Which of the following processes is expected to have a positive change in entropy (deltaS > 0)?
Sublimation of solid CO2 to gaseous CO2
Condensation of water vapor to liquid water
Freezing of liquid water to ice
Dissolving a gas into a liquid
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WhySublimation of dry ice converts an ordered solid into a gas, greatly increasing the number of accessible microstates, so deltaS > 0.
2Multiple choice · Easy
For a reaction in which deltaH is negative and deltaS is positive, the reaction is:
Thermodynamically favorable at all temperatures
Favorable only at high temperatures
Favorable only at low temperatures
Never thermodynamically favorable
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WhyWith deltaG = deltaH - T*deltaS, a negative deltaH minus a positive T*deltaS term gives a negative deltaG at all temperatures, so the reaction is always thermodynamically favorable.
3Multiple choice · Easy
In a galvanic (voltaic) cell, oxidation occurs at the:
Anode
Cathode
Salt bridge
Voltmeter
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WhyBy definition, oxidation always occurs at the anode and reduction at the cathode; in a galvanic cell the anode is the negative electrode.
4Multiple choice · Easy
A reaction has a positive standard cell potential (E_cell > 0). What is the sign of deltaG for this reaction?
Negative
Positive
Zero
Cannot be determined from E_cell
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WhySince deltaG = -n*F*E_cell, a positive E_cell makes deltaG negative, meaning the reaction is thermodynamically favorable and the cell is galvanic.
5Multiple choice · Easy
Which statement about an electrolytic cell is correct?
It requires an external power source to drive a nonspontaneous reaction
It produces electrical energy from a spontaneous reaction
It has a positive cell potential without an external source
Reduction occurs at the anode
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WhyAn electrolytic cell uses an external power source to drive a nonspontaneous reaction (deltaG > 0, E_cell < 0), the opposite of a galvanic cell.
6Fill in the blank · Easy
For a reaction at equilibrium, the value of deltaG (in kJ/mol) is exactly .
Answer:
0 / zero
WhyAt equilibrium there is no net driving force, so deltaG = 0; this also corresponds to deltaG = deltaG_standard + R*T*ln(Q) with Q = K.
7Multiple choice · Medium
A reaction has deltaH = +120 kJ/mol and deltaS = +250 J/(mol*K). Above approximately what temperature does the reaction become thermodynamically favorable?
480 K
208 K
370 K
830 K
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WhySetting deltaG = 0 gives T = deltaH/deltaS = 120000 J/mol / 250 J/(mol*K) = 480 K; above this temperature deltaG becomes negative.
8Multiple choice · Medium
For a reaction with deltaG_standard < 0, what can be concluded about the equilibrium constant K?
K > 1
K < 1
K = 1
K = 0
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WhyFrom deltaG_standard = -R*T*ln(K), a negative deltaG_standard requires ln(K) > 0, so K > 1 and products are favored at equilibrium.
9Multiple choice · Medium
A galvanic cell is constructed using Zn2+/Zn (E = -0.76 V) and Cu2+/Cu (E = +0.34 V). What is the standard cell potential E_cell?
+1.10 V
-1.10 V
+0.42 V
-0.42 V
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WhyE_cell = E_cathode - E_anode = (+0.34 V) - (-0.76 V) = +1.10 V; copper is reduced at the cathode and zinc is oxidized at the anode.
10Multiple choice · Medium
Which best describes a coupled reaction in biological systems?
A favorable reaction drives an unfavorable reaction so their combined deltaG is negative
Two favorable reactions are added to make an unfavorable one
A reaction at equilibrium is forced to a new equilibrium by heat alone
An unfavorable reaction proceeds because its activation energy is low
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WhyA coupled reaction uses an energy-releasing (favorable, deltaG < 0) reaction such as ATP hydrolysis to drive an otherwise unfavorable reaction; the sum of the deltaG values must be negative.
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Thermodynamics: The study of energy changes, heat flow, and the direction in which chemical and physical processes naturally proceed.
Entropy: A thermodynamic property, symbol S, that measures the dispersal of energy and the number of accessible microscopic arrangements of a system.
Microstate: One specific arrangement of the positions and energies of all particles in a system that is consistent with its overall macroscopic state.
Energy Dispersal: The spreading out of energy among more particles or over more available energy levels, which corresponds to higher entropy.
Positional Disorder: The increase in entropy that arises when particles have more available spatial arrangements, such as during expansion or mixing.
Absolute Entropy: The total entropy of a substance measured relative to a perfectly ordered crystal at absolute zero, reported as a standard molar value.
Standard Molar Entropy: The absolute entropy of one mole of a substance under standard conditions, symbol S degrees, expressed in J per mol per K.
Third Law of Thermodynamics: The principle that the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero.