Zinc from a Carbonate Ore
A student is given 5.0 g of an ore that is mainly zinc carbonate, ZnCO₃, mixed with sand. She strongly heats a sample in a hard-glass test tube and observes a yellow solid when hot, which turns white on cooling, while the gas produced turns limewater milky. She then mixes the cooled white solid with carbon powder and heats the mixture strongly. A grey metal forms, but the product is brittle and mixed with white powder. The mass of grey metal collected is 1.8 g. The teacher says zinc boils at about 907 °C and that zinc oxide is reduced by carbon at high temperature. The student wants to know whether she has actually extracted zinc, why the product is brittle, and how the yield could be improved. The labels give M(ZnCO₃) = 125 g/mol and M(Zn) = 65 g/mol.
1. Identify the yellow-hot, white-cold solid and the gas. Write equations for the two heating stages.
Read the worked answer
The gas turns limewater milky, so it is carbon dioxide. Heating zinc carbonate decomposes it: ZnCO3(s) -> ZnO(s) + CO2(g). The solid is zinc oxide, ZnO. Zinc oxide is white when cold but yellow when hot, which explains the colour change. In the second stage, carbon reduces zinc oxide: ZnO(s) + C(s) -> Zn(s) + CO(g). The grey metal is zinc. Carbon is oxidised to carbon monoxide, so carbon acts as the reducing agent.
2. Calculate the theoretical mass of zinc from 5.0 g of pure ZnCO3 and the percentage yield based on 1.8 g. Why is this yield only an apparent value for the ore?
Read the worked answer
M(ZnCO3) = 125 g/mol; amount = 5.0/125 = 0.040 mol. The equations show 1 mol ZnCO3 -> 1 mol Zn. M(Zn) = 65 g/mol; theoretical mass = 0.040 × 65 = 2.60 g. Percentage yield = (1.8/2.60) × 100 = 69.2%. However, the ore contains sand, so 5.0 g is not pure ZnCO3. The true theoretical yield is below 2.60 g, so the true percentage yield is higher than 69%. The calculation gives a minimum-comparison value, not the real purity-corrected yield.
3. Explain why the product is brittle and mixed with white powder, and describe two changes to improve the extraction.
Read the worked answer
The white powder is probably unreacted ZnO or ash from the carbon. Impurities and oxide particles break up the metal lattice, making the zinc brittle rather than a clean ingot. Some zinc may also have burned in air or escaped as vapour, since zinc boils near 907 °C. Improvements: grind the ore and mix it thoroughly with excess carbon; heat strongly in a covered crucible or tube with a cooler section to condense zinc vapour away from air. Avoid opening the tube while hot. Collect the zinc in a cool receiver, and test the product with dilute acid to see if unreacted carbonate or oxide remains.