OH-

Definition

OH- is the hydroxide ion, a negatively charged particle composed of one oxygen atom and one hydrogen atom. This ion is a key component in understanding acid-base chemistry, as it plays a central role in the concepts of pH, pOH, relative strengths of acids and bases, hydrolysis of salts, buffers, acid-base titrations, Lewis acids and bases, and coupled equilibria.

5 Must Know Facts For Your Next Test

1. The hydroxide ion (OH-) is a key component in the determination of pH and pOH, as the concentration of H+ and OH- ions in a solution are inversely related.
2. Strong bases, such as sodium hydroxide (NaOH), readily dissociate in water to produce a high concentration of OH- ions, resulting in a high pH.
3. The hydrolysis of salts can produce OH- ions, leading to a basic solution, or H+ ions, leading to an acidic solution, depending on the nature of the salt.
4. Buffers are solutions that resist changes in pH and often contain conjugate acid-base pairs, where one member of the pair can accept or donate a proton to maintain a relatively constant pH.
5. In acid-base titrations, the endpoint is reached when the concentration of OH- ions equals the concentration of H+ ions, resulting in a neutral solution with a pH of 7.

Review Questions

• Explain how the concentration of OH- ions affects the pH and pOH of a solution.
  • The concentration of OH- ions is inversely related to the concentration of H+ ions in a solution, as described by the autoionization of water. When the concentration of OH- ions is high, the pH of the solution will be high (basic), and the pOH will be low. Conversely, when the concentration of OH- ions is low, the pH of the solution will be low (acidic), and the pOH will be high. This relationship is expressed by the equation: pH + pOH = 14, which holds true for all aqueous solutions at 25°C.
• Describe the role of OH- ions in the hydrolysis of salts and the formation of buffer solutions.
  • The hydrolysis of salts can produce OH- ions, leading to a basic solution, or H+ ions, leading to an acidic solution, depending on the nature of the salt. For example, the hydrolysis of a salt formed from a weak acid and a strong base, such as sodium acetate (CH3COONa), will produce OH- ions and result in a basic solution. Conversely, the hydrolysis of a salt formed from a strong acid and a weak base, such as ammonium chloride (NH4Cl), will produce H+ ions and result in an acidic solution. Buffer solutions, which resist changes in pH, often contain conjugate acid-base pairs, where one member of the pair can accept or donate a proton to maintain a relatively constant pH.
• Analyze the role of OH- ions in the determination of the endpoint during an acid-base titration and how this relates to the concept of Lewis acid-base theory.
  • In an acid-base titration, the endpoint is reached when the concentration of OH- ions equals the concentration of H+ ions, resulting in a neutral solution with a pH of 7. This is the point where the acid and base have completely reacted, and no excess of either is present. The OH- ion, as the conjugate base of water, can act as a Lewis base by donating a pair of electrons to a Lewis acid, such as H+, to form a neutral water molecule. This Lewis acid-base reaction is the fundamental principle underlying acid-base titrations and the determination of the equivalence point, where the moles of H+ and OH- are equal.

Related terms

pH:

pH is a measure of the hydrogen ion (H+) concentration in a solution, with lower pH values indicating higher acidity and higher pH values indicating higher basicity or alkalinity.

pOH:

pOH is a measure of the hydroxide ion (OH-) concentration in a solution, with lower pOH values indicating higher basicity and higher pOH values indicating higher acidity.

Conjugate Base:

A conjugate base is the species formed when an acid loses a proton (H+), and it has a higher pH than the original acid.