Question

If a base is added to pure water, why does the concentration of \(\mathrm{H}_{3} \mathrm{O}^{+}\) decrease?

Short answer

Adding a base increases \( \text{OH}^{-} \) ions, which react with \( \text{H}_{3}\text{O}^{+} \) ions, reducing their concentration.

Step-by-step solution

Understanding Pure Water

Pure water has a neutral pH of 7, which means the concentrations of \(\text{H}_{3}\text{O}^{+}\) and \(\text{OH}^{-}\) ions are equal, each being \1 \times 10^{-7}\ M.

Effect of Adding a Base

A base is a substance that increases the concentration of \(\text{OH}^{-}\) ions in a solution. When added to water, it reacts with \(\text{H}_{3}\text{O}^{+}\) ions.

Neutralization Reaction

The base reacts with \(\text{H}_{3}\text{O}^{+}\) ions to form water, which reduces the concentration of \(\text{H}_{3}\text{O}^{+}\). The reaction can be written as \[\text{H}_{3}\text{O}^{+} + \text{OH}^{-} \rightarrow 2 \text{H}_{2}\text{O}\].

Resulting Concentration Change

As the concentration of \(\text{OH}^{-}\) increases, it further drives the neutralization reaction, thus decreasing the concentration of \(\text{H}_{3}\text{O}^{+}\) in the solution.

Key concepts

pH

The term 'pH' stands for 'potential of hydrogen' and measures the acidity or basicity of a solution. The pH scale ranges from 0 to 14.
A pH of 7 is considered neutral, like pure water.
pH values below 7 represent acidic solutions, while values above 7 indicate basic (alkaline) solutions.
The mathematical expression for pH is: \( \text{pH} = -\log[\text{H}_3\text{O}^+] \).
This means that as the concentration of hydronium ions (\(\text{H}_3\text{O}^+\)) increases, the pH value decreases, making the solution more acidic. Conversely, a decrease in \(\text{H}_3\text{O}^+\) ions means the solution becomes more basic.
Understanding pH is crucial for analyzing chemical reactions and the overall behavior of a solution.

Neutralization Reaction

A neutralization reaction occurs when an acid and a base react to form water and a salt.
This reaction is fundamental in chemistry and environmental science.
In the context of the given exercise, the neutralization reaction is described by the equation: \[ \text{H}_3\text{O}^+ + \text{OH}^- \rightarrow 2 \text{H}_2O\ \]
This process is significant because it explains how the hydronium ion (\( \text{H}_3\text{O}^+ \)) concentration is reduced when a base (which provides \(\text{OH}^-\) ions) is added to water.
The \(\text{H}_3\text{O}^+\) ions react with the \(\text{OH}^-\) ions to form water, effectively removing \(\text{H}_3\text{O}^+\) ions from the solution and thus lowering its acidity. Neutralization reactions are not only important for maintaining pH balance in various systems but also for processes such as wastewater treatment and digestion in the human body.

Hydronium Ion (\( \text{H}_3\text{O}^+ \))

The hydronium ion (\(\text{H}_3\text{O}^+\)) is crucial in acid-base chemistry.
It forms when a hydrogen ion (\(\text{H}^+\)) combines with a water molecule.
The presence of \(\text{H}_3\text{O}^+\) ions in a solution is what makes it acidic.
High \(\text{H}_3\text{O}^+\) concentrations result in low pH, leading to acidity, whereas low \(\text{H}_3\text{O}^+\) concentrations lead to higher pH levels and less acidity.
In pure water, the concentration of \(\text{H}_3\text{O}^+\) is exactly \( 1 \times 10^{-7} \) M, resulting in a neutral pH of 7.
When a base is added, \(\text{H}_3\text{O}^+\) reacts with \(\text{OH}^-\) ions to form water.
This neutralization reduces the concentration of \(\text{H}_3\text{O}^+\), thus increasing the pH and making the solution more basic.

Hydroxide Ion (\(\text{OH}^-\))

The hydroxide ion (\(\text{OH}^-\)) plays a key role in basic (alkaline) chemistry.
It forms when a base dissolves in water, producing \(\text{OH}^-\) ions.
A high concentration of \(\text{OH}^-\) ions results in a high pH, making the solution basic.
When \(\text{OH}^-\) ions are added to pure water, they neutralize \(\text{H}_3\text{O}^+\) ions, reducing the solution's acidity.
The overall chemical equation for this neutralization is: \[ \text{H}_3\text{O}^+ + \text{OH}^- \rightarrow 2 \text{H}_2O\ \]
This equation demonstrates the consumption of \(\text{H}_3\text{O}^+\) ions by \(\text{OH}^-\) ions, leading to a decrease in \(\text{H}_3\text{O}^+\) concentration. As a result, the solution becomes less acidic and more basic. Understanding the role of \(\text{OH}^-\) ions helps in grasping the behavior of bases and their impact on pH levels.