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Grains contain gold in forms that seem tailor-made for industrial use. Rumpelstiltskin, the fairy-tale rogue who spun straw into gold, has nothing on Miguel Yacaman and Jorge Gardea-Torresdey. The gold appears as particles mere billionths of a meter wide. ...
Answer:
Explanation:
We must look up the standard reduction potentials for the half-reactions.
<u> ℰ° </u>
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ ⇌ 2Cr³⁺ + 7H₂O 1.36
Cl₂ + 2e⁻ ⇌ 2Cl⁻ 1.35827
2IO₃⁻ + 12H⁺ + 10e⁻ ⇌ I₂ + 6H₂O 1.195
Fe³⁺ + e⁻ ⇌ Fe²⁺ 0.771
(a) Cr³⁺/Cl₂
We reverse the sign of ℰ° for the oxidation half-reaction. Then we add the two half-reactions and their ℰ° values.
<u> ℰ°/V </u>
2Cr³⁺ + 7H₂O ⇌ Cr₂O₇²⁻ + 14H⁺ + 6e⁻ -1.36
<u>Cl₂ + 2e⁻ ⇌ 2Cl⁻ </u> <u>1.358 27
</u>
2Cr³⁺ + 3Cl₂ + 7H₂O ⇌ Cr₂O₇²⁻ + 6Cl⁻ + 14H⁺ 0.00
(b) Fe²⁺/IO₃⁻
<u> ℰ°/V
</u>
Fe²⁺ ⇌ Fe³⁺ + e⁻ -0.771
<u>2IO₃⁻ + 12H⁺ + 10e⁻ ⇌ I₂ + 6H₂O </u> <u> 1.195
</u>
10Fe²⁺ + 2IO₃⁻ + 12H⁺ ⇌ 10Fe³⁺ + I₂ + 6H₂O 0.424
The ℰ° values for the cells are
Assuming that the ammonium sulfide formula is (NH4)2<span>S then you can see that there are 2 nitrogen, 8 hydrogen and 2 sulfur atoms for every ammonium sulfide. If the amount of ammonium sulfide is 8.9 moles, then the number of hydrogen atoms should be: 8/1 * 8.9 mol= 71.2 moles.
If you are asked the number of individual atoms you need to multiply it with Avogadro number like this : </span>71.2 moles * 6.02 * 10^23= 4.28 *10^25