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, copper metal would oxidize under standard conditions in the presence of oxygen and hydrogen ions, based on the standard reduction potentials. Specifically, copper will oxidize in the presence of oxygen more so than in the presence of hydrogen ions, as the reaction process has a positive cell potential of 0.89V. (b) Teflon spacers play an essential role in preventing galvanic corrosion between the iron skeleton and the copper surface of the Statue of Liberty. By placing Teflon spacers between the iron and copper, an insulating barrier is created to prevent electrical contact. This barrier protects the iron skeleton from corroding in the presence of the more noble copper metal.

Step 1: (Step 1: Obtain the standard reduction potentials)

(Look up the standard reduction potentials for oxygen, hydrogen, and copper. You can find these in a reference material or online.) For oxygen and hydrogen, the reactions are as follows: \(O_2(g) + 4H^+ + 4e^- \rightarrow 2H_2O(l)\) \(E^{\circ} = 1.23V\) \(2H^+ + 2e^- \rightarrow H_2(g)\) \(E^{\circ} = 0V\) For copper: \(Cu^{2+} + 2e^- \rightarrow Cu(s)\) \(E^{\circ} = 0.34V\) We will use these reduction potentials to determine the feasibility of reactions involving copper and the given substances.

Step 2: (Step 2: Analyze the reaction between copper, oxygen, and hydrogen ions)

(Based on the reduction potentials, we can now analyze if copper metal would oxidize in the presence of oxygen and hydrogen ions.) If copper metal were to oxidize, the reaction should be: \(Cu(s) \rightarrow Cu^{2+} + 2e^-\) To find the feasibility of this reaction, we need to pair it with another half-reaction involving either oxygen or hydrogen ions. The most favorable pair would be the one with the highest positive value for the overall cell potential (calculated as the difference between the reduction potentials of the two half-reactions). When pairing with oxygen reduction: \(E^{\circ}_{cell} = E^{\circ}_{(O_2/H_2O)} - E^{\circ}_{(Cu^{2+}/Cu)} = 1.23 - 0.34 = 0.89V\) When pairing with hydrogen reduction: \(E^{\circ}_{cell} = E^{\circ}_{(H^+/H_2)} - E^{\circ}_{(Cu^{2+}/Cu)} = 0 - 0.34 = -0.34V\) As per the first pair, the cell's potential is positive (0.89 V), which means the reaction involving copper oxidation and oxygen reduction is feasible.

Step 3: (Part a: Answer for Part a)

(Now we can answer Part a of the exercise.) Yes, copper metal would oxidize under standard conditions in the presence of oxygen and hydrogen ions, based on the standard reduction potentials. Specifically, copper will oxidize in the presence of oxygen more so than in the presence of hydrogen ions, as the reaction process has a positive cell potential of 0.89V.

Step 4: (Step 3: Analyze the role of Teflon spacers in the refurbishment of the Statue of Liberty)

(To answer this question, we need to consider the electrochemical properties of iron and copper.) When two different metals come into contact with each other, galvanic corrosion can occur. Galvanic corrosion is an electrochemical process where one metal corrodes more readily when in electrical contact with a more noble metal. In the case of the Statue of Liberty, the iron skeleton (less noble metal) could corrode in the presence of copper (more noble metal) and the surrounding humid and saline environment.

Step 5: (Part b: Answer for Part b)

(Now, let's answer Part b of the exercise referring to the role of Teflon spacers.) Teflon spacers play an essential role in preventing galvanic corrosion between the iron skeleton and the copper surface of the Statue of Liberty. By placing Teflon spacers between the iron and copper, an insulating barrier is created to prevent electrical contact. This barrier protects the iron skeleton from corroding in the presence of the more noble copper metal.

Key Concepts

Standard Reduction Potentials

Standard reduction potentials refer to the tendency of a chemical species to gain electrons and be reduced under standard conditions, which are typically at 25°C, 1 M concentration for solutions, and 1 atm pressure for gases. The standard reduction potential (\(E^{ ect} \)) is measured in volts (V) and compared to the standard hydrogen electrode which is assigned a potential of 0 V.
Reduction potentials indicate how likely a substance is to be reduced. The more positive the potential, the more likely it is to gain electrons and undergo reduction. For example:

• The reduction potential for \(O_2(g) + 4H^+ + 4e^- \rightarrow 2H_2O(l)\) is 1.23 V, indicating a strong tendency towards reduction.
• For \(Cu^{2+} + 2e^- \rightarrow Cu(s)\), the reduction potential is 0.34 V, which is less than that of oxygen but still positive.

These values are pivotal for predicting the direction of redox reactions. Thus, by comparing these potentials, chemists can determine which species will be reduced and which will be oxidized in a reaction.

Galvanic Corrosion

Galvanic corrosion occurs when two different metals are electrically connected in a corrosive environment, leading to accelerated corrosion of the more reactive (less noble) metal. This process happens because when metals with differing standard reduction potentials make electrical contact, electrons tend to flow from the metal that is less noble to the more noble metal, causing the former to corrode more rapidly.
In practical terms:

• For the Statue of Liberty, the iron skeleton, being less noble than copper, would corrode when in electrical contact with copper.
• To prevent this, Teflon spacers were used during refurbishments. These spacers act as insulating barriers that prevent electrical contact between iron and copper.

By doing so, the Teflon spacers effectively protect the iron from galvanic corrosion, thereby extending the life of the statue's structural integrity.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between two chemical species. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons.
In a redox reaction:

• The substance that loses electrons (undergoes oxidation) is said to be the reducing agent.
• The substance that gains electrons (undergoes reduction) is known as the oxidizing agent.

In the case of the copper oxidation with oxygen, the copper is oxidized:\[Cu(s) \rightarrow Cu^{2+} + 2e^-\]And simultaneously, oxygen is reduced:\[O_2(g) + 4H^+ + 4e^- \rightarrow 2H_2O(l)\]To sum up, understanding these reactions and the standard reduction potentials helps predict whether these reactions will proceed under given conditions, such as in our air where copper can oxidize in the presence of oxygen and hydrogen ions.