Which would react faster? 1.0M HCl or 2.0M HCl?

• A. 1.0M HCl
• B. 2.0M HCl
• C. Both equally fast
• D. None react

Answer: (B)

Reaction rate depends on how many reacting H+ ions are present. Since HCl is a strong acid, it fully dissociates in water, so the hydrogen ion concentration [H+] equals the acid concentration. Doubling the acid concentration from 1.0 M to 2.0 M doubles the available H+ ions, so the rate of reaction is faster under the same conditions. Hence, 2.0 M HCl reacts faster than 1.0 M HCl. The idea that they react at the same rate would only hold if concentration didn’t affect rate, which isn’t the case here; and the option that none reacts is incorrect because the acid will react.