Three unknown metals P , Q , and R are tested for their reactivity. It is observed that metals P and R liberate H ₂ gas when reacted with dilute HCl , whereas metal Q does not. Furthermore, when metal P is placed in an aqueous salt solution of R , it displaces R from the solution. The correct increasing order of the standard reduction potentials ( E^ _ M ²⁺/ M ) for the three metals is:

Step-by-step solution

For a metal to liberate H ₂ gas from dilute acids, its standard reduction potential must be negative. Since P and R liberate H ₂ , we have E^ _ P Since Q does not liberate H ₂ from dilute acid, its standard reduction potential must be positive. Thus, E^ _ Q > 0 . Metal P displaces R from its aqueous salt solution, which means P is a stronger reducing agent than R . A stronger reducing agent has a more negative standard reduction potential. Therefore, E^ _ P Combining these observations, the increasing order of stan