Question

1)Determine which of the following pairs of reactants will result in a spontaneous reaction at 25°C.(explain)

a)Na+ (aq) + Cr (s)

b)Fe3+ (aq) +Fe (s)

c)Ag+ (aq) + Br− (aq)

d)Pb2+ (aq) + Ag (s)

e)None of the above pairs will react. 2)Decide whether or not each of the following metals dissolves in 1 M HCl . For these metals that do dissolve, write a balanced redox reaction showing what happens when the metal dissolves.(Express your answer as a net ionic equation. Enter noreaction if there is no reaction. Identify all of the phases in your answer). a)Ag b)Hg c)Pb

Answer 1

A reaction is spontaneous when its net E0 > 0 or delta G < 0

Na+ and Cr will not react as sodium is more electropositive than Cr and will not gain electron from Cr to disturb its already noble state configuration. Also, the standard electrode potentital of the reaction is negative due to Na+ +e -> Na and thus its not feasible.

Fe⁺³ and Fe reacts to form 2Fe⁺². It is a feasible reaction as Fe⁺³ + e -> Fe⁺² has E0 >0 and Fe -> Fe⁺² + 2e also has E0 > 0 so, net E0 for the reaction Fe⁺³ + Fe -> 2Fe⁺² must also have E0 > 0

Ag⁺ + Br^- ->AgBr . E0 = 0.8 for Ag+ + e -> Ag and for Br^- -> e + 1/Br₂ E0 = -1.09/2 = -0.545 so, for net reaction E0 = 0.8-0.545 = 0.255 which is positive so, it is also feasible.

Pb⁺² + Ag is not spontaneous as Ag to Ag⁺ has a large E0 < 0 and Pb⁺² to Pb also has E0 < 0 so net E0 < 0.

Answer 2)

In reactivity table Ag comes amongst the least reactive metals and thus won't be able to react with HCl so, no reaction.

HgCl and PbCl₂ are highly insoluble and both forms precipitate in water.

Mercury gives a classical metal + acid reaction with HCl to produce H₂ gas . The reaction is -

Hg(l) + HCl(aq) -> HgCl(s) + H₂(g)

H₂ gas can be confirmed by a pop sound when we bring burning match stick close to the test tube mouth.

Pb slowly reacts with HCl to form the same H₂ gas and PbCl₂. Slowly because generally, Pb suface is protected by a thin layer which doesn't allow Pb to react freely.

Pb(s) + 2HCl(aq) -> PbCl₂(s) + H₂(g)