Write the balanced redox equation for AsO33- + Cu2+ AsO43- + Cu
The Correct Answer and Explanation is:
Balanced Redox Equation
Unbalanced reaction:
AsO₃³⁻ + Cu²⁺ → AsO₄³⁻ + Cu
We will balance the redox reaction in an acidic solution using the half-reaction method.
Step 1: Write oxidation and reduction half-reactions
Oxidation (Arsenic changes from +3 in AsO₃³⁻ to +5 in AsO₄³⁻):
AsO₃³⁻ → AsO₄³⁻
Reduction (Copper changes from +2 to 0):
Cu²⁺ → Cu
Step 2: Balance atoms other than O and H
Already balanced for arsenic and copper.
Step 3: Balance oxygen using H₂O
AsO₃³⁻ has 3 oxygen atoms
AsO₄³⁻ has 4 oxygen atoms
We add 1 H₂O on the left side:
AsO₃³⁻ + H₂O → AsO₄³⁻
Step 4: Balance hydrogen using H⁺
1 H₂O = 2 H atoms
Add 2 H⁺ on the right:
AsO₃³⁻ + H₂O → AsO₄³⁻ + 2 H⁺
Step 5: Balance charge using electrons
Left: AsO₃³⁻ (−3), H₂O (0) → total: −3
Right: AsO₄³⁻ (−3), 2 H⁺ (+2) → total: −1
To balance charges:
Add 2 e⁻ to the right
Oxidation half-reaction:
AsO₃³⁻ + H₂O → AsO₄³⁻ + 2 H⁺ + 2 e⁻
Reduction half-reaction (Cu²⁺ gains 2 e⁻):
Cu²⁺ + 2 e⁻ → Cu
Step 6: Add half-reactions
AsO₃³⁻ + H₂O + Cu²⁺ → AsO₄³⁻ + 2 H⁺ + Cu
Final Balanced Redox Equation
AsO₃³⁻ + H₂O + Cu²⁺ → AsO₄³⁻ + 2 H⁺ + Cu
Explanation
This redox reaction involves arsenite (AsO₃³⁻) and copper ions (Cu²⁺). The key to balancing redox reactions is identifying the oxidation states of the elements involved and determining which species is oxidized and which is reduced.
In this reaction, arsenic in AsO₃³⁻ has an oxidation state of +3, while in AsO₄³⁻ it becomes +5. This increase in oxidation number indicates that arsenic is oxidized. On the other hand, copper goes from Cu²⁺ to elemental copper (Cu⁰), meaning it is reduced.
We used the half-reaction method in acidic solution to balance the reaction. The oxidation half-reaction shows the conversion of arsenite to arsenate, balanced by adding water and hydrogen ions to match the oxygen and hydrogen atoms. Electrons are added to balance the charge. The reduction half-reaction shows Cu²⁺ gaining two electrons to form solid copper.
Combining the two balanced half-reactions cancels out the electrons, which confirms the conservation of charge and mass. Water and hydrogen ions are balanced appropriately for acidic conditions.
This balanced equation shows the fundamental concept of redox: the transfer of electrons. Arsenic loses electrons (oxidation), and copper gains them (reduction). Understanding how to separate and balance these processes helps in interpreting chemical reactions that involve electron exchange, which are common in electrochemistry, corrosion, and biological systems.
