Chemical Thermodynamics Flashcards
At STP, liquid spontaneously vaporizes, even though rxn is endothermic. The most likely explanation is:
A. Change in enthalpy has no effect on rxn spontaneity
B. Temperature has no effect on rxn spontaneity
C. This rxn results in an increase in entropy
D. Endothermic rxns are always spontaneous
C.
Since G must be neg to be spontaneous, S must be positive when endothermic.
According to the laws of thermodynamics, an endothermic rxn is most likely to be spontaneous if the:
A. Change in entropy is pos and temp is high
B. Change in entropy is negligible and temp is low
C. Change in entropy is pos and temp is low
D. Change in entropy is neg and temp is high
A
Endothermic means pos H, there S must also be pos with high temp.
Sample of solid NH4NO3 dissolved in water, flask becomes cold to touch. What can be concluded about the given thermodynamic values for teh solvation process?
A. ΔG < 0, ΔH < 0, ΔS > 0
B. ΔG > 0, ΔH < 0, ΔS > 0
C. ΔG < 0, ΔH > 0, ΔS > 0
D. ΔG < 0, ΔH > 0, ΔS < 0
C
Rxn is observed, so spontaneous. ΔG < 0. Since cold, enthalpy must be pos and endothermic (H > 0). Dissolving solid in liquid increases disorder, so S > 0
A substance has a standard heat of formation of 0. This is best
A. Substance is an element in its standard state
B. At low temps, entropy makes virtually no contribution to changes in Gibb’s
C. H is a state function
D. Bond energies within the substance are very neg
Which of the following rxns has the most positive Srxn?
A. C(diamond) C(graphite)
B. 6 H2O(g) 6 H2O(l)
C. 6 CO2(g) + 6 H2O(g) C6H12O6(s)
D. CO2(s) CO2(g)
D
Pos S means entropy of products > rxns. Gases more entropic. Gases as products and s/l as reactant most pos.
The formation of CO2(g) from the reaction of CO(g) and O2(g) is spontaneous at room temperature. What must also be true for this reaction?
A. It is exothermic.
B. It is endergonic.
C. It is endothermic.
D. It is isentropic.
A
Decrease in moles indicates entropy (-S). Spontaneous at room temp (-G). H must also be neg, so exothermic.
Perspiration is important in maintaining normal body temp. Compared to water a boiling point, which of the following is true about water at normal human body temp?
A. Intermolecular forces are weaker
B. Average kinetic energy is greater
C. More energy is required for gas expansion
D. The heat required for vaporization is higher
D.
Heat required to vaporize is higher when water is at lower temperatures. IE: On skin vs boiling
Which of the following lists hydrogen halides in terms of increasing standard heats of formation?
A. HF < HBr < HCl < HI
B. HI < HBr < HCl < HF
C. HBr < HF < HCl < HI
D. HF < HI < HCl < HBr
A
Flourine is smallest, most EN, not a strong acid. HF most stable halide, and most neg heat of formation.
At 100 K, a certain reaction has the following values for ΔH and ΔS: ΔH = 25 kJ and ?S = 50 J/K. Neglecting any variation in ΔH and ΔS, at what temperature will ΔG = 0?
A. 100 K
B. 500 K
C. 1000 K
D. 250 K
B
When ΔG = 0, ΔH – TΔS = 0 and T=H/S=25kJ/0.05Kj/K=500K
If H denotes enthalpy and S denotes entropy, then a chemical system will always react spontaneously when:
A. ΔH > 0 and ΔS < 0.
B. ΔH < 0 and ΔS < 0.
C. ΔH < 0 and ΔS > 0. Correct Answer
D. ΔH > 0 and ΔS > 0.
C
A -H and +S will always lead to spontaneous rxn
At 298 K, ?H° = 436 kJ and ?S° = 100 J/K for the reaction H2(g) 2 H(g). Therefore, this reaction:
A. is at equilibrium at 298 K.
B. is spontaneous at 298 K.
C. is not spontaneous at 298 K but will be spontaneous at high enough temperatures.
D. will never be spontaneous, regardless of the temperature.
C
Not spontaneous, but would be when T is high enough to make G positive.
Metal-organic frameworks (MOFs) are systems of metals tethered together by organic linkers forming porous solids with exposed, active metal centers. MOFs are often used to bind and hold gases such as H2, often allowing for a more dense packing of gas than allowed in the pure liquid state. If the binding of gas to metals in the MOF is spontaneous, which must be true?
A. The enthalpy of binding is positive.
B. The enthalpy of binding is negative.
C. The change in free energy associated with binding is positive.
D. The entropy of reaction is positive.
B
Spontaneous, so G is negative, S neg b/c gas to solid, so H must be negative
Determine ΔH for the reaction CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) given the following information:
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
ΔH = –802 kJ/mol
2 H2O(g) 2 H2O(l)
ΔH = –88 kJ/mol
A. 714 kJ/mol
B. –714 kJ/mol
C. –890 kJ/mol
D. 890 kJ/mol
C
Overall rxn is just sum of 2 rxns.
Which of the following must result in a neg free energy change for a rxn?
A. The enthalpy change is negative, and the entropy change is negative
B. The entropy change is positive
C. The enthalpy change is negative
D. The enthalpy change is neg, and entropy change is pos.
D
T is always pos, so a neg H and pos S always gives neg G
A reaction occurs that results in a set of products with more stable bonds and more orderly arrangement than were present in the reactants. Which of the following is true of this reaction?
A. The enthalpy and entropy changes are negative.
B. The enthalpy change is positive, and the entropy change is negative.
C. The enthalpy change is negative, and the entropy change is positive.
D. The enthalpy and entropy changes are positive.
A
To be more stable, must have lost energy. H must be neg. To be more orderly, S must be neg
