Spontaneous reaction?

Question

Which substance will react spontaneously with Cu 2+ under standard conditions?
a) Fe (s)
b) Zn 2+ (aq)
c)Br2 (l)
d) Ce 3+ (aq)

Answer 1

You need the table with the standard reduction potentials to answer this.
I use values from http://www.northland.cc.mn.us/Chemistry/standard_reduction_potentials.htm

Cu(+2) +2e -> Cu(0) E01(Cu)= + 0.342 V
Cu(+2) + e -> Cu(+) E02(Cu)= +0.153 V

In order for the reaction to occur spontansously you need to have E>0 for the reaction. For the total reaction you just add the standard potentials of the half-reactions. However, since the potentials are always given for reduction half-reaction you'll have to invert the sign when you want the respective oxidation reaction.

E.g. for iron you have
Fe(+3) + 3e -> Fe(s) E01(Fe)=-0.037 Volt but you are interested in the inverse half reaction, so

Fe(s) -> Fe(+3) +3e E'01(Fe)= +0.037 V.
For the overall reaction E=+0.342+0.037= 0.379 Volt or 0.153+0.037= 0.190 Volt if Cu takes only 1e. The more positive the E the more favoured that reaction, so the reaction
2Fe(0) + 3Cu(+2) <=> 2Fe(+3) + 3Cu(0) is more favoured than
Fe(0) + 3Cu(2+) <=> Fe(+3) + 3Cu(+)

However there is also the possibility
Fe(0) -> Fe(+2) +2e E'0(Fe)= +0.447
Then you would get
Fe(0) +Cu(+2) <=>Fe(+2) + Cu(0) with E=0.447+0.342=0.789 V
(the reaction where Cu2+ becomes Cu+ is less favoured as above). So from all these possible spontaneous reactions reactions with Fe(s) the most favourable is
Fe(0) +Cu(+2) <=>Fe(+2) + Cu(0)

For Zn(+2) we don't need to look at the E0. Zn+2 and Cu+2 are both at their highest oxidation level so none of them can give electrons to the other. Thus no reaction takes place.

For Br2 you would need reduction to Br-. This means that Cu+2 would have to give electrons to Br2, but as we said Cu+2 cannot give more electrons, so the reaction doesn't occur.

Ce+3 cannot be reduced by Cu+2 (Cu+2 as we said cannot give any more electrons thus it cannot reduce anything). Let's see if it can be oxidized.

Ce(+3) <=> Ce(+4)+e E'0(Ce)= -1.720 Volt
so for
Ce(3) + Cu(+2) <=> Ce(+4) + Cu(0) E=0.342+(-1.720) = -1.378 <0 so no it will not happen spontaneously.

Answer 2

c - Bromine 2 br- will react with Cu 2+ - remember Br F Cl I are all 1- and tend to be both soluble and reactive - Zn and Ce are both positively charged and wont tend to react with Cu2+ under standard conditions. Iron (Fe) would also need sufficient energy to react with Copper so bromine seems the most logical answer. also- in this case Fe is noted as being solid not in ionized format with a negative analogue.

Answer 3

Copper ions and bromine will react to produce CuBr2

If you put an iron nail in CuSO4 the nail will become copper colored and the solution will become lighter blue. So Fe also reacts with Cu+2

This reaction I have done. I am only assuming on the Br

Answer 4

A)

As far as C) , Br2 is a strong oxidizer, but Cu++ is already in an oxidized state, so Br2 cannot oxidize it further.