[Paulo1913]

please help me as I can't understand how to figure out this question...

(a)   Many elements exhibit variable oxidation numbers. The oxidation numbers of copper are 0, +1 and +2.

   The element copper is present in many metallic minerals. These minerals include chalcopyrite (CuFeS2), malachite (Cu2(CO3)(OH)2), and copper metal (Cu).

   (i)   State the oxidation number of copper in copper metal (Cu)   

   (ii)   Determine the oxidation number of copper in malachite, Cu2(CO3)(OH)2. Include your working to show how the oxidation number was determined.

   (iii)   Sulfur has an oxidation number of –2 in chalcopyrite, CuFeS2.

      Discuss the possible oxidation numbers of both copper and iron in chalcopyrite, CuFeS2.

(b)   Copper ores that contained the compound copper(II) sulfide, CuS, were first mined by ancient miners. First they roasted the CuS in air as shown by Equation A. Then the product was reacted with carbon as shown by Equation B.

   Roasting in air:   2CuS    +    3O2    →   2CuO    +    2SO2   Equation A

   Reacting the product with carbon:    CuO    +    C   →   Cu    +    CO   Equation B

   Both these reactions are examples of redox equations.

   (i)   In Equation A circle the element which is oxidised.

   (ii)   Justify your answer.

   (iii)   In terms of oxidation number, explain why Equation B is a redox reaction.

[nexisrocks]

For parts (ii) and (iii) in (a), consider the charges on the non-copper parts of the solution (i.e. the charges on CO₃ and OH in (ii)).  You know that the compound is neutral, so the total positive charges of elements and ions in the compound must equal the total negative charges of elements and ions in the compound.

For (iii), also consider the possible oxidation states of Fe.

For (b), remember that oxidation is the loss of electrons, leading to a change in oxidation number.  Also think about the definition of a redox reaction, and apply that to oxidation numbers for (iii).

[AWK]

(i)   State the oxidation number of copper in copper metal (Cu)

Check definition of oxidation number for elements