Question

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

Short answer

The spontaneous redox reactions that can cause corrosion between copper and steel pipes are: a) Copper corrodes while iron is oxidized to Fe(2+), with a standard emf value of 0.78V. b) Copper corrodes while iron is oxidized to Fe(3+) first, then reduced to Fe(2+), with a standard emf value of 1.11V. These reactions are spontaneous due to their positive standard emf values and can lead to corrosion. Using an insulating fitting prevents these reactions from occurring, avoiding corrosion.

Step-by-step solution

1. Identify possible redox reactions

In this step, we need to identify the oxidation states of copper and iron and determine the reactions that could occur. Copper can exist in the oxidation states of Cu (0) and Cu (2+), while iron can exist in Fe (0), Fe (2+), and Fe (3+) oxidation states. Possible redox reactions between these metals are: 1. Cu (0) -> Cu (2+) + 2e- (Oxidation of copper) 2. Fe (0) -> Fe (2+) + 2e- (Oxidation of iron) 3. Fe (2+) -> Fe (3+) + e- (Oxidation of iron)

2. Determine standard reduction potentials

Next, we will find the standard reduction potentials of these reactions using a standard reduction potential table. 1. Cu (2+) + 2e- -> Cu (0) E° = +0.34V (reduction potential of copper) 2. Fe (2+) + 2e- -> Fe (0) E° = -0.44V (reduction potential of iron) 3. Fe (3+) + e- -> Fe (2+) E° = +0.77V (reduction potential of iron) Now, we will have to reverse the reactions 2 and 3 to obtain their oxidation potentials. 2. Fe (0) -> Fe (2+) + 2e- E° = +0.44V (oxidation potential of iron) 3. Fe (2+) -> Fe (3+) + e- E ° = -0.77V (oxidation potential of iron)

3. Calculate standard emf values for possible reactions

The standard emfs of the reactions can be calculated by using the Nernst equation: Ecell = E°cathode - E°anode Possible redox reactions: a) Copper corrodes while iron is oxidized to Fe(2+) Ecell = E°Cu(2+)/Cu - E°Fe/Fe(2+) Ecell = 0.34V - (-0.44V) Ecell = 0.78V b) Copper corrodes while iron is oxidized to Fe(3+) first, then reduced to Fe(2+) Ecell = E°Cu(2+)/Cu - E°Fe(2+)/Fe(3+) Ecell = 0.34V - (-0.77V) Ecell = 1.11V

4. Justify the answer with standard emf calculations

Comparing the standard emf values calculated from step 3, both reactions a and b have positive emf values (0.78V and 1.11V) and are therefore spontaneous. However, reaction b has a higher standard emf value, which means it is more spontaneous. This analysis shows that connecting a copper pipe directly to a steel pipe can lead to corrosion due to the spontaneous redox reactions between the metals. The most spontaneous reaction involves the oxidation of copper to Cu(2+) and the oxidation of iron to Fe(3+), followed by reduction to Fe(2+). Using an insulating fitting helps to prevent these reactions from occurring, avoiding corrosion.