HKDSE Chemistry Question Review (20240701)
Question:
John and Irene are a pair of douches. One day, Irene bought a gold jewelry and say that it is made of pure gold, but John digressed and maintained that it must be infiltrated with copper which makes an air of shiny color. Describe how John's doubt can be justified.
Solution:
Recall the reactivity of metal:
Cu>Ag>Au
Since gold is an inert metal, a pure gold must be very soft and slippery so it is virtually impossible to craft a jewelry made of 100% gold.
In addition, since gold has the least reactivity, if we pour a dilute AgNO3 there will be no chemical reaction, but since Copper is more reactive that gold and silver, by pouring AgNO3 displacement reaction will be observed as blue Cu(NO3)2 solution will result alongside with silvery deposit spotted:
2AgNO3 (aq) + Cu(s) = Cu(NO3)2 (aq) +2Ag (s)
By this token, burning a pure gold will have no observable changes until it melts with sufficient heat. Burning copper will lead to black deposits on the metal surface:
2Cu(s) + O2(g) = 2CuO (s)
No doubt, Irene is stupid this time.
Question:
John and Irene are a pair of douches. One day, Irene bought a gold jewelry and say that it is made of pure gold, but John digressed and maintained that it must be infiltrated with copper which makes an air of shiny color. Describe how John's doubt can be justified.
Said*
To make an aqueous Au(NO3)3 implies that strong oxidizing agents such as concentrated HNO3 and concentrated HCl, or a blend of both, must be deployed.
Also, as gold has a big metallic radius, AuSO4 will be also soluble in water because the lattice energy of gold cations with sulphate anions will be lowered. The hydration enthalpy of AuSO4 is also high because in aqueous state it is quite stable.