Problem 84 - Chemistry: The Central Science

Question

(a) Based on standard reduction potentials, would you expect copper metal to oxidize under standard conditions in the presence of oxygen and hydrogen ions? (b) When the Statue of Liberty was refurbished, Teflon spacers were placed between the iron skeleton and the copper metal on the surface of the statue. What role do these spacers play?

Step-by-Step Solution

Answer

(a) Yes, we expect copper metal to oxidize under standard conditions in the presence of oxygen and hydrogen ions. The overall cell potential for the combination of copper oxidation and oxygen reduction is positive (\(E^{0}_{cell} = 0.89\,V\)), indicating that the oxidation of copper is favored in this case. (b) Teflon spacers placed between the iron skeleton and the copper metal on the Statue of Liberty prevent direct contact between the two metals, which eliminates the possibility of a galvanic cell. This helps prevent corrosion of the iron skeleton and preserves the structural integrity of the statue. Teflon is inert, electrically non-conductive, and does not promote corrosion, making it an ideal material for this purpose.

Step 1: Identifying the Half-Reactions

We are given the components: copper metal (Cu), oxygen (O2) and hydrogen ions (H+). We need to identify the possible half-reactions involving each component: 1. Copper: \[ Cu^{2+} + 2e^- \rightarrow Cu \] (Reduction) 2. Oxygen: \[ O_2 + 4H^+ + 4e^- \rightarrow 2H_2O \] (Reduction) 3. Hydrogen ions: \[ 2H^+ + 2e^- \rightarrow H_2 \] (Reduction) We need to consider if any of these reactions could occur as an oxidation, which means going in reverse direction.

Step 2: Find the standard reduction potentials

The standard reduction potentials are given for the following half-reactions (values from a standard reduction potential table): 1. Copper: \[ Cu^{2+} + 2e^- \rightarrow Cu : E^{0} = +0.34\,V \] 2. Oxygen: \[ O_2 + 4H^+ + 4e^- \rightarrow 2H_2O : E^{0} = +1.23\,V \] 3. Hydrogen ions: \[ 2H^+ + 2e^- \rightarrow H_2 : E^{0} = 0\,V \]

Step 3: Determine if oxidation of copper is favored

In order for the oxidation of copper to occur, the reverse reaction for copper should be coupled to one of the other two reactions to give a positive overall cell potential. Let's try to combine copper oxidation with oxygen reduction: 1. Copper oxidation: \[ Cu \rightarrow Cu^{2+} + 2e^- : E^{0} = -0.34\,V \] (Oxidation) 2. Oxygen reduction: \[ O_2 + 4H^+ + 4e^- \rightarrow 2H_2O : E^{0} = +1.23\,V \] The overall cell potential for this combination would be: \[ E^{0}_{cell} = E^{0}_{Oxygen} + E^{0}_{Copper,oxidation} = -0.34\,V + 1.23\,V = 0.89\,V \] Since the cell potential is positive, the oxidation of copper is favored in the presence of oxygen and hydrogen ions under standard conditions. #b) Role of Teflon Spacers#

Step 1: Identify the possible galvanic cell

If the iron skeleton and copper metal surface are in direct contact without the Teflon spacers, a galvanic cell can be created, with iron as anode and copper as cathode. A galvanic cell occurs when two different metals are in contact with each other and an electrolyte.

Step 2: Understand the corrosive effect of galvanic cells

In a galvanic cell, the anode (iron in this case) will undergo corrosion as it loses electrons, leading to deterioration of the metal structure. The corrosion of iron can be detrimental to the structural integrity of the statue.

Step 3: Explain the role of Teflon spacers

By placing Teflon spacers between the iron and copper, direct contact between the two metals is prevented, thus eliminating the possibility of a galvanic cell. This prevents corrosion of the iron skeleton and helps to preserve the structural integrity of the Statue of Liberty. Teflon is inert, electrically non-conductive and does not promote corrosion, making it an ideal material for this purpose.