急!!F5.Chem,double decomposition,prepare salt,plating

double decomposition will happen when two ionic compounds react
the two ionic compounds must not be both solids or insoluble each other
e.g. AgNO3(aq) + KCl(aq) → KNO3(aq) + AgCl(s) (double decomposition occurs)
e.g. AgCl(s) + CaSO4(s) → AgCl(s) + CaSO4(s) (double decomposition do not occur)

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making soluble salt:
method 1: acid + metal/insoluble base/insoluble carbonate
e.g. Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
e.g. ZnO(s) + H2SO4(aq) → ZnSO4(aq) + H2O(l)
e.g. ZnCO3(s) + H2SO4(aq) → ZnSO4(aq) + H2O(l) + CO2(g)
method 2: titration (acid + alkali/soluble carbonate)
e.g. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
e.g. K2CO3(aq) + 2HCl(aq) → 2KCl(aq) + H2O(l) + CO2(g)

making insoluble salt: precipitation
e.g. Pb(NO3)2(aq) + 2NaCl(aq) → PbCl2(s) + 2NaNO3(aq)

differences in adding together and titration:
for the reaction of NaOH and HCl
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
if we know the volume of NaOH and HCl used
and we know the molarity of NaOH
we can know the molarity of HCl,
the amount of NaCl and H2O formed
so titration let us know the volume of reactants to undergo complete reaction
it is very useful for calculation
if we just add NaOH and HCl together
of course the reaction is the same
but we don't know actually how much reactants are needed
and we don't know how much products are formed
maybe one of the reactants is in excess

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why Ag anode will dissolve and Ag+ discharges in cathode?

we may discuss the second part first
cathode is somewhere for ions to undergo reduction
in the electrolyte, there are two ions will discharge in cathode
they are Ag+ and H+
as Ag+ is a stronger oxidizing agent than H+
it tends to accept a electron in order to have reduction
and it is discharged first
for the anode
Ag will dissolve and it is due to the nature of electrode
"Ag is a stronger reducing agent than OH-"
this sentence is only a comparison between Ag and OH-
Ag is a stronger reducing agent than OH-
and it is a weaker reducing agent than potassium
since Ag is a stronger reducing agent
it tends to undergo oxidation and give electrons
so it dissolves