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General Chemistry Principles, Patterns, and Applications, 2011

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when Cr(II) is oxidized to Cr(III) <strong>and</strong> Fe is oxidized to Fe(II). Explain how this film prevents the corrosion of Fe<br />

to rust, which has the formula Fe 2 O 3 .<br />

8. ♦ Ion-selective electrodes are powerful tools for measuring specific concentrations of ions in solution. For<br />

example, they are used to measure iodide in milk, copper-ion levels in drinking water, fluoride concentrations<br />

in toothpastes, <strong>and</strong> the silver-ion concentration in photographic emulsions <strong>and</strong> spent fixing solutions.<br />

Describe how ion-selective electrodes work <strong>and</strong> then propose a design for an ion-selective electrode that can<br />

be used for measuring water hardness (Ca 2+ , Mg 2+ ) in water-conditioning systems.<br />

9. ♦ Enzymes are proteins that catalyze a specific reaction with a high degree of specificity. An example is the<br />

hydrolysis of urea by urease:<br />

NH2CONH2 + 2H2O+ H + - ® - -urease2NH 4 + +HCO3-<br />

An enzyme electrode for measuring urea concentrations can be made by coating the surface of a glass electrode<br />

with a gel that contains urease.<br />

a. Explain what occurs when the electrode is placed in contact with a solution that contains<br />

urea.<br />

b. What species diffuses through the gel?<br />

c. What would be an effective reference electrode?<br />

10. Gas-sensing electrodes can be constructed using a combination electrode that is surrounded by a gaspermeable<br />

membrane. For example, to measure CO 2 , a pH electrode <strong>and</strong> a reference electrode are placed in<br />

solution on the “inner” side of a CO 2 -permeable membrane, <strong>and</strong> the sample solution is placed on the<br />

“external” side. As CO 2 diffuses through the membrane, the pH of the internal solution changes due to the<br />

reaction CO 2 (g) + H 2 O(l) → HCO − 3 (aq) + H + (aq). Thus the pH of the internal solution varies directly with the<br />

CO 2 pressure in the external sample. Ammonia electrodes operate in the same manner. Describe an<br />

electrode that would test for ammonia levels in seawater.<br />

11. US submarines that are not nuclear powered use a combination of batteries <strong>and</strong> diesel engines for their<br />

power. When submerged, they are battery driven; when on the surface, they are diesel driven. Why are<br />

batteries not used when submarines are on the surface?<br />

12. List some practical considerations in designing a battery to power an electric car.<br />

13. ♦ It is possible to run a digital clock using the power supplied by two potatoes. The clock is connected to two<br />

wires: one is attached to a copper plate, <strong>and</strong> the other is attached to a zinc plate. Each plate is pushed into a<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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