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Chapter 19: Problem 42

How does the tendency of iron to rust depend on the \(\mathrm{pH}\) of solution?

Short Answer

Expert verified
The tendency of iron to rust increases in acidic conditions (lower pH) and decreases in basic or neutral conditions (higher pH). This is because acidic conditions facilitate the removal of electrons from iron (oxidation), promoting rust formation, whereas basic conditions slow down the rust formation by creating a protective layer on iron's surface and the low concentration of hydrogen ions slows down the oxidation process.

Step by step solution

01

Understanding pH

pH is a measure of how acidic or basic water is. It ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. Acidic solutions have high concentration of hydrogen ions (H+), while basic solutions have high concentration of hydroxide ions (OH鈭�).
02

Effect of pH on rusting process

When iron reacts with oxygen and water, it forms hydrated iron(III) oxide, commonly known as rust. This reaction (corrosion process) be expressed as: \(4Fe(s) + 3O2(g) + 6H_2O(l) \rightarrow 4Fe(OH)_3(s)\), where \(Fe(s)\) is iron, \(O2(g)\) is oxygen, \(H2O(l)\) is water, and \(Fe(OH)_3(s)\) is hydrated iron(III) oxide or rust. This process is faster in acidic conditions (lower pH) because in acidic conditions, the concentration of hydrogen ions (which are positively charged) is increased and these ions promote the removal of electrons from iron (oxidation process), hence helping to form rust.
03

pH Levels and Corrosion Rate

Research studies have found out that the corrosion rate of iron is slower in neutral or basic conditions (higher pH). There are two reasons for this. Firstly, at higher pH, the concentration of hydrogen ions is lower, so the removal of electrons from iron is slower. Secondly, basic conditions often produce a protective layer on the iron's surface that inhibit further corrosion.

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Most popular questions from this chapter

A galvanic cell is constructed as follows. One halfcell consists of a platinum wire immersed in a solution containing \(1.0 M \mathrm{Sn}^{2+}\) and \(1.0 M \mathrm{Sn}^{4+} ;\) the other half-cell has a thallium rod immersed in a solution of \(1.0 M \mathrm{TI}^{+}\). (a) Write the half-cell reactions and the overall reaction. (b) What is the equilibrium constant at \(25^{\circ} \mathrm{C} ?\) (c) What is the cell voltage if the \(\mathrm{TI}^{+}\) concentration is increased tenfold? \(\left(E_{\mathrm{T} 1^{+} / \mathrm{T} 1}^{\circ}=-0.34 \mathrm{~V} .\right)\)

In a certain electrolysis experiment, \(1.44 \mathrm{~g}\) of \(\mathrm{Ag}\) were deposited in one cell (containing an aqueous \(\mathrm{AgNO}_{3}\) solution), while \(0.120 \mathrm{~g}\) of an unknown metal X was deposited in another cell (containing an aqueous \(\mathrm{XCl}_{3}\) solution) in series with the \(\mathrm{AgNO}_{3}\) cell. Calculate the molar mass of \(\mathrm{X}\).

The zinc-air battery shows much promise for electric cars because it is lightweight and rechargeable: The net transformation is \(\mathrm{Zn}(s)+\frac{1}{2} \mathrm{O}_{2}(g) \rightarrow \mathrm{ZnO}(s)\) (a) Write the half-reactions at the zinc-air electrodes and calculate the standard emf of the battery at \(25^{\circ} \mathrm{C}\). (b) Calculate the emf under actual operating conditions when the partial pressure of oxygen is 0.21 atm. (c) What is the energy density (measured as the energy in kilojoules that can be obtained from \(1 \mathrm{~kg}\) of the metal) of the zinc electrode? (d) If a current of \(2.1 \times 10^{5} \mathrm{~A}\) is to be drawn from a zinc-air battery system, what volume of air (in liters) would need to be supplied to the battery every second? Assume that the temperature is \(25^{\circ} \mathrm{C}\) and the partial pressure of oxygen is \(0.21 \mathrm{~atm} .\)

"Galvanized iron" is steel sheet that has been coated with zinc; "tin" cans are made of steel sheet coated with tin. Discuss the functions of these coatings and the electrochemistry of the corrosion reactions that occur if an electrolyte contacts the scratched surface of a galvanized iron sheet or a tin can.

Describe the basic features of a galvanic cell. Why are the two components of the cell separated from each other?
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