Which element will reduce Mg2+ to Mg?
a) Fe b) Ba c) Pb d) Ag

Ba

Explanation:

Let us consider the electrode potential of one of the options listed in the question,

Ba2+(aq) + 2 e- > Ba(s). -2.90V

It is very important to recall that a metal can only displace another metal from aqueous solution if that metal has a more negative reduction potential that the metal it displaced. This implies that if there are two metals A and B, A can only displace B from an aqueous solution of B if A has a more negative reduction potential than B.

The reduction potential of Mg^2+ is -2.37V. This is less negative than that of Ba^2+ which is -2.90V. Hence barium can reduce Mg^2+ to metallic Mg.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

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