The Correct Answer and Explanation is:
Net Ionic Equation:
CN−(aq)+H+(aq)→HCN(aq)\text{CN}^-(aq) + \text{H}^+(aq) \rightarrow \text{HCN}(aq)
Explanation: This reaction is a classic acid-base reaction involving strong and weak electrolytes. To write the correct net ionic equation, we need to understand how substances dissociate in aqueous solution. In the molecular equation:
NaCN(aq) + HBr(aq) → NaBr(aq) + HCN(aq)
we are told that HBr is a strong electrolyte and HCN is a weak electrolyte. This means HBr dissociates completely in water to form H⁺(aq) and Br⁻(aq), while HCN remains mostly as intact molecules in solution. Sodium salts like NaCN and NaBr are also strong electrolytes, so they dissociate completely.
Breaking down the full ionic equation: Na⁺(aq) + CN⁻(aq) + H⁺(aq) + Br⁻(aq) → Na⁺(aq) + Br⁻(aq) + HCN(aq)
We now eliminate the spectator ions. Spectator ions appear unchanged on both sides of the reaction and do not participate in the chemical change. In this case, Na⁺ and Br⁻ are present on both sides, so we remove them.
That leaves us with: CN⁻(aq) + H⁺(aq) → HCN(aq)
This is the net ionic equation. It shows the actual chemical change occurring, which is the protonation of the cyanide ion by the hydrogen ion to form hydrogen cyanide, a weak acid that stays mostly undissociated in aqueous solution.
The importance of identifying strong versus weak electrolytes is to understand which compounds dissociate and how to properly construct ionic equations. Strong electrolytes, like HBr and NaCN, break apart into ions, while weak electrolytes, like HCN, largely remain as molecules in solution. This distinction guides which species remain in the equation.
