reduction half-reactions have the same # of e – CrO 4 2– > Cr 3+ Step 4: Write and balance the reduction half reaction First balance the mass – since Cr is already balanced, balance the O by adding 4 H 2 O. Next add e – to balance the charge – left side is +6 total, right side is +3, so add 3 e– CrO 4 2– > Cr H 2 O Finish balancing the mass – by balancing the H by adding 8 H + 8 H + + 3e – + CrO 4 2– > Cr H 2 O 8 H + + CrO 4 2– > Cr H 2 O 8 H + + 3e – + CrO 4 2– > Cr H 2 O (X4) 2 H 2 O + Cl – > HClO H e – (X3) 32 H e – + 4 CrO 4 2– > 4 Cr H 2 O 6 H 2 O + 3 Cl – > 3 HClO H e – Step 6: Combine the two half-reactions together, to make the net ionic equation. Remember to cancel out the items on opposite sides of arrow (water, H + ions, and e – ). In this case in addition to the e –, 9H+ on each side, and 6 H 2 O on both sides cancel out. 23 H CrO 4 2– + 3 Cl – > 3 HClO Cr H 2 O

Step 7: Add back spectator ions and combine with other ions to write complete compounds and the balanced overall equation. Start with H + and Cl –. Since there are more H + than Cl –, need to add an extra 20 Cl – spectator ions. 23 HCl + 4 CrO 4 2– > 3 HClO CrCl H 2 O + 8 Cl – Next add back spectator Na HCl + 4 Na 2 CrO > 3 HClO CrCl H 2 O + 8NaCl