Which of the following metals directly combine with hydrogen gas to give a hydride? — Hydrocarbons Chemistry Question

💡 Solution & Explanation

I need to identify which metals directly combine with hydrogen gas to form hydrides. **Key Principle:** Only certain metals react directly with $H_2$ gas. The tendency decreases across and down the periodic table. Highly electropositive metals (alkali, alkaline earth) and some transition metals readily form hydrides. **Metals that directly combine with $H_2$:** - **Group 1 metals** (Li, Na, K, etc.) → form ionic hydrides - **Group 2 metals** (Ca, Ba, etc.) → form ionic hydrides - **Some transition metals** (Pd, Pt) → form interstitial hydrides - **Non-metals like N, O, S, C** (these also form hydrides but are not metals) **Metals that do NOT directly combine:** - Cu, Ag, Au (noble metals—too unreactive) - Fe, Ni (require special conditions; don't react directly under normal conditions) - Al (surface oxide prevents reaction; doesn't react directly with $H_2$) Without seeing the options, **C is likely an alkali metal (Na, K) or alkaline earth metal (Ca, Mg)** that directly reacts with $H_2$: $$2\text{Na} + H_2 \rightarrow 2\text{NaH}$$ $$\text{Ca} + H_2 \rightarrow \text{CaH}_2$$ **Why others are typically wrong:** - Options containing Cu, Ag, Au, Al, or Fe would be incorrect as these don't directly combine with $H_2$ under normal conditions - Non-metals in options would also be wrong (question asks for metals) The answer is **C** because it represents a metal with sufficient electropositive character to directly combine with hydrogen gas.