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Autoionization of Water: Exploring Kw and Chemical Balance
Dive into the fascinating world of water autoionization. Understand its equation, endothermic nature, and impact on pH levels. Enhance your chemistry knowledge with our comprehensive lesson.

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Now Playing:Autoionization of water – Example 0a
Intros
  1. What is auto-ionization?
  2. What is auto-ionization?
    Autoionization of water.
  3. What is auto-ionization?
    The autoionization expression.
Examples
  1. Understand the Kw expression for the ionic product of water.
    1. Write the expression for the ionic product of water, Kw, at 25oC. Include the equation for the dissociation of water.

    2. Explain why, at 45oC, a sample of water has a pH below 7 but it is still neutral.

Introduction to acid-base theory
Jump to:NotesConceptFAQsPrerequisites
Notes

In this lesson, we will learn:

  • To recall the equation for the autoionization of water.
  • How to use the autoionization expression to find the concentration of ions.
  • How the autoionization expression changes with temperature and gives the pH value of pure water.

Notes:

  • Recall that when acids and bases react, a salt and water is formed. When strong bases and strong acids react, the reaction is more vigorous and gives off more heat energy because neutralization is an exothermic process.
    Removing the salt-forming ions which are spectator ions, this can be expressed in the equation:

    H+(aq) + OH- (aq) ⇌\rightleftharpoons H2O (aq)

  • The reverse of this reaction is known as the autoionization of water, symbol Kw (also known as the ionic product of water) – there is an equilibrium between neutral water and the dissociated H3O+ and OH- ions in solution. See the equation:

    2 H2O (l) ⇌\rightleftharpoons H3O+ (aq) + OH- (aq)

  • The ionic product Kw is a very small value (almost all the equilibrium mixture is still water, the reactant) but it is noticeable and has a constant value at 25oC:

    Kw = [H3O+ (aq)] [OH-(aq)] = 1.00 * 10 -14 at 25oC

    This ionic product can be indexed using a negative logarithm:

    pKw = -log(Kw) = 14 at 25oC

    This is used just like pH can be used to get the following equation:

    pKw = pH + pOH = 14 at 25oC

  • The Kw expression is just an equilibrium constant expression, with [H2O(l)] cancelled out. As [H3O+] increases, [OH-] decreases so their product stays equal to 1 * 10-14 constant value at 25oC.
    • This is chemically sound too; if an acid HX was added to neutral water, dissociation into protons, H+, and the conjugate base X- would occur.
    • These protons would quickly react to form H3O+ by protonating neutral H2O molecules and the very small amount of OH- ions, decreasing [OH-] further. As [H3O+] increases, a decrease in [OH-] would therefore be observed.
    • The addition of acid (or base) will neutralize the majority of the base (or acid) present in neutral water, causing the equilibrium to shift to the left and form more water. However, the initial amounts of [H3O+] and [OH-] are extremely small, orders of magnitude smaller than any standard solutions of acid/base to be added to neutral water. As such, the concentration of the added acid/base is effectively unchanged by initial [H3O+] or [OH-]. Assume [H3O+] = [HX] or [OH-] = [:B] where [HX] or [:B] is the concentration of a strong acid or base added to neutral water.

  • Remember that the Kw constant holds at 25oC only.
    This means in neutral water at 25oC, [H3O+] = 1 * 10-7 M and [OH-] = 1 * 10-7 M.
    As stated above though, neutralization is an exothermic process which means that the autoionization of water is an endothermic process. At higher temperatures, the equilibrium shifts right (Le Chatelier’s principle!), leading to greater Kw as the product of [H3O+] * [OH-] increases. Even though the water is still neutral, this greater dissociation at higher temperatures leads to higher Kw and lower pH, because pH is defined as –log [H3O+].
Concept

Introduction to Autoionization of Water

Welcome to our lesson on the autoionization of water, a fundamental concept in chemistry. We'll begin with an introductory video that provides a visual representation of this process, helping you grasp its significance in understanding water's unique properties. Autoionization is a crucial phenomenon where water molecules react with each other, forming hydronium and hydroxide ions. This lesson will delve into the autoionization equation, explaining how water molecules dissociate and recombine continuously. We'll explore the endothermic nature of this reaction, which plays a vital role in maintaining water's chemical balance. Additionally, we'll discuss how autoionization impacts pH levels, a critical factor in various biological and chemical processes. By understanding autoionization, you'll gain insights into water's behavior in different environments and its importance in countless chemical reactions. Let's dive into this fascinating aspect of water chemistry!

