- The Arrhenius theory defines an acid as a substance that increases the \(\text{H}^+\) ion concentration in an aqueous solution.
- \(BF_3\) acts as a Lewis acid but does not produce \(\text{H}^+\) in water.
- Both \(NH_3\) and \(Mg(OH)_2\) increase the \(OH^-\) concentration in water, making them bases.
- \(\text{HCN}\) dissociates to yield \(\text{H}^+\) ions, identifying it as the Arrhenius acid.
- The final answer is \(\boxed{\text{HCN}}\).
Explanation
- Defining and Analyzing the Problem
The objective is to identify which substance among \(BF_3\), \(\text{HCN}\), \(NH_3\), and \(Mg(OH)_2\) qualifies as an Arrhenius acid. According to the Arrhenius theory, an acid is a substance that dissociates in aqueous solution to increase the concentration of hydrogen ions, \(\text{H}^+\). We will evaluate each compound based on this definition.
- Evaluating the Chemical Substances
We examine each substance individually:
- \(BF_3\) (Boron trifluoride): This molecule has a boron atom with an incomplete octet. It acts as an electron pair acceptor, making it a Lewis acid, but it does not dissociate in water to produce \(\text{H}^+\) ions.
- \(\text{HCN}\) (Hydrogen cyanide): When dissolved in water, \(\text{HCN}\) dissociates to release hydrogen ions: \(\text{HCN}(aq) \rightarrow \text{H}^+(aq) + \text{CN}^-(aq)\). Since it increases the concentration of \(\text{H}^+\), it fits the Arrhenius definition of an acid.
- \(NH_3\) (Ammonia): In an aqueous solution, ammonia reacts with water: \(NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)\). By increasing the concentration of hydroxide ions (\(OH^-\)), it acts as an Arrhenius base.
- \(Mg(OH)_2\) (Magnesium hydroxide): This compound is a strong electrolyte that dissociates into magnesium ions and hydroxide ions: \(Mg(OH)_2(s) \rightarrow Mg^{2+}(aq) + 2OH^-(aq)\). Because it releases \(OH^-\) ions, it is classified as an Arrhenius base.
- Conclusion
Based on the analysis, only \(\text{HCN}\) satisfies the definition of an Arrhenius acid, as it is the only compound in the list that increases the concentration of hydrogen ions in water. Thus, \(\text{HCN}\) is the correct answer. The final answer is \(\boxed{\text{HCN}}\).
Examples
In water treatment processes, scientists must carefully monitor the acidity of the water to ensure safety. An Arrhenius acid like hydrogen cyanide, while toxic and needing specific management, demonstrates the fundamental principle of how acids release hydrogen ions in solution. Understanding these classifications helps chemists neutralize acidic or basic waste effectively before it is returned to the environment, ensuring ecological health and compliance with safety standards.