2 quick multiple choice question need explanation (1 Viewer)

[kooltrainer]

1) Which of the species below will spontaneously react with iron?

a)Cu(s)
b)H2O (l)
c)Mn 2+
d) Pb 2+



2) A galvanic cell was constructed as follows: in one half cell hydrogen gas is bubbled over an inert electrode in as basic solution. The other half cell contained metal X and X 2+ (aq). The overall potential was between 0.58V and 0.71V. Select the correct option.

a)The lead cathode increased in mass
b)The nickel anode decreased in mass
c)The colour of the solution in the reduction half cell became paler blue.
d)Copper was oxidised.



The answer to 1) is D
The andwer to 2) is A
whys that

 

[minijumbuk]

1. Pb 2+ is the only substance in that list that is a stronger oxidising agent than iron (refer to the data sheet), therefore Fe gets oxidised to Fe 2+ and Pb 2+ becomes Pb.

Case a) Cu(s) will not react with Fe, because the reaction must be a redox reaction: for every oxidation half, there is also a reduction half. Cu(s) and Fe(s) cannot both oxidise at the same time.
Case b) Potential of reduction of H2O liquid into 1/2 H2 + OH- = -0.83V, while oxidation of Fe into Fe 2+ and 2e = 0.44. The overall potential is -0.83 + 0.44, which gives -0.39 V. This means that it will not occur spontaneously, as extra input energy is required.
Case c) Mn 2+ reduction potential is -1.18V, while oxidation potential of Fe is 0.44V. Again, this gives a negative value, so there will be no spontaneous reaction.
That leaves us case d)
Pb 2+ reduction potential is -0.13V, while oxidation potential of Fe is 0.44. The total is 0.31 V. This value is above 0, so it will react spontaneously.

2. I'm not 100% certain on this one, but look at the data sheet.
The cell potential of 1/2 H2 + OH- --> H2O + e is 0.83 V, while the cell potential for Pb2+ + 2e --> Pb is -0.13V.
The overall potential is therefore 0.83 - 0.13 = 0.70 V, which satisfies the given data.
So, if this assumption was correct, then it can be deduced that lead cathode increases in mass, because the Pb 2+ are reduced to Pb solid.