Kw, Ionic Product of Water (A-Level Chemistry)

Summary

This video explains the ionic product of water (KW), covering key concepts such as water's dissociation into H+ and OH- ions, and how this affects pH and acidity. It explores the relationship between the dissociation of water and the temperature-dependent value of KW, which influences the concentration of hydrogen ions in a solution. The video also details how to calculate the pH of solutions, including both acidic and alkaline, by using the KW value. It concludes by demonstrating how changes in temperature can impact water’s pH, with practical examples.

Takeaways

• 😀 Water has the ability to dissociate into H+ and OH- ions, forming a neutral solution in pure water.
• 😀 The ionic product of water (Kw) represents the concentration of H+ and OH- ions in pure water at equilibrium.
• 😀 The Kw value at 298K is 1 x 10^-14, meaning that the concentration of H+ and OH- ions in pure water is 1 x 10^-7 M each.
• 😀 pH is a logarithmic scale that measures the concentration of H+ ions in a solution, with pH = -log[H+].
• 😀 For pure water at 298K, pH = 7 because the concentration of H+ and OH- ions are equal, making the solution neutral.
• 😀 The dissociation of water is endothermic, meaning it increases at higher temperatures, leading to more H+ ions and a lower pH.
• 😀 Kw is temperature-dependent, and as temperature increases, Kw increases, shifting the dissociation equilibrium to favor more H+ and OH- ions.
• 😀 When calculating pH for solutions like sodium hydroxide, Kw can be used to find the H+ ion concentration from the OH- ion concentration.
• 😀 At higher temperatures (e.g., 328K), Kw increases to 7.23 x 10^-14, resulting in a higher concentration of H+ ions and a lower pH.
• 😀 For a solution to be acidic, the concentration of H+ ions must be higher than that of OH- ions, and for a solution to be alkaline, the concentration of OH- ions must be higher.
• 😀 When dealing with alkaline solutions, Kw and the OH- ion concentration can be used to calculate the pH of the solution.

Q & A

What is the ionic product of water (KW)?

The ionic product of water (KW) is the product of the concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻) in pure water. It is represented by the equation KW = [H⁺][OH⁻], and its value at 298 K is 1 × 10⁻¹⁴.

How does temperature affect the value of KW?

The value of KW is temperature-dependent. As the temperature increases, more water molecules dissociate into H⁺ and OH⁻ ions, causing the value of KW to increase. For example, at 328 K, KW is 7.23 × 10⁻¹⁴, compared to 1 × 10⁻¹⁴ at 298 K.

Why does the pH of pure water change with temperature?

The pH of pure water changes with temperature because the dissociation of water into H⁺ and OH⁻ ions increases with temperature. As the concentration of H⁺ ions increases, the pH decreases. However, water remains neutral as the concentrations of H⁺ and OH⁻ are still equal at any given temperature.

What is the relationship between pH and H⁺ concentration?

pH is the negative logarithm of the concentration of hydrogen ions (H⁺) in a solution. The formula is pH = -log[H⁺], meaning that as the concentration of H⁺ ions increases, the pH decreases, and vice versa.

How do you calculate the pH of an alkaline solution?

To calculate the pH of an alkaline solution, you first use the ionic product of water (KW) to find the concentration of H⁺ ions. The equation KW = [H⁺][OH⁻] allows you to solve for [H⁺] if the concentration of OH⁻ is known. Then, you can calculate the pH using the formula pH = -log[H⁺].

What happens to the pH of water when the temperature increases?

When the temperature of water increases, the dissociation of water into H⁺ and OH⁻ ions increases, which raises the concentration of H⁺ ions. This leads to a lower pH value. However, the solution remains neutral, meaning the concentration of H⁺ ions equals the concentration of OH⁻ ions.

Why is pure water considered neutral at 298 K?

Pure water is considered neutral at 298 K because the concentrations of H⁺ ions and OH⁻ ions are equal, both being 1 × 10⁻⁷ M. This gives a pH of 7, which is defined as neutral on the pH scale.

How does the dissociation of water compare to the dissociation of a weak acid?

The dissociation of water is similar to the dissociation of a weak acid because both are reversible reactions. In water, the dissociation produces equal amounts of H⁺ and OH⁻ ions, just as a weak acid dissociates into H⁺ and its conjugate base.

How can you calculate the pH of a sodium hydroxide (NaOH) solution?

To calculate the pH of a sodium hydroxide (NaOH) solution, first determine the concentration of OH⁻ ions, which is equal to the concentration of NaOH. Then, use the equation KW = [H⁺][OH⁻] to solve for the concentration of H⁺ ions. Finally, use the pH formula pH = -log[H⁺] to find the pH.

What is the significance of KW in understanding acid-base equilibria?

KW is crucial for understanding acid-base equilibria because it links the concentrations of H⁺ and OH⁻ ions in water. It allows for the calculation of pH in both acidic and alkaline solutions, and helps explain the behavior of water as it dissociates into ions. KW also changes with temperature, affecting the equilibrium of acid-base reactions.