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 reaction that might cause corrosion when a copper pipe is connected directly to a steel pipe involves the reduction of copper ions and the oxidation of iron. The standard emf for this reaction, $E_{cell}^o$, is calculated as $E_{cell}^o=E_{Cu}^o-E_{Fe}^o=(+0.34V)-(-0.44V)=+0.78V$. Since the standard emf is positive, the reaction is spontaneous and will cause corrosion. To prevent this, an insulating fitting should be used to connect the copper and steel pipes.

Step-by-step solution

Identify the half-reactions for both copper and steel

First, we need to identify the half-reactions for both copper and iron (since steel is mostly composed of iron). The relevant half-reactions for copper and iron are: For Copper (Cu): $$C{u}^{2+}(aq)+2e^{-}\to Cu(s)$$$$E_{Cu}^o=+0.34V$$ For Iron (Fe): $$F{e}^{2+}(aq)+2e^{-}\to Fe(s)$$$$E_{Fe}^o=-0.44V$$ Now, let's determine if any combinations of these half-reactions result in a spontaneous redox reaction.

Combine the half-reactions and calculate the standard emf

To calculate the standard emf value for a redox reaction, we will use the formula: $$E_{cell}^o=E_{cathode}^o-E_{anode}^o$$ When combining the half-reactions, the species with the higher standard reduction potential (more positive) will act as the cathode, and the species with the lower standard reduction potential (less positive or more negative) will act as the anode. In this case, copper has a higher standard reduction potential than iron, so it will act as the cathode and iron will act as the anode. Hence, the redox reaction involves the reduction of copper ions and the oxidation of iron: Anode (oxidation): $$Fe(s)\to F{e}^{2+}(aq)+2e^{-}$$ Cathode (reduction): $$C{u}^{2+}(aq)+2e^{-}\to Cu(s)$$ Now, let's calculate the standard emf value for this reaction using the given standard reduction potentials: $$E_{cell}^o=E_{Cu}^o-E_{Fe}^o$$ $$E_{cell}^o=(+0.34V)-(-0.44V)$$ $$E_{cell}^o=+0.78V$$

Determine if the reaction is spontaneous

A positive value for the standard emf indicates that the redox reaction is spontaneous. Our calculated standard emf for the reaction between copper and iron is +0.78V, which means that the reaction between copper and iron is spontaneous. Therefore, when a copper pipe is connected directly to a steel pipe, electrochemical reactions between the two metals will cause corrosion due to this redox reaction. In conclusion, the plumber's handbook is correct in recommending the use of an insulating fitting to connect copper and steel pipes to avoid corrosion caused by this spontaneous redox reaction between copper and iron.

Key concepts

Redox Reactions

Redox reactions, or reduction-oxidation reactions, are a type of chemical reaction that involve the transfer of electrons between two substances. In these reactions, one substance loses electrons (oxidation), while the other gains electrons (reduction).
Redox reactions are fundamental in explaining many chemical processes, including those involved in corrosion. By understanding the movement of electrons, we can predict how different substances will react, especially metals like copper and steel.
In the context of copper and steel, when they are in contact, a redox reaction can occur due to their difference in electrical potentials. This is because copper and iron, a component of steel, have different affinities for electrons, leading to a transfer of electrons when in contact.

Standard EMF Calculations

Understanding standard emf (electromotive force) calculations is key to predicting whether a redox reaction will occur spontaneously. The standard emf of a cell is calculated using the formula:

• $E_{cell}^o=E_{cathode}^o-E_{anode}^o$

In this formula, $E^o$ represents the standard reduction potential of the cathode and anode.
To determine the cell's emf, identify the half-reactions for each metal involved. For example, copper and iron have the following half-reactions:

• Copper: $C{u}^{2+}(aq)+2e^{-}\to Cu(s)$, with $E^o=+0.34V$
• Iron: $F{e}^{2+}(aq)+2e^{-}\to Fe(s)$, with $E^o=-0.44V$

When these potentials are applied, the positive value of $E_{cell}^o=+0.78V$ indicates a spontaneous reaction, driving the corrosion when copper is in contact with steel.

Corrosion Prevention

Corrosion is a natural phenomenon that occurs when metals oxidize in the presence of moisture and other chemical conditions. Preventing corrosion is crucial to extend the life of metal structures and components. Metals like steel are particularly susceptible due to their active redox potentials when coupled with other metals, like copper.
To mitigate corrosion, an insulating fitting is recommended when connecting pipes of different metals. This fitting acts as a barrier, preventing the direct transfer of electrons between the metals.
This isolation is an effective strategy to reduce the risk of corrosion by preventing electrochemical reactions, such as those discussed in standard emf calculations. It's an essential practice in plumbing and construction to ensure structural integrity and longevity.

Copper and Steel Interaction

Copper and steel can engage in electrochemical interactions due to the differing properties and behaviors of their respective electrons.
When copper and steel, primarily composed of iron, come into contact, they create a galvanic cell. In this cell, copper acts as the cathode with a higher reduction potential than iron, which acts as the anode.
This interaction leads to the oxidation of iron, catalyzing rust, while copper remains relatively protected because it undergoes reduction. The differential in electron potential makes steel the metal that corrodes in this setup.
Practical applications always consider these interactions to minimize damage. In fields like plumbing, careful material selection and protective measures are necessary to avoid the detrimental effects of corrosion, ensuring systems remain operational and safe.