Water and its Autoionization

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Water's essential role in the human body encompasses temperature regulation, nutrient transport, and waste removal. It constitutes about 60% of an adult's body weight and is crucial for cellular function and cognitive performance. The text delves into the autoionization of water, a chemical reaction that produces hydronium and hydroxide ions, influencing the pH of solutions. Understanding this process is vital for various scientific and medical applications, as it affects the equilibrium constant, Kw, and the pH scale.

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Water's role in temperature regulation

Water absorbs and redistributes body heat, maintaining stable internal temperature.

Water's function in nutrient transport and waste removal

Water dissolves nutrients for cellular uptake and flushes out metabolic waste via excretory systems.

Effects of mild dehydration on cognitive performance

Mild dehydration can lead to impaired attention, memory, and motor coordination.

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The Chemical Equation of Water Autoionization

The autoionization of water can be represented by the chemical equation: 2H2O(l) ⇌ H3O+(aq) + OH−(aq). This equation shows that two water molecules interact to form one hydronium ion and one hydroxide ion, which are produced in equimolar amounts. The double arrow indicates that the reaction is reversible and that the system reaches a state of dynamic equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction.

Simplifying the Autoionization Equation

In many chemical contexts, the hydronium ion (H3O+(aq)) is often represented simply as the hydrogen ion (H+(aq)) for convenience. This leads to the simplified form of the autoionization equation: H2O(l) ⇌ H+(aq) + OH−(aq). This notation is widely used in acid-base chemistry and will be adopted in the following discussion to clarify the concepts without compromising the scientific accuracy of the information.

The Autoionization Constant of Water (Kw)

The equilibrium constant for the autoionization of water is known as the ion-product constant, or Kw. It is expressed as Kw = [H+][OH−], where the square brackets denote the molar concentrations of the ions at equilibrium. Since water is a pure liquid, its concentration is not included in the expression. Kw is temperature-dependent and has a value of approximately 1.0 × 10^-14 at 25°C, indicating that the autoionization of water is a relatively rare event under standard conditions.

The Relationship Between Autoionization, Kw, and pH

The constant value of Kw at a given temperature has direct implications for the pH of aqueous solutions. At 25°C, the product of the molar concentrations of hydrogen and hydroxide ions in any aqueous solution is always approximately 1.0 × 10^-14. This relationship is used to calculate the pH, which is the negative logarithm of the hydrogen ion concentration. In pure water at 25°C, the concentration of hydrogen ions is about 1.0 × 10^-7 M, resulting in a neutral pH of 7.0. The pH scale ranges from 0 to 14, with values below 7 indicating acidic conditions and values above 7 indicating basic conditions.

Calculating pH from Autoionization Examples

To calculate pH from autoionization, one can use the known value of Kw and the concentration of one type of ion to find the concentration of the other ion. For example, if the concentration of hydroxide ions (OH−) is 5.6 × 10^-4 M at 25°C, the concentration of hydrogen ions (H+) can be calculated, and subsequently, the pH can be determined to be 10.7, indicating a basic solution. Conversely, given a pH of 3.5, the concentration of hydroxide ions can be calculated to be 3.16 × 10^-11 M, reflecting an acidic solution.

Temperature Effects and Limitations in Autoionization Calculations

The value of Kw and the pH of pure water are affected by temperature changes. For instance, at 75°C, Kw increases to approximately 2.0 × 10^-13, and the pH of pure water decreases to about 6.3, which is less neutral than at 25°C. This is because the autoionization of water is an endothermic reaction; thus, an increase in temperature drives the equilibrium towards the formation of more ions. However, when dealing with very dilute acids or bases, the contribution of ions from the autoionization of water must be considered, as it can significantly affect pH calculations.

Key Takeaways on Water Autoionization

In conclusion, water autoionization is a critical chemical process that generates hydronium and hydroxide ions in equal amounts, establishing a dynamic equilibrium. The equilibrium constant, Kw, is essential for understanding the pH of aqueous solutions and varies with temperature. The pH of a solution is determined by the molar concentrations of H+ and OH− ions, with neutrality defined by their equal concentrations. However, temperature influences and the limitations of the autoionization concept in highly dilute solutions must be taken into account when performing pH calculations.

Water and its Autoionization

Concept Map

Summary

Outline

Water and its Autoionization

Importance of Water in the Human Body

Composition of the Human Body

Functions of Water in the Body

Autoionization of Water in the Body

Autoionization of Water

Definition and Process

Amphoteric Nature of Water

Equilibrium and pH

Equilibrium Constant and pH

Ion-Product Constant (Kw)

Calculation of pH

Limitations of Autoionization in pH Calculations

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