Question

Lab procedure:

This laboratory exercise involves the use of dilute sulfuric acid (H₂SO₄) to leach copper from a synthetic copper-containing ore, malachite. This technique is known in the mining industry as solution mining. The soluble copper is then filtered to remove unreacted ore (the residue) from the filtrate containing the copper⁺² ion. The copper in the filtrate is then treated with metallic magnesium to precipitate the copper as the familiar orange-brown metal. This copper is dried and weighed. From these data the percent copper in the original ore can be determined. In this experiment you will use many of the concepts and techniques you have already learned in class including: molar mass calculations, separation by dissolution, oxidation-reduction, precipitation, equation writing and balancing, reaction types, activity series, percent composition, and percent error.

-Using an electronic balance weigh out approximately 2.5-3.0 grams of the green malachite ore. The ore may be dusty so avoid breathing in the dust. Place this ore in a 250-mL beaker and add very slowly 20 mL of 2M H₂SO₄. Since the malachite contains a metal carbonate, you can expect significant foaming and gas evolution. Using a glass stirring rod mix the ore with the acid. Dilute this copper-containing solution with 25 mL of deionized water.

- Decant this solution into a funnel equipped with a 12.5 cm filter paper circle. Place another 250 mL beaker beneath the funnel. Once most of the blue-green colored copper solution has passed through the filter, wash the residue (unreacted ore) in the beaker with 10-15 mL of deioned water. Add this washing to the filter paper in the funnel. You may want to test the residue in the beaker by adding 5 mL of 2M H₂SO₄ to see if any additional copper can be removed from the ore. If the acid solution in the beaker turns blue-green, more copper has been extracted and you will need to add this solution to the filter. Once the filter has completely drained, discard the filter paper.

-To the filtrate, add.0.40-0.50 grams of magnesium turnings. You will notice gas evolution and an immediate precipitation of copper metal. The colored solution will fade to a colorless solution. The magnesium will react with any unreacted sulfuric acid in the filtrate to liberate a gas. If a blue-green color persists, add an additional piece of magnesium turnings until the solution has been completely decolorized. If after the evolution of gas has ceased and there appears to be be any magnesium remaining, add 1-2 mL of 2M H₂SO₄ to completely remove residual magnesium.

-Transfer the precipitated copper to a piece of pre-weighed filter paper in a funnel using a stirring rod equipped with a rubber policeman. Wash the copper with 15-25 mL of deionized water and discard the filtrate. Place the copper on the filter paper on a watch glass and place it in an oven at 100 ºC.

-After drying the copper for 15-20 minutes, weigh the copper and filter paper and determine the mass of copper you have recovered.

Lab questions:

What gas was evolved when the ore was treated with the acid?

Did the copper appear the same before and after drying? If not, what can account for the color change? Write the equation for this color change. How will this affect the percent of the recovered copper?

Answer 1

Malachite, one of the common copper ores which is composed of copper carbonate, CuCO3.Cu(OH)2 mainly ,when sulfuric acid is added to malachite,it produces copper oxide along with the release of carbon dioxide.

CuCO₃.Cu(OH)₂ → 2 CuO + CO₂+ H₂O

and 2 CuO + C → 2 Cu + CO₂

Hence the gas was evolved when the ore was treated with the acid was CO_2.

No, the copper did not appeared the same before and after drying.If the sample appears to be black in color, then it means that the copper is too much heated and copper (II) oxide has been formed.

The reaction involved for the color change;

2 Cu + O₂ → 2 CuO

this will affect the percent of the recovered copper by increasing the mass of Cu of the final product. As we know

percent recovered =  (mass of final product / mass of initial sample ) 100

so if mass of final product is increased due to copper (II) oxide, the percent recovered will be more ,hence giving wrong result of the experiment.

hope this is helpful!