99. A reaction has a positive AH and a positive AS. Which of the following is true? a. It will be spontaneous at all temperatures. b. It will be spontancous at low temperatures. C. It will be nonspontaneous at all temperatures. d. It will be spontaneous at high temperatures. 60. Predict the signs of AH, AS and AG for the reaction: Mg(s)- Mg(g) L AH=-, AS =+ and A G = - (all temperature) b. AH=+,AS=+ and A G = - (At low temperature) C. AH=+,AS=+ and A G =- (At all temperature) d. AH=+,AS=+ and A G =- (At high temperature)

99. A reaction has a positive AH and a positive AS. Which of the following is true? a. It will be spontaneous at all temperatures. b. It will be spontancous at low temperatures. C. It will be nonspontaneous at all temperatures. d. It will be spontaneous at high temperatures. 60. Predict the signs of AH, AS and AG for the reaction: Mg(s)- Mg(g) L AH=-, AS =+ and A G = - (all temperature) b. AH=+,AS=+ and A G = - (At low temperature) C. AH=+,AS=+ and A G =- (At all temperature) d. AH=+,AS=+ and A G =- (At high temperature)

Chemistry for Engineering Students

4th Edition

ISBN:9781337398909

Author:Lawrence S. Brown, Tom Holme

Publisher:Lawrence S. Brown, Tom Holme

Chapter10: Entropy And The Second Law Of Thermodynamics

Section: Chapter Questions

Problem 10.50PAE: For the reaction NO(g)+NO2(g)N2O3(g) , use tabulated thermodynamic data to calculate H and S. Then...

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A reaction has H298=100 kj/mol and S298=250 J/mol K. Is the reaction spontaneous at room temperature? If not, under what temperature conditions will it become spontaneous?
Methane can be produced from CO and H2.The process might be done in two steps, as shown below, with each step carried out in a separate reaction vessel within the production plant. Reaction 1 CO(g)+2H2(g)CH3OH(l) S = -532 J/K Reaction 2 CH3OH(l)CH4(g)+12O2(g) S= +162 J/K NOTE: You should be able to work this problem without using any additional tabulated data. (a) Calculate H for reaction 1. (b) Calculate Gf for CO(g). (c) Calculate S° for O2(g). (d) At what temperatures is reaction 1 spontaneous? (e) Suggest a reason why these two steps would need to be carried out separately. Substance Hf (kJ mol-1) Gf (kJ mol-1) S (J mol-1 K-1) CO(s) -110.5 197.674 CH3OH( l ) -238.7 -166.4 126.8 CH4(g) -74.8 186.2
For the reaction NO(g)+NO2(g)N2O3(g) , use tabulated thermodynamic data to calculate H and S. Then use those values to answer the following questions. (a) Is this reaction spontaneous at 25°C? Explain your answer. (b) If the reaction is not spontaneous at 25°C, will it become spontaneous at higher temperatures or lower temperatures? (c) To show that your prediction is accurate, choose a temperature that corresponds to your prediction in part (b) and calculate G . (Assume that both enthalpy and entropy are independent of temperature.)
The free energy change, G, for a process at constant temperature and pressure is related to Suniv and reflects the spontaneity of the process. How is G related to Suniv? When is a process spontaneous? Nonspontaneous? At equilibrium? G is a composite term composed of H, T, and S. What is the G equation? Give the four possible sign combinations for H and S. What temperatures are required for each sign combination to yield a spontaneous process? If G is positive, what does it say about the reverse process? How does the G = H TS equation reduce when at the melting-point temperature of a solid-to-liquid phase change or at the boiling-point temperature of a liquid-to-gas phase change? What is the sign of G for the solid-to-liquid phase change at temperatures above the freezing point? What is the sign of G for the liquid-to-gas phase change at temperatures below the boiling point?
At 298 K, G = 70.52 kJ for the reaction 2NO(g) + O2(g) 2NO2(g) (a) Calculate _G at the same temperature when PNO = 1.0 104 atm, PO2=2.0103 atm, and PNO2=0.30 atm. (b) Under the conditions in part a, in which direction is the reaction spontaneous?
On the basis of your experience, predict which of the following reactions are spontaneous. (a) CO2(s)CO2(g) at 25°C (b) NaCl(s)NaCl(l) at 25°C (c) 2NaCl(s)2Na(s)+Cl2(g) (d) CO2(g)C(s)+O2(g)
For the reaction CH3OH(l)+32O2(g)2H2O(l)+CO2(g) the value of G is 702.2 kJ at 25C. Other data are as follows: Hf (kJ/mol) at 25C S (J/molK) at 25C CH3OH(l) 238.7 126.8 H2O(l) 285.8 70.0 CO2(g) 393.5 213.7 Calculate the standard entropy, S, per mole of O2(g).
What is the standard free energy change, G, for a reaction? What is the standard free energy of formation, Gf for a substance? How are Gf values used to calculate Grxn? How can you use Hesss law to calculate Grxn? How can you use H and S values to calculate Grxn? Of the functions H, S, and G, which depends most strongly on temperature? When G is calculated at temperatures other than 25Cwhat assumptions are generally made concerning H and S?
The reaction CO2(g)+H2(g)CO(g)+H2O(g) is not spontaneous at room temperature but becomes spontaneous at a much higher temperature. What can you conclude from this about the signs of H and S , assuming that the enthalpy and entropy changes are not greatly affected by the temperature change? Explain your reasoning.
The reaction shown below is involved in the refining of iron. (The table that follows provides all of the thermodynamic data you should need for this problem.) 2Fe2O3(s)+3C(s,graphite)4Fe(s)+3CO2(g) (a) Find H for the reaction. (b) S for the reaction above is 557.98 J/K. Find S° for Fe2O3(s). (c) Calculate G for the reaction at the standard temperature of 298 K. (There are two ways that you could do this.) (d) At what temperatures would this reaction be spontaneous? Compound Hf (kJ mol-1) S° (kJ mol-1) Gf (J mol-1 K-1) Fe2O3(s) -824.2 ? -742.2 C(s, graphite) 0 5.740 0 Fe(s) 0 27.3 0 CO2(g) -393.5 213.6 -394.4.83
What is the third law of thermodynamics? What are standard entropy values, S, and how are these S values (listed in Appendix 4) used to calculate S for a reaction? How would you use Hesss law to calculate S for a reaction? What does the superscript indicate? Predicting the sign of S for a reaction is an important skill to master. For a gas-phase reaction, what do you concentrate on to predict the sign of S? For a phase change, what do you concentrate on to predict the sign of S? That is, how are Ssolid, Sliquid, and Sgas related to one another? When a solute dissolves in water, what is usually the sign of S for this process?
Calculate H and G for the following reactions at 25C, using thermodynamic data from Appendix C; interpret the signs of H and G. a 2PbO(s)+N2(g)2Pb(s)+2NO(g)\ b CS2(l)+2H2O(l)CO2(g)+2H2S(g)