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 reactions that might cause corrosion when a brass pipe (mixture of copper and zinc) is connected to a galvanized steel pipe are: 1. Cu²⁺|Cu (cathode) and Zn²⁺|Zn (anode), with E°(cell) = 1.10 V 2. Cu²⁺|Cu (cathode) and Fe²⁺|Fe (anode), with E°(cell) = 0.78 V These reactions are spontaneous based on the positive standard emf values, which justifies the recommendation in the plumber's handbook to use an insulating fitting to connect brass to galvanized steel pipes.

Step-by-step solution

Write possible half-reactions for each metal

First, we should write down the possible reduction half-reactions for each metal involved: copper (Cu), zinc (Zn), and iron (Fe, as galvanized steel is mostly composed of iron and coated with zinc). Reduction half-reactions: Cu²⁺ + 2e⁻ → Cu Zn²⁺ + 2e⁻ → Zn Fe²⁺ + 2e⁻ → Fe

Find standard reduction potentials

Next, we need to find the standard reduction potentials for each half-reaction. Standard reduction potentials can be found in standard reference tables. Standard reduction potentials (E°): Cu²⁺|Cu: +0.34 V Zn²⁺|Zn: -0.76 V Fe²⁺|Fe: -0.44 V

Calculate emf for each possible redox reaction

Now, for each possible redox reaction, we need to calculate the standard emf. Subtract each anode's standard reduction potential from the reduction potential of each cathode. 1. Cu²⁺|Cu (cathode) and Zn²⁺|Zn (anode) E°(cell) = E°(cathode) - E°(anode) = 0.34 - (-0.76) = 1.10 V 2. Cu²⁺|Cu (cathode) and Fe²⁺|Fe (anode) E°(cell) = E°(cathode) - E°(anode) = 0.34 - (-0.44) = 0.78 V 3. Zn²⁺|Zn (cathode) and Fe²⁺|Fe (anode) E°(cell) = E°(cathode) - E°(anode) = -0.76 - (-0.44) = -0.32 V

Identify spontaneous redox reactions

A positive emf indicates that the reaction is spontaneous. Based on our calculations, the spontaneous redox reactions are: 1. Cu²⁺|Cu (cathode) and Zn²⁺|Zn (anode), with E°(cell) = 1.10 V 2. Cu²⁺|Cu (cathode) and Fe²⁺|Fe (anode), with E°(cell) = 0.78 V The reaction between Zn²⁺|Zn (cathode) and Fe²⁺|Fe (anode) is not spontaneous, with E°(cell) = -0.32 V.

Conclusion

Spontaneous redox reactions that might cause corrosion when a brass pipe (mixture of copper and zinc) is connected to a galvanized steel pipe are: 1. Cu²⁺|Cu (cathode) and Zn²⁺|Zn (anode), with E°(cell) = 1.10 V 2. Cu²⁺|Cu (cathode) and Fe²⁺|Fe (anode), with E°(cell) = 0.78 V These reactions are spontaneous based on the positive standard emf values. This provides the justification for not connecting brass pipes directly to galvanized steel pipes.