CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
In the manufacture of brass, molten copper and molten zinc are combined to produce a uniform solid solution once the alloy cools. Over time, brass objects exposed to moist air may develop a surface coating of basic copper(II) carbonate (“patina”) according to the simplified reaction
2 Cu(s) + H₂O(l) + CO₂(g) + O₂(g) → Cu₂CO₃(OH)₂(s)
(a) Classify the formation of brass from molten copper and molten zinc as a physical change, a chemical change, or both. Justify your answer, explicitly discussing the nature of the metallic bonding before and after mixing.
(b) Contrast your classification from part (a) with the classification you assign to the patina-forming reaction shown above. Support your choice with bonding and stoichiometric arguments.
(c) Propose a method to separate copper and zinc atoms from a brass sample without decomposing either element chemically. Outline the physical principle exploited and explain why the procedure does not constitute a chemical change.
(d) Predict how the electrical conductivity of freshly cast brass compares with that of pure copper. Provide a particulate-level explanation that accounts for the identity and arrangement of the atoms in the alloy.