Question

An important source of copper is from the copper ore, chalcocite, a form of copper(I) sulfide. When heated, the Cu2S decomposes to form copper and sulfur described by the following equation:

Cu₂S(s)⟶Cu(s)+S(s)

(a) Determine AG₂₉₈^◦ for the decomposition of Cu₂S(s).

(b) The reaction of sulfur with oxygen yields sulfur dioxide as the only product. Write an equation that describes this reaction, and determine ΔG₂₉₈^◦ for the process.

c) The production of copper from chalcocite is performed by roasting the Cu₂S in air to produce the Cu. By combining the equations from parts (a) and (b), write the equation that describes the roasting of the chalcocite, and explaine why coupling these reactions together makes for a more efficient process for the production of the copper.

Answer 1

The decomposition of Cu₂S(s) is favorable and the ΔG° value would be negative. The reaction of sulfur with oxygen to form sulfur dioxide can be spontaneous if ΔG° is negative. Coupling the reactions together in the roasting process makes the production of copper from chalcocite more efficient.

Step-by-step explanation:

(a) The standard Gibbs free energy change (ΔG°) for a reaction can be determined using the formula: ΔG° = ΔH° - TΔS°, where ΔH° is the change in enthalpy and ΔS° is the change in entropy at standard conditions. In the given decomposition reaction, Cu₂S(s) ⟶ Cu(s) + S(s), the formation of Cu(s) and S(s) leads to an increase in entropy. Since the reaction is exothermic, it releases energy, resulting in a negative ΔH° value. Therefore, the decomposition of Cu₂S(s) is favorable and the ΔG° value would be negative.

(b) The reaction of sulfur with oxygen to form sulfur dioxide can be represented by the equation: S(s) + O₂(g) ⟶ SO₂(g). To determine ΔG° for this process, the standard Gibbs free energy change can be calculated using the same formula as in part (a). The spontaneity of this reaction can be predicted by comparing the ΔG° value to zero. If ΔG° is negative, the reaction is spontaneous.

(c) The equation that describes the roasting of chalcocite, Cu₂S, in air which combines the reactions from parts (a) and (b) is: Cu₂S(s) + O₂(g) ⟶ Cu(s) + S(s) + SO₂(g). By coupling these reactions together, the production of copper from chalcocite becomes more efficient because the heat given off by the exothermic reaction in part (a) can be used to drive the reaction in part (b) without the need for additional external energy.