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💏intro to chemistry review

OH⁻

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

OH⁻ is the hydroxide ion, a negatively charged species consisting of one oxygen atom and one hydrogen atom. It is an important concept in the context of Brønsted-Lowry acid-base theory, as it acts as a base by accepting protons to form water.

5 Must Know Facts For Your Next Test

  1. The hydroxide ion, OH⁻, is a Brønsted-Lowry base because it can accept a proton to form water (H₂O).
  2. The reaction between a Brønsted-Lowry acid and a Brønsted-Lowry base is called a neutralization reaction, where water and a salt are formed.
  3. The strength of a base is determined by its ability to accept protons, with stronger bases having a greater tendency to accept protons.
  4. The pH of a solution is determined by the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) present, with pH = -log[H⁺].
  5. In a neutral solution, the concentration of H⁺ and OH⁻ are equal, resulting in a pH of 7.

Review Questions

  • Explain the role of the hydroxide ion (OH⁻) in the Brønsted-Lowry acid-base theory.
    • In the Brønsted-Lowry acid-base theory, the hydroxide ion (OH⁻) acts as a base by accepting protons (H⁺) to form water (H₂O). This proton acceptance is a key characteristic of a Brønsted-Lowry base. The reaction between a Brønsted-Lowry acid and a Brønsted-Lowry base, such as the reaction between H⁺ and OH⁻ to form H₂O, is known as a neutralization reaction. The strength of a base, such as OH⁻, is determined by its ability to accept protons, with stronger bases having a greater tendency to do so.
  • Describe the relationship between the hydroxide ion (OH⁻) and the pH of a solution.
    • The pH of a solution is determined by the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) present. In a neutral solution, the concentration of H⁺ and OH⁻ are equal, resulting in a pH of 7. When the concentration of OH⁻ is greater than the concentration of H⁺, the solution is considered basic, and the pH will be greater than 7. Conversely, when the concentration of H⁺ is greater than the concentration of OH⁻, the solution is considered acidic, and the pH will be less than 7. The hydroxide ion (OH⁻) is a key player in determining the pH of a solution, as it can accept protons to form water, thereby influencing the overall hydrogen ion concentration.
  • Analyze the role of water autoionization in the context of the hydroxide ion (OH⁻) and Brønsted-Lowry acid-base theory.
    • Water autoionization is the process where water molecules can undergo self-ionization to form hydrogen ions (H⁺) and hydroxide ions (OH⁻). This is a crucial concept in the Brønsted-Lowry acid-base theory, as the hydroxide ion (OH⁻) is a key Brønsted-Lowry base that can accept protons (H⁺) to form water. The equilibrium between H⁺ and OH⁻ ions in water determines the pH of the solution, with a neutral solution having equal concentrations of H⁺ and OH⁻. Understanding the role of water autoionization and the behavior of the hydroxide ion (OH⁻) as a Brønsted-Lowry base is essential for predicting and analyzing acid-base reactions in chemical systems.

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