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 are between copper (from brass) with zinc (from galvanized steel) and copper (from brass) with iron (from galvanized steel). The standard EMFs for these reactions are +1.10 V and +0.78 V, respectively, indicating that they are spontaneous and can lead to corrosion. Therefore, it is essential to use an insulating fitting to connect brass and galvanized steel pipes to prevent corrosion due to these electrochemical reactions.

Step-by-step solution

Identify potential redox reactions

Since brass contains copper and zinc, and galvanized steel contains iron coated in zinc, we can look into the possible redox reactions between copper (Cu), zinc (Zn), and iron (Fe).

Refer to the standard reduction potentials table

Using the standard reduction potentials table, we can find the reduction half-reactions and standard reduction potentials for the three metals: 1. Cu^2+ + 2e^- -> Cu ; E° = +0.34 V 2. Zn^2+ + 2e^- -> Zn ; E° = -0.76 V 3. Fe^2+ + 2e^- -> Fe ; E° = -0.44 V

Calculate standard EMF for each possible redox reaction

To determine which reaction is spontaneous, we will calculate the standard EMF (E°) for each potential half-cell reaction pairing: - Cu with Zn - Cu with Fe - Zn with Fe The standard EMF (E°) can be calculated using the formula: E° = E°(cathode) - E°(anode).

Calculate the standard EMF for Cu with Zn

Let's consider Cu as the cathode and Zn as the anode: E°(Cu-Zn) = E°(Cu) - E°(Zn) = +0.34 V - (-0.76 V) = +1.10 V Since the standard EMF is positive, the reaction is spontaneous.

Calculate the standard EMF for Cu with Fe

Now, let's consider Cu as the cathode and Fe as the anode: E°(Cu-Fe) = E°(Cu) - E°(Fe) = +0.34 V - (-0.44 V) = +0.78 V Since the standard EMF is positive, the reaction is also spontaneous.

Calculate the standard EMF for Zn with Fe

Lastly, let's consider Zn as the cathode and Fe as the anode: E°(Zn-Fe) = E°(Zn) - E°(Fe) = -0.76 V - (-0.44 V) = -0.32 V Since the standard EMF is negative, the reaction is non-spontaneous.

Conclude the possible redox reactions causing corrosion

Based on our calculations, the spontaneous redox reactions that might cause corrosion are: 1. Copper (from brass) with zinc (from galvanized steel) 2. Copper (from brass) with iron (from galvanized steel) The non-spontaneous reaction is between zinc and iron. These results justify the plumber's handbook recommendation to use an insulating fitting to connect brass and galvanized steel pipes, as the spontaneous redox reactions could lead to corrosion.

Key concepts

Standard Reduction Potentials

To understand why certain metals corrode when in contact, we need to dive into the concept of standard reduction potentials. These values, often symbolized as E°, represent the inherent tendency of a substance to gain electrons and reduce. You can picture it as a measure of the 'hunger' a substance has for electrons. In a tabulated form, they allow us to predict the direction of redox reactions under standard conditions, which typically means solutions at 1 M concentration, a pressure of 1 atm, and a temperature of 25°C (298 K).
These potentials are key in calculating whether one metal will corrode when in contact with another. If you connect two different metals, the one with the lower (more negative) standard reduction potential will tend to lose electrons (oxidize) and corrode, acting as an anode. It's akin to a sacrificial process where the 'weakest' metal in terms of electron affinity gives itself up. Understanding these values is crucial in fields like construction and plumbing to prevent unintentional damage and extend the life of metal structures.

Spontaneous Redox Reactions

In the realm of chemistry, a spontaneous redox reaction is one that naturally occurs without external energy input. Think of it like a waterfall - once conditions are right, the water flows without needing to be pushed. When metals come into contact, such as copper and iron, a redox reaction may spontaneously take place based on their reduction potentials, causing one metal to corrode over time.
The spontaneity of redox reactions is determined by the standard electromotive force (EMF), calculated from the standard reduction potentials. If the standard EMF is positive, electrons flow naturally from the anode to the cathode, and the reaction will spontaneously take place. Conversely, a negative EMF indicates that the reverse reaction is spontaneous under standard conditions. An easy way to remember this is: positive EMF equals natural occurrence; negative EMF means it won’t happen on its own.

Galvanized Steel

Galvanized steel is a superhero in the construction and plumbing industries due to its enhanced resistance to corrosion. Galvanized doesn’t mean a special type of steel but refers to a protective zinc coating applied to ordinary steel. Zinc serves as a sacrificial layer. Its job is to take the hit instead of the steel. This relates to the 'galvanic' part of our story.
When you have different metals in contact, such as in a water pipe system with galvanized steel and brass, the less noble metal (zinc in this case) will corrode preferentially to protect the underlying steel. This process is exploited in galvanization to extend the lifespan of steel structures. However, if the insulating layer is breached, or if the zinc is entirely consumed over time, the underlayer, steel, could start to corrode. This is where understanding galvanic relationships and using precautionary measures, like insulating fittings between dissimilar metals, becomes essential.

Electrochemical Series

The electrochemical series is a grand ranking of various elements based on their standard reduction potentials. Picture it as a leaderboard where chemicals compete based on how eagerly they acquire electrons. This series is incredibly useful for predicting the outcomes of galvanic reactions, such as corrosion in mixed-metal plumbing.
In galvanic corrosion, metals high on the series (with more positive or less negative standard reduction potentials) act as cathodes, while those lower down (with more negative potentials) become anodes. The electrochemical series tells us that copper will not corrode when in contact with iron or zinc because it sits higher in the series - it’s the stronger electron-wanter. By referring to this series, plumbers and engineers can predict and prevent potential corrosion issues by keeping incompatible metals apart or by using safeguards like insulators to protect integrity where metals meet.