Q.2-2-13P - Organic Chemistry (Mcmurry)

Question

Question: Amide ion,H₂N₂, is a much stronger base than hydroxide ion, HO^-. Which is the stronger acid, NH₃ or H₂O? Explain.

Step-by-Step Solution

Answer

Answer:

H-OH is a stronger acid than H₂N-H .because the pK_a value of water is less than ammonia.The stronger acid has a smaller  pK_a and a weaker acid has a larger  pK_a .

Step 1: Acid and base strength

Acids differ in their ability to donate H⁺. Stronger acids, such as HCl, react almost completely with water, whereas weaker acids, such as acetic acid (CH₃COOH), react only slightly. The exact strength of a given acid HA in water solution is described using the acidity constant (K_a) for the acid-dissociation equilibrium.

Example:

                                               $$\begin{gathered} {\text{HA+H}}_2{\text{O⇄A}}^{-}{\text{H}}_3\text{O} \\ {K}_a=\frac{\left[H_3{O}^{+}\right]\left[A^{-}\right]}{\left[HA\right]} \end{gathered}$$

Acid strengths normally uses pK_a values rather than K_a values, where the pK_a   is the negative common logarithm of K_a  :

pK_a =-log K_a 

The stronger acid has a smaller pK_a   and a weaker acid has a larger K_a  

Step 2: Bronsted-Lowry definition of acid and bases

In the Bronsted-lowry definition of acid and bases, an acid is a proton (H⁺) donor, and base is a proton acceptor.When Bronsted-Lowry acid loses a proton, a conjugate base is formed. Similarly, when a Bronsted-Lowry base gains a proton, a conjugate acid is formed.

 

Step 3: Determining the strongest acid between NH 3 and H 2 O

H-OH is a stronger acid than H₂N-H. Since H₂N-H is a stronger base than HO^-, the conjugate acid of is a weaker acid H₂N^- (H₂N-H) than the conjugate acid of HO^- (HO-H).H-OH is a stronger acid than H₂N-H  because the pK_a  value of water is less than ammonia.The stronger acid has a smaller pK_a  and a weaker acid has a larger pK_a  .