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Unformatted text preview: tudents (for students and parents). AP® CHEMISTRY
2007 SCORING GUIDELINES (Form B)
2 H2(g) + O2(g) → 2 H2O(l)
In a hydrogen-oxygen fuel cell, energy is produced by the overall reaction represented above.
(a) When the fuel cell operates at 25°C and 1.00 atm for 78.0 minutes, 0.0746 mol of O2(g) is consumed.
Calculate the volume of H2(g) consumed during the same time period. Express your answer in liters
measured at 25°C and 1.00 atm. (0.0746 mol O2) × V= n H 2 RT
P = 2 mol H 2
= 0.149 mol H2
1 mol O2 One point is earned for
calculation of moles of H 2 . (0.149 mol H2 )(0.0821 L atm mol −1 K −1 )(298 K)
1.00 atm One point is earned for
substitution into PV = nRT .
One point is earned for the
answer. = 3.65 L H2 (b) Given that the fuel cell reaction takes place in an acidic medium,
(i) write the two half reactions that occur as the cell operates,
O2 + 4 H+ + 4 e− → 2 H2O One point is earned for each
of the two half reactions. H2 → 2 H+ + 2 e−
(ii) identify the half reacti...
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This note was uploaded on 10/01/2009 for the course OC 9876 taught by Professor Dq during the Spring '09 term at UC Merced.
- Spring '09