Reactivity trends in Groups I and VII

Steps:

• Group I metals increase in reactivity down the group due to decreasing ionization energy.
• Group VII halogens decrease in reactivity down the group, so higher ones displace lower ones.
• In electrolysis of aqueous halides, H2 forms at cathode; halide ions oxidize to halogens at anode.
• Displacement and reactivity follow these periodic trends.

Why C is correct:

• Down Group I, atomic radius increases and ionization energy decreases, making it easier for larger atoms like Rb to lose electrons and form +1 ions compared to K.

Why the others are wrong:

• A: Chlorine is more reactive than bromine, so it displaces Br- from bromides, not the reverse.
• B: In electrolysis of aqueous KI, I2 forms at the anode (oxidation), while H2 forms at the cathode.
• D: Reactivity increases down Group I, so K reacts more violently with water than Na.

Final answer: C