15.8
Mélanges d'acides
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Q1: Why does a strong acid suppress the dissociation of a weak acid in a mixture?
According to Le Châtelier's principle, when a strong acid produces excess hydronium ions, it shifts the weak acid's equilibrium toward reactants, reducing its dissociation. The high hydronium concentration disturbs the equilibrium, causing the reaction to reverse until balance is restored. This suppression effect means the weak acid contributes negligibly to the overall pH.
Q2: How do you calculate the pH of a mixture containing a strong acid and a weak acid?
Calculate pH using only the strong acid's concentration, since it produces nearly all hydronium ions. For example, a mixture with 0.15 M hydrochloric acid and 0.30 M hydrocyanic acid has pH determined solely by the 0.15 M strong acid concentration. The weak acid's contribution is negligible due to suppressed dissociation.
Q3: What role does the acid dissociation constant play when comparing two weak acids?
The acid dissociation constant (Ka) determines which weak acid dominates pH in a mixture. The acid with the significantly higher Ka value produces more hydronium ions and controls the mixture's pH. For instance, hydrofluoric acid with Ka = 3.5 × 10−4 is nearly a million times stronger than hydrocyanic acid with Ka = 4.9 × 10−10.
Q4: How can you use an ICE table to find hydronium ion concentration from a weak acid in a mixture?
Set up an ICE table with initial concentrations, changes denoted by x, and equilibrium values. Apply the 5% rule to simplify calculations when x is small. Substitute equilibrium concentrations into the Ka expression to solve for x, which represents the hydronium ions produced by the weak acid.
Q5: In a mixture of two weak acids with equal concentrations, which acid determines the pH?
The relatively stronger weak acid, with the higher Ka value, determines the pH. In a mixture of nitrous acid (Ka = 4.6 × 10−4) and hypochlorous acid (Ka = 2.9 × 10−8), nitrous acid controls pH because its Ka is approximately 10,000 times larger, suppressing the dissociation of the weaker acid.
Q6: Why is the contribution of a weak acid to total hydronium concentration often negligible in acid mixtures?
The weak acid's partial dissociation is further suppressed by hydronium ions from the stronger acid. This suppression, explained by Le Châtelier's principle, reduces the weak acid's dissociation so significantly that its hydronium contribution becomes negligible compared to the stronger acid's contribution.
Q7: What is the relationship between acid strength and pH determination in mixtures?
The strongest acid in a mixture dominates pH calculation because it produces the highest hydronium ion concentration. Whether mixing a strong with a weak acid or two weak acids, the acid with the highest dissociation constant or complete dissociation controls the overall pH of the solution.