Consider the sublimation of iodine at \(25.0^{\circ} \mathrm{C}\) :
\(\mathrm{I}_{2}(s) \longrightarrow \mathrm{I}_{2}(g)\)
a. Find \(\Delta G_{\mathrm{rxn}}^{\circ}\) at \(25.0^{\circ} \mathrm{C}\).
b. Find \(\Delta G_{\mathrm{rxn}}^{\circ}\) at \(25.0^{\circ} \mathrm{C}\) under the following nonstandard conditions:
i. \(P_{\mathrm{l}_{2}}=1.00 \mathrm{mmHg}\)
ii. \(P_{\mathrm{I}_{2}}=0.100 \mathrm{mmHg}\)
c. Explain why iodine spontaneously sublimes in open air at \(25.0^{\circ} \mathrm{C}\).
Text Transcription:
25^circ C
I2(s) longrightarrow I2( g)
Delta G_rxn^circ
PI2 = 1.00 mmHg
PI2 = 0.100 mmHg
Chem 222 notes Week 13 Sample problem: Calculate ΔG for the following reaction at standard conditions: 2Ag +(aq) Cu (s)> 2Ag (s)+ Cu 2+(aq) The standard reduction potential for silver is 0.799, and the standard reduction potential for copper is 0.337. First step: figure out what you have, what you need, and what relationship gets you from what you have to what you need. You have the E for your two reactions, and 0 you need ΔG. ΔG is related to E by the following equation: ΔG=-nFE. E for the complete reaction is the sum of the E of the two half-reactions. One half reaction is going forward, and the other is going backward. In the equation at the top, silver is written as a reduction; thus, its reduction potential is the same as given in the table. DO NOT multiply by the balancing coefficient. Copper is being oxidized in the above reaction, not reduced, meaning that its E will have an opposite sign of the number given in the table. Add the two half-reactions to get the E for the full reaction: Ecell +0.799 + (-0.337) = 0.462V. Plug this value into the expression for ΔG (n is 2 because you have 2 electrons being transferred in the above reaction, and F is Faraday’s constant, 96485): ΔG=-nFE = -(2)(96485)(.462)= -89000 J In an electrolytic cell, an electric current drives a nonspontaneous reaction. Electrolytic cells have a negative E, this is how you distinguish an electrolytic cell from a galvanic cell. Positive E means spontaneous, negative E means nonspontaneous. (Notice that this is the opposite of what the sign means when
