Which sequence correctly balances a redox reaction in acidic solution?

• A. Assign oxidation states, balance atoms with H2O, balance H with H+, balance charge with electrons.
• B. Balance electrons first, then balance atoms with H2O and H+, then add electrons.
• C. Balance O with CO2, then balance H with H+, then add electrons.
• D. Assign oxidation states, balance atoms with H2O, balance H with H+, balance charge with electrons, then add electrons to the appropriate side.

Answer: (D)

Balancing redox reactions in acidic solution uses the half-reaction method, with water and hydrogen ions serving to balance oxygen and hydrogen, and electrons balancing charge. Start by assigning oxidation states to see which species are oxidized and reduced. This identifies the two half-reactions you’ll work with. Next, balance the atoms in each half-reaction, leaving O and H to be fixed later: add water to supply or remove oxygen as needed. Then balance hydrogen by adding H+ in acidic solution. After the atoms are balanced, balance the charge by adding electrons to the side that loses electrons in each half-reaction (and to the other side as appropriate so the two halves can be combined). Finally, combine the half-reactions so the electrons cancel.

The sequence that begins with assigning oxidation states, then balances atoms with H2O, then balances hydrogen with H+, then balances charge with electrons, and finally adds the electrons to the appropriate side, correctly follows this procedure. It includes all the essential steps in the proper order and reflects how acidic conditions guide the use of H2O and H+.