The number of ions from the following that have the ability to liberate hydrogen from a dilute acid is _________.
Ti²⁺, Cr²⁺ and V²⁺

Solution

Chemistry ma’am · 3 steps

Compare each ion's M³⁺/M²⁺ potential with hydrogen's zero, and all three come out negative.

1. What liberating H2 needs. To give H₂ from dilute acid, the ion must reduce H⁺. It does this by getting oxidised itself, from M²⁺ to M³⁺.

   2M²⁺ + 2H⁺ → 2M³⁺ + H₂

2. The test. E°(H+/H₂) = 0 V. If E°(M³⁺/M²⁺) is negative, M²⁺ is a stronger reducing agent than H₂, and the reaction above goes forward.

   Ion	E°(M³⁺/M²⁺)	Reduces H⁺ to H₂?
   Ti²⁺	-0.37 V	Yes
   V²⁺	-0.26 V	Yes
   Cr²⁺	-0.41 V	Yes

3. So. All three potentials are below zero, so Ti²⁺, V²⁺ and Cr²⁺ all liberate hydrogen. The count is 3, option (c).

The idea to keep. An ion frees H₂ from dilute acid only if its M³⁺/M²⁺ potential is negative. Remember Ti, V and Cr as the 2+ ions that do this, and Mn, Fe and Co as the ones that do not.

Answer from the NTA final answer key. Solution by PrepWiser.