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Chapter 17: Problem 25

For a concentration cell, briefly explain why corrosion occurs at that region having the lower concentration.

Short Answer

Expert verified
Answer: In a concentration cell, corrosion occurs at the region with the lower concentration. This is because the region with the lower concentration loses electrons and becomes the anode, experiencing oxidation and a loss of mass, leading to its eventual corrosion.

Step by step solution

01

Concentration Cell and Corrosion

In a concentration cell, two electrodes, typically made of the same material, are exposed to different concentrations of an electrolyte solution. The difference in concentration creates a potential difference between the two electrodes, driving a flow of electrons from one electrode to the other. One of the electrodes becomes the anode, which experiences oxidation and releases electrons, while the other acts as the cathode, which experiences reduction and gains electrons.
02

Explanation of Corrosion at Lower Concentration Region

Corrosion occurs at the region with the lower concentration because the region with the higher concentration gains electrons, making it the cathode and preventing the corrosion. On the other hand, the region with the lower concentration loses electrons and becomes the anode.
03

Electrochemical Reactions in a Concentration Cell

Oxidation occurs at the anode, with metal ions entering the electrolytic solution and losing electrons. The process can be represented by the equation: \(M_{(s)} \rightarrow M^{n+}_{(aq)} + ne^{-}\) At the cathode, the reduction of the metal ions from the electrolyte solution occurs. They receive electrons from the electrode, depositing metal onto the electrode surface. This process can be represented by the equation: \(M^{n+}_{(aq)} + ne^{-} \rightarrow M_{(s)}\)
04

Lower Concentration Region as Anode and Greater Concentration Region as Cathode

As mentioned earlier, the higher concentration region becomes the cathode, where reduction of ions occurs. In contrast, the lower concentration region becomes the anode, where metal ions are oxidized and dissolve into the solution. This causes depletion of mass in the anode region and its eventual corrosion. In conclusion, corrosion occurs at the lower concentration region in a concentration cell because the loss of electrons and oxidation processes make it an anode, leading to a loss of mass and its eventual corrosion.

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Most popular questions from this chapter

(a) From the galvanic series (Table 17.2), cite three metals or alloys that may be used to galvanically protect 304 stainless steel in the active state. (b) As Concept Check 17.4(b) notes, galvanic corrosion is prevented by making an electrical contact between the two metals in the couple and a third metal that is anodic to the other two. Using the galvanic series, name one metal that could be used to protect a copper-aluminum galvanic couple.

(a) Compute the voltage at \(25^{\circ} \mathrm{C}\) of an electrochemical cell consisting of pure cadmium immersed in a \(2 \times 10^{-3} M\) solution of \(\mathrm{Cd}^{2+}\) ions, and pure iron in a \(0.4 M\) solution of \(\mathrm{Fe}^{2+}\) ions. (b) Write the spontaneous electrochemical reaction.

Briefly describe the phenomenon of passivity. Name two common types of alloy that passivate.

In the following table, weight gain-time data for the oxidation of some metal at an elevated temperature are given. \begin{tabular}{cc} \hline\(W\left(m g / c m^{2}\right)\) & Time (min) \\ \hline \(1.90\) & 25 \\ \(3.76\) & 75 \\ \(6.40\) & 250 \\ \hline \end{tabular} (a) Determine whether the oxidation kinetics obey a linear, parabolic, or logarithmic rate expression. (b) Now compute \(W\) after a time of 3500 min.

(a) Write the possible oxidation and reduction half-reactions that occur when magnesium is immersed in each of the following solutions: (i) HCl, (ii) an HCl solution containing dissolved oxygen, and (iii) an HCl solution containing dissolved oxygen and, in addition, \(\mathrm{Fe}^{2+}\) ions. (b) In which of these solutions would you expect the magnesium to oxidize most rapidly? Why?
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