Which of the following is a Bronsted-Lowry acid?

• A. CH₄
• B. HCN
• C. NH₃
• D. Br₂

Answer: (B)

Verified step by step guidance

1. Understand the definition of a Bronsted-Lowry acid: A Bronsted-Lowry acid is a substance that can donate a proton (H⁺) to another substance.
2. Examine each compound to determine if it can donate a proton. Start with CH₄ (methane): CH₄ is a hydrocarbon and does not have a hydrogen atom that can be easily donated as a proton.
3. Next, consider HCN (hydrogen cyanide): HCN has a hydrogen atom that can be donated as a proton, making it a potential Bronsted-Lowry acid.
4. Evaluate NH₃ (ammonia): NH₃ is known to accept a proton to form NH₄⁺, which makes it a Bronsted-Lowry base rather than an acid.
5. Finally, look at Br₂ (bromine): Br₂ is a diatomic molecule and does not have a hydrogen atom to donate, so it cannot be a Bronsted-Lowry acid.