Rank the given compounds based on their relative Bronsted acidities.
strongest Bronsted acid
H – OH
H – NH2
H – Cl
H – F
H – CH3
weakest Bronsted acid
The Correct Answer and Explanation is:
To rank the given compounds based on their Brønsted acidities, we are comparing their ability to donate a proton (H⁺). The stronger the Brønsted acid, the more easily it donates a proton. Let’s analyze the compounds:
Compounds:
- H–Cl (hydrogen chloride)
- H–F (hydrogen fluoride)
- H–OH (water)
- H–NH₂ (ammonia)
- H–CH₃ (methane)
Correct Ranking (from strongest to weakest Brønsted acid):
H–Cl > H–F > H–OH > H–NH₂ > H–CH₃
Explanation (Approx. 300 words):
Brønsted acids are substances that donate protons (H⁺). The strength of a Brønsted acid depends on how stable the conjugate base is after losing a proton. The more stable the conjugate base, the stronger the acid.
- H–Cl (Hydrogen chloride):
This is a strong acid. When it loses a proton, it forms Cl⁻, a very stable anion due to its large size and high electronegativity. HCl fully dissociates in water, indicating strong acidity. - H–F (Hydrogen fluoride):
HF is weaker than HCl but still a relatively strong acid. Its conjugate base, F⁻, is very electronegative but smaller in size, leading to strong H–F bonding and less complete dissociation. It is weaker than HCl because the H–F bond is much stronger and harder to break. - H–OH (Water):
Water can donate a proton to become OH⁻. It is a much weaker acid than HCl or HF, and its conjugate base (OH⁻) is a stronger base, indicating weaker acidity. - H–NH₂ (Ammonia):
This group actually represents ammonia (NH₃) when protonated. Its acidity is weaker than water because NH₂⁻ is a very strong base and not stable, making NH₃ a very weak acid. - H–CH₃ (Methane):
Methane is extremely weak as a Brønsted acid. Its conjugate base CH₃⁻ (methyl anion) is highly unstable and extremely basic. Thus, methane practically never donates a proton in normal conditions.
Summary:
| Compound | Conjugate Base | Relative Acid Strength |
|---|---|---|
| H–Cl | Cl⁻ | Strongest |
| H–F | F⁻ | Strong |
| H–OH | OH⁻ | Moderate |
| H–NH₂ | NH₂⁻ | Weak |
| H–CH₃ | CH₃⁻ | Weakest |
Strongest acid: H–Cl
Weakest acid: H–CH₃