Question

What happens to the hydroxide ion concentration in aqueous solutions when we increase the hydrogen ion concentration by adding an acid? What happens to the hydrogen ion concentration in aqueous solutions when we increase the hydroxide ion concentration by adding a base? Explain.

Short answer

When an acid is added to an aqueous solution, the hydrogen ion (H+) concentration increases, resulting in a decrease in the hydroxide ion (OH-) concentration to maintain the constant ion product of water (Kw). Conversely, when a base is added to an aqueous solution, the hydroxide ion (OH-) concentration increases, leading to a decrease in the hydrogen ion (H+) concentration to maintain the constant ion product of water (Kw).

Step-by-step solution

Introduction to the Ion Product of Water

The ion product of water (Kw) is a constant value representing the product of the concentrations of hydrogen ions (H+) and hydroxide ions (OH-) in a solution at a given temperature. Mathematically, this relationship can be expressed as: \[K_w = [H^+][OH^-]\] At 25°C, the value of Kw is approximately \(1.0 \times 10^{-14}\). This constant value plays a crucial role in understanding the relationship between the concentrations of H+ and OH- ions when adding an acid or a base to an aqueous solution.

Effect of Adding Acid on Hydroxide Ion Concentration

When we add an acid to an aqueous solution, the hydrogen ion (H+) concentration increases. Since the ion product of water (Kw) must remain constant, an increase in H+ concentration will lead to a decrease in hydroxide ion (OH-) concentration to maintain the balance. In other words, as the solution becomes more acidic, the concentration of hydroxide ions decreases: \[K_w = [H^+][OH^-] \implies [OH^-] = \frac{K_w}{[H^+]}\] As [H+] increases, the fraction on the right decreases, which indicates a decrease in [OH-].

Effect of Adding Base on Hydrogen Ion Concentration

When we add a base to an aqueous solution, the hydroxide ion (OH-) concentration increases. Since the ion product of water (Kw) must remain constant, an increase in OH- concentration will lead to a decrease in hydrogen ion (H+) concentration to maintain the balance. In other words, as the solution becomes more basic, the concentration of hydrogen ions decreases: \[K_w = [H^+][OH^-] \implies [H^+] = \frac{K_w}{[OH^-]}\] As [OH-] increases, the fraction on the right decreases, which indicates a decrease in [H+].

Conclusion

In summary, when we add an acid to an aqueous solution and increase the hydrogen ion concentration, the hydroxide ion concentration decreases to maintain the constant ion product of water (Kw). Similarly, when we add a base to an aqueous solution and increase the hydroxide ion concentration, the hydrogen ion concentration decreases to maintain the constant ion product of water (Kw).

Key concepts

pH and pOH

Understanding the concepts of pH and pOH is a quintessential part of acid-base chemistry. In simple terms, the pH is a measure that represents the acidity or alkalinity of a solution. It is defined as the negative logarithm to the base 10 of the hydrogen ion \( [H^+] \) concentration. Mathematically, it is expressed as \( pH = -\log_{10} [H^+] \).

On the other side, we have the pOH, which serves a similar function but is focused on the hydroxide ion \( [OH^-] \) concentration. It is also the negative logarithm to the base 10 of the hydroxide ion concentration: \( pOH = -\log_{10} [OH^-] \).

Together, pH and pOH are related through the ion product of water \( K_w \), stating that at 25°C, \( pH + pOH = 14 \). This implies that when one increases, the other must decrease to maintain the balance, which is fundamental when adding acids or bases to a solution.

Acid-Base Chemistry

Acid-base chemistry revolves around the transfer and reaction of protons \( H^+ \) ions between substances. Acids are substances that release \( H^+ \) ions into a solution, while bases are chemicals that can accept them. As you add an acid to water, the concentration of hydrogen ions increases, consequently decreasing the pH of the solution, making it more acidic.

Conversely, adding a base to water results in an increase in hydroxide ion \( [OH^-] \) concentration and a corresponding increase in pOH, rendering the solution more basic or alkaline. This behavior is guided by the principles of Le Chatelier's principle, which predicts how the position of equilibrium will shift to maintain balance when a stress is applied to a system.

Hydrogen Ion Concentration

The hydrogen ion concentration in a solution is a critical aspect that determines its pH level. As the concentration of \( [H^+] \) ions increases, substances in water become more acidic, resulting in a lower pH. This important relationship underlies many reactions in acid-base chemistry, including buffer systems in biological organisms and the dissolution of substances in environmental water.

The preexisting concentration of \( [H^+] \) can be altered by adding acids, which release more \( H^+ \) ions into the solution, or by adding bases, which consume the \( H^+ \) ions, thereby reducing their concentration. The equilibrium dynamic here is critical for various chemical applications, from industrial processes to physiological functions.

Hydroxide Ion Concentration

The hydroxide ion concentration \( [OH^-] \) dictates the pOH of a solution and its basicity. When a base is dissolved in water, it contributes to an increase in \( [OH^-] \) ions, which elevates the solution's pOH and makes it more basic. On the flip side, adding an acid reduces the \( [OH^-] \) ion concentration as the \( H^+ \) ions from the acid react with the \( OH^- \) ions to form water.

Thus, the manipulation of \( [OH^-] \) is a fundamental aspect of controlling a solution's properties and is essential in processes such as neutralization reactions, where acids and bases are combined to form water and a salt, leading to a change in \( [OH^-] \) concentration.