IB Chemistry SL R3.2 Electron transfer reactions questions

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Question 1

Multiple choiceMedium1 mark

A voltaic cell operates by the reaction Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\mathrm{Mg(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2(g)} Initially, 0.0040 mol0.0040\ \mathrm{mol} Mg and 0.0060 mol0.0060\ \mathrm{mol} HCl are available. What amount of H2\mathrm{H_2} forms, and what is the sign of the electrode where it forms?

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Answer: D. The equation requires 2 mol2\ \mathrm{mol} HCl per 1 mol1\ \mathrm{mol} H₂, so HCl limits the reaction: n(H2)=0.0060/2=0.0030 moln(\mathrm{H_2})=0.0060/2=0.0030\ \mathrm{mol} H⁺ is reduced at the hydrogen electrode, making it the cathode; the cathode is positive in a voltaic cell.
A: gives the correct amount but assigns the hydrogen electrode the negative sign.
B: uses the amount of Mg as though it all reacts, despite insufficient HCl.
C: uses the Mg amount without checking the limiting reagent and also assigns the cathode a negative sign.
Remember: in a voltaic cell, reduction occurs at the positive cathode.

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