15.2
L'eau : un acide et une base selon Bronsted-Lowry
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Q1: What does it mean for water to be amphoteric?
Water is amphoteric because it can act as both an acid and a base. In acidic solutions, water accepts protons from acids, functioning as a base. In basic solutions, water donates protons to bases, functioning as an acid. This dual capability allows water to participate in acid-base reactions regardless of the solution composition.
Q2: How does water autoionization work?
Water autoionization occurs when one water molecule donates a proton to another water molecule. The donating molecule acts as an acid, while the accepting molecule acts as a base. This produces a hydronium ion and a hydroxide ion. Although the process occurs to only a slight extent, it establishes the ion-product constant for water, KW, which equals 1 × 10−14 at 25 °C.
Q3: What is the ion-product constant for water and why is it important?
The ion-product constant for water, KW, equals the concentration of hydronium ions multiplied by the concentration of hydroxide ions, with a value of 1 × 10−14 at 25 °C. KW is important because it allows calculation of either ion concentration when the other is known, and it determines whether a solution is acidic, basic, or neutral based on relative ion concentrations.
Q4: How do you determine if a solution is acidic or basic using ion concentrations?
A solution is acidic when hydronium ion concentration exceeds hydroxide ion concentration. A solution is basic when hydroxide ion concentration exceeds hydronium ion concentration. A solution is neutral when both concentrations are equal. Using KW and the known concentration of one ion, you can calculate the other and classify the solution accordingly.
Q5: What are the hydronium and hydroxide ion concentrations in pure water at 25 °C?
In pure water at 25 °C, the concentrations of hydronium ions and hydroxide ions are equal, each at 1 × 10−7 M. This equality results from water's autoionization, which produces one hydronium ion for every hydroxide ion formed. These equal concentrations make pure water neutral.
Q6: How can you calculate hydroxide ion concentration if you know the hydronium ion concentration?
Use the ion-product constant expression: KW = [H3O+][OH−]. Rearrange to solve for hydroxide ion concentration: [OH−] = KW / [H3O+]. For example, if hydronium concentration is 5 × 10−7 M, then [OH−] = (1 × 10−14) / (5 × 10−7) = 2 × 10−8 M, indicating an acidic solution.
Q7: Does the autoionization of water change with temperature?
Yes, water autoionization is endothermic, so it increases with temperature. At 25 °C, KW equals 1 × 10−14, but at 100 °C, KW increases to approximately 5.6 × 10−13, roughly 50 times larger. This temperature dependence means hydronium and hydroxide ion concentrations increase at higher temperatures.