Question

A plumber's handbook states that you should not connect a brass pipe directly to a galvanized steel pipe because electrochemical reactions between the two metals will cause corrosion. The handbook recommends you use instead an insulating fitting to connect them. Brass is a mixture of copper and zinc. What spontaneous redox reaction(s) might cause the corrosion? Justify your answer with standard emf calculations.

Short answer

The spontaneous redox reaction that might cause corrosion is between copper (present in brass) and zinc, with a standard emf of +1.10 V \( (Cu^{2+}(aq) + Zn(s) \rightarrow Cu(s) + Zn^{2+}(aq)) \). This electrochemical reaction can occur when connecting a brass pipe directly to a galvanized steel pipe. Using an insulating fitting prevents direct contact between the metals, avoiding the corrosion process.

Step-by-step solution

Identify the half-reactions

First, we need to identify the possible half-reactions for the metals involved: copper, zinc, and iron (the main component of steel). We will consider the following half-reactions: 1. Copper (Cu): \( Cu^{2+}(aq) + 2e^- ⟶ Cu(s) \) 2. Zinc (Zn): \( Zn^{2+}(aq) + 2e^- ⟶ Zn(s) \) 3. Iron (Fe): \( Fe^{2+}(aq) + 2e^- ⟶ Fe(s) \)

Determine the standard reduction potentials

Next, we should look up the standard reduction potentials (Eº) for each half-reaction. These values can be found in a standard reduction potentials table. We have: 1. Copper (Cu): \( E^\circ_{Cu} = +0.34 \, V \) 2. Zinc (Zn): \( E^\circ_{Zn} = -0.76 \, V \) 3. Iron (Fe): \( E^\circ_{Fe} = -0.44 \, V \)

Determine the possible redox reactions

Now, we will examine the possible combinations of half-reactions between these metals, considering the two half-reactions with the higher standard reduction potentials to be reduction reactions and the one with the lower standard reduction potential to be the oxidation reaction: 1. Copper and Zinc: Cu reduction and Zn oxidation 2. Copper and Iron: Cu reduction and Fe oxidation 3. Zinc and Iron: Zn reduction and Fe oxidation We can now calculate the standard emf for each possible reaction to determine which one is spontaneous.

Calculate the standard emf for each reaction

The standard emf is given by the difference in standard reduction potentials, represented by: \( E^\circ_{cell} = E^\circ_{reduction} - E^\circ_{oxidation} \) For each possible reaction: 1. Copper and Zinc: \( E^\circ_{cell} = E_{Cu}^\circ - E_{Zn}^\circ = (+0.34 \, V) - (-0.76 \, V) = +1.10 \, V \) 2. Copper and Iron: \( E^\circ_{cell} = E_{Cu}^\circ - E_{Fe}^\circ = (+0.34 \, V) - (-0.44 \, V) = +0.78 \, V \) 3. Zinc and Iron: \( E^\circ_{cell} = E_{Zn}^\circ - E_{Fe}^\circ = (-0.76 \, V) - (-0.44 \, V) = -0.32 \, V \)

Determine the spontaneous redox reaction

From the calculated standard emf values, we can see that the reactions with positive values are spontaneous: 1. Copper and Zinc: \( E^\circ_{cell} = +1.10 \, V \) 2. Copper and Iron: \( E^\circ_{cell} = +0.78 \, V \) The reaction with the highest standard emf is the most spontaneous, in this case, the reaction between copper and zinc: \( Cu^{2+}(aq) + Zn(s) \rightarrow Cu(s) + Zn^{2+}(aq) \) So, the spontaneous redox reaction that might cause corrosion is between copper (present in brass) and zinc. This is why it's not recommended to connect a brass pipe directly to a galvanized steel pipe. Using an insulating fitting prevents direct contact between the two metals, thus avoiding the corrosion process.