Qual è la costante di equilibrio per la reazione di NH3 con l'acqua?

The equilibrium constant for the reaction of NH₃ with water is 1.76 × 10⁻⁵.

In aqueous solution, ammonia acts as a base. It accepts hydrogen ions from H₂O to yield ammonium and hydroxide ions.

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

The base ionization constant is

#K_"b" = (["NH"_4^+]["OH"^-])/(["NH"_3])#

We can determine the #K_"b"# valore da pH misurazioni.

Esempio

The pH of a 0.100 mol/L solution of NH₃ is 11.12. What is the #K_"b"# for NH₃?

Soluzione

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

pH 11.12

pOH = 14.00 – 11.12 = 2.88

[OH⁻] = #10^"-pOH" = 10^-2.88# = 1.32 × 10⁻³ mol/L
[NH₄⁺]= 1.32 × 10⁻³ mol/L
[NH₃] = (0.100 -1.32 × 10⁻³) mol/L = 0.099 mol/L

#K_"b" = (["NH"_4^+]["OH"^-])/(["NH"_3])# = (1.3×10⁻³ × 1.32×10⁻³)/0.099 = 1.76 × 10⁻⁵