Which of the following reactions could be used to produce hydrogen gas?

• A. Zn + 2HCl → ZnCl_2 + H_2
• B. 2NaCl → 2Na + Cl_2
• C. CaCO_3 + 2HCl → CaCl_2 + CO_2 + H_2O
• D. 2K + Br_2 → 2KBr

Answer: (A)

Verified step by step guidance

1. Identify the type of reaction that produces hydrogen gas (H_2). Typically, hydrogen gas is produced when a metal reacts with an acid, releasing H_2 as a product.
2. Examine the first reaction: \(\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\). Here, zinc (a metal) reacts with hydrochloric acid, producing zinc chloride and hydrogen gas. This fits the pattern for hydrogen gas production.
3. Look at the second reaction: \$2\text{NaCl} \rightarrow 2\text{Na} + \text{Cl}_2$. This is a decomposition reaction producing sodium metal and chlorine gas, but no hydrogen gas is formed.
4. Consider the third reaction: \(\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{CO}_2 + \text{H}_2\text{O}\). This reaction produces calcium chloride, carbon dioxide, and water, but not hydrogen gas.
5. Review the fourth reaction: \$2\text{K} + \text{Br}_2 \rightarrow 2\text{KBr}$. This is a synthesis reaction forming potassium bromide, with no hydrogen gas produced.