FAQs

Here are some frequently asked questions about the autoionization of water:

1. What is the meaning of autoionization?

Autoionization, in the context of water, refers to the process where water molecules spontaneously dissociate into hydronium (H3O+) and hydroxide (OH-) ions. This process occurs continuously in pure water and aqueous solutions, maintaining a dynamic equilibrium.

2. Why is Kw always 10^-14?

The ion product of water (Kw) is approximately 10^-14 at 25°C (room temperature). This value represents the product of the concentrations of H+ and OH- ions in pure water. It's important to note that while Kw remains constant at a given temperature, it can change with temperature variations.

3. How do you write an autoionization equation?

The autoionization equation for water is typically written as: H2O + H2O H3O+ + OH-. This equation shows two water molecules reacting to form a hydronium ion and a hydroxide ion. Alternatively, it can be simplified to: H2O H+ + OH-.

4. Is the autoionization of water endothermic or exothermic?

The autoionization of water is an endothermic process, meaning it absorbs heat from the surroundings. This is why the extent of autoionization increases with temperature, leading to a higher Kw value at higher temperatures.

5. How does autoionization affect the pH of water?

Autoionization establishes the baseline for pH in water. In pure water at 25°C, the concentration of H+ ions from autoionization results in a pH of 7, which we define as neutral. Changes in temperature can affect the extent of autoionization, slightly altering the pH of pure water.

Prerequisites

Understanding the autoionization of water is a crucial concept in chemistry, but to fully grasp its significance, it's essential to have a solid foundation in related topics. Two key prerequisite concepts that play a vital role in comprehending the autoionization of water are dynamic equilibrium and the equilibrium constant.

The concept of dynamic equilibrium is fundamental to understanding the autoionization of water. In the context of water, dynamic equilibrium refers to the constant process of water molecules splitting into hydrogen ions (H+) and hydroxide ions (OH-), while simultaneously recombining to form water molecules. This ongoing process occurs at a balanced rate, maintaining a steady concentration of ions in pure water. Grasping the principles of dynamic equilibrium helps students visualize the continuous molecular dance happening in water, even when it appears static on a macroscopic level.

Equally important is the understanding of the equilibrium constant, which is crucial when studying the autoionization of water. In this context, the equilibrium constant is specifically referred to as Kw, the ion product constant for water. This constant quantifies the extent of water's autoionization at a given temperature. By mastering the concept of equilibrium constants, students can better interpret the relationship between the concentrations of hydrogen and hydroxide ions in water, and how this relationship remains constant in pure water regardless of dilution.

The interplay between dynamic equilibrium and the equilibrium constant forms the backbone of understanding water's autoionization. The dynamic equilibrium of water explains the process, while the equilibrium constant Kw provides a quantitative measure of this phenomenon. Together, these concepts allow students to comprehend why pure water always maintains a neutral pH of 7, and how the addition of acids or bases disrupts this delicate balance.

Moreover, these prerequisite topics extend beyond just explaining the autoionization of water. They serve as foundational knowledge for more advanced concepts in chemistry, such as acid-base reactions, buffer solutions, and pH calculations. By thoroughly understanding dynamic equilibrium and the equilibrium constant, students are better equipped to tackle more complex chemical systems and reactions.

In conclusion, mastering these prerequisite topics is not just about understanding the autoionization of water; it's about building a strong foundation for advanced chemistry concepts. The principles of dynamic equilibrium and the equilibrium constant are essential tools in a chemist's arsenal, enabling a deeper understanding of chemical processes and paving the way for more sophisticated analysis in various fields of chemistry.