In balancing the acidic solution half-reaction CN− → CNO−, what is the coefficient of CN−?

• A. 2
• B. 5
• C. 1
• D. 3

Answer: (B)

Balancing in acidic solution packs oxygen by adding water and hydrogen by adding H+, and then makes up any charge difference with electrons. Here, carbon in CN− goes from +2 in CN− to +4 in CNO−, so each cyanide ion loses 2 electrons. That means the half-reaction must include 2 e− for every CN−.

A clean balanced form starts as: CN− + H2O → CNO− + 2 H+ + 2 e−. This satisfies both atoms and charge. If you want the equation with only whole-number coefficients when you later combine this half-reaction with another one that involves more electrons, you can multiply everything by 5, giving 5 CN− + 5 H2O → 5 CNO− + 10 H+ + 10 e−. In this scaled version, the coefficient in front of CN− is 5. So the 5 comes from choosing a form that matches the electron count for combining with the other half-reaction, while the simplest form has a coefficient of 1.