Write a balanced half-reaction for the reduction of aqueous arsenic acid H3AsO4 to gaseous arsine AsH3 in basic aqueous solution. Be sure to add physical state symbols where appropriate.

H₃AsO₄(aq) + 4 H₂O(l) + 8 e⁻ ⇒ AsH₃(g) + 8 OH⁻(aq)

Explanation:

Let's consider the half-reaction for the reduction of aqueous arsenic acid to gaseous arsine in a basic aqueous solution.

H₃AsO₄(aq) ⇒ AsH₃(g)

We see that there is an excess of 4 oxygen atoms on the left side. So, we add 4 molecules of water to the left side and 8 hydroxyl ions to the right side.

H₃AsO₄(aq) + 4 H₂O(l) ⇒ AsH₃(g) + 8 OH⁻(aq)

We need to add 8 electrons to the left side to balance the reaction electrically.

H₃AsO₄(aq) + 4 H₂O(l) + 8 e⁻ ⇒ AsH₃(g) + 8 OH⁻(aq)

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.