Which of these equations correctly expresses the self-ionization of water?
A. H₂O+H₂O → H₂O + OH
B. H₂O + H₂O 20H™
C. H₂O+H₂O → 2H₂O*
D. H₂O+H₂O H₂O + OH
The Correct Answer and Explanation is:
The correct answer is D. H₂O + H₂O → H₃O⁺ + OH⁻.
Explanation:
Water undergoes self-ionization, which refers to the process where two water molecules interact, one donating a proton (H⁺) to the other, resulting in the formation of hydronium ions (H₃O⁺) and hydroxide ions (OH⁻). This is a reversible reaction, and it is important for understanding the concept of pH and the balance of acids and bases in aqueous solutions.
The self-ionization of water can be written as:
[
H₂O + H₂O \rightleftharpoons H₃O⁺ + OH⁻
]
Here’s a breakdown of the process:
- Proton Transfer: A water molecule (H₂O) donates a proton (H⁺) to another water molecule, forming a hydronium ion (H₃O⁺) and leaving behind a hydroxide ion (OH⁻).
- Dynamic Equilibrium: This reaction occurs in equilibrium, meaning that the rate of ionization (formation of H₃O⁺ and OH⁻) is equal to the rate of recombination (H₃O⁺ and OH⁻ ions combining to form H₂O). This ensures that the concentrations of H₃O⁺ and OH⁻ in pure water are equal.
- Concentrations in Pure Water: In pure water at 25°C, the concentration of H₃O⁺ and OH⁻ ions is both (1.0 \times 10^{-7}) M. This results in a neutral pH of 7 because pH is defined as the negative logarithm of the hydrogen ion concentration, and in pure water, the concentrations of H₃O⁺ and OH⁻ are equal.
Why the other options are incorrect:
- A. H₂O + H₂O → H₂O + OH⁻: This equation is incorrect because it does not show the formation of the hydronium ion (H₃O⁺), which is a product of the ionization.
- B. H₂O + H₂O → 20H™: This equation is nonsensical and does not represent a valid chemical reaction or species.
- C. H₂O + H₂O → 2H₂O: This suggests a reaction that results in the same molecules as reactants, which is not the process of ionization. It doesn’t show any ion formation.
Thus, D is the correct expression for the self-ionization of water.