Problem 221 - Introductory Chemistry Atoms First

Question

An aqueous solution containing \(\mathrm{CN}^{-}\) ions turns litmus blue. Write an equation showing how \(\mathrm{CN}^{-}\) makes water basic.

Step-by-Step Solution

Answer

The cyanide ion \(\mathrm{CN}^{-}\) makes water basic by accepting a proton (\(\mathrm{H}^+\)) from water, resulting in the formation of hydrocyanic acid (\(\mathrm{HCN}\)) and hydroxide ion (\(\mathrm{OH}^-\)). The balanced chemical equation for this reaction is \(\mathrm{CN}^{-} + \mathrm{H_2O} \rightleftharpoons \mathrm{HCN} + \mathrm{OH}^-\). The increased concentration of \(\mathrm{OH}^-\) ions makes the solution basic, causing the litmus paper to turn blue.

Step 1: Identify the reaction participants

The given problem states cyanide ions \(\mathrm{CN}^{-}\) cause the aqueous solution to be basic and the experiment involves water. So, the participants of the reaction are \(\mathrm{CN}^{-}\) and water (\(\mathrm{H_2O}\)).

Step 2: Write the balanced chemical equation

Now, using the known properties of CN⁻ ion and water, let's write a balanced chemical equation for the reaction between them. We know that \(\mathrm{CN}^-\) can act as a Lewis base. It accepts a proton (\(\mathrm{H}^+\)) from water. The equation for this reaction is: \[ \mathrm{CN}^{-} + \mathrm{H_2O} \rightleftharpoons \mathrm{HCN} + \mathrm{OH}^- \]

Step 3: Identify why the water turns basic

Upon interaction with CN⁻ ions, some water molecules get converted into \(\mathrm{OH}^-\) ions, increasing the concentration of hydroxide ions. A higher concentration of \(\mathrm{OH}^-\) ions makes the solution basic, and that is why the litmus paper turns blue, indicating a basic solution.

Key Concepts

Cyanide Ion

The cyanide ion, represented by the chemical symbol \(\mathrm{CN}^{-}\), is a negative ion composed of one carbon atom triple-bonded to a nitrogen atom. This ion is famous for its role in various chemical reactions due to its ability to attract protons and function as a Lewis base.

• Lewis bases are substances that can donate an electron pair. In the case of the cyanide ion, it is the nitrogen atom that primarily participates in proton acceptance.
• When \(\mathrm{CN}^{-}\) is present in a solution, it tends to interact with water molecules, facilitating an essential chemical equilibrium.

Understanding the nature of \(\mathrm{CN}^{-}\) ions helps explain why certain solutions become basic when they contain this ion.

Basic Solutions

Basic solutions are characterized by a higher concentration of hydroxide ions (\(\mathrm{OH}^-\)) compared to hydrogen ions (\(\mathrm{H}^+\)). This is typical in solutions with a pH greater than 7. The presence of basic or alkaline conditions can be demonstrated in various ways, one of which includes turning red litmus paper blue.

• When the concentration of \(\mathrm{OH}^-\) ions increases in a solution, it results in the solution being basic.
• Basic solutions often feel slippery and are typically associated with compounds such as soaps and detergents.

The concept of basicity is important in chemistry because it helps to predict the behavior of different substances in reactions.

Hydroxide Ions

Hydroxide ions, \(\mathrm{OH}^-\), play a pivotal role in defining the pH level of a solution. These ions are generated through the dissociation of bases in water or through reactions like the one between \(\mathrm{CN}^{-}\) ions and water.

• In the presence of \(\mathrm{CN}^{-}\), the chemical equation: \( \mathrm{CN}^{-} + \mathrm{H_2O} \rightleftharpoons \mathrm{HCN} + \mathrm{OH}^- \) illustrates the generation of \(\mathrm{OH}^-\) ions.
• The increase in \(\mathrm{OH}^-\) concentration leads to a basic solution, as shown by the equation's equilibrium shifting towards the right, favoring the formation of \(\mathrm{OH}^-\) ions.

This action is responsible for altering the properties of a solution, making it alkaline in nature.

Litmus Test

Litmus tests are a simple and effective way to determine the acidity or basicity of a solution. They involve the use of litmus paper, which contains a dye that changes color in response to pH changes.

• In acidic solutions, blue litmus paper turns red.
• In basic solutions, like those containing \(\mathrm{OH}^-\) ions, red litmus paper turns blue.

This color change provides a quick visual indication of the presence of a basic or acidic environment, making it a valuable tool in chemistry education and experiments.