Wishboy is correct regarding the kinetics question.
Regarding the electrochem qn, B is the correct answer.
Precipitating Mg2+ out as Mg(OH)2 ppt lowers the molarity of Mg2+,
thus making the oxidation potential @ anode more positive (as
predicted by Le Chatelier's principle of system opposing the
change). Since cell potential = reduction potential @ cathode
+ oxidation potential @ anode, hence cell potential becomes
more positive.
