Chemical Thermodynamics Iit Part 2

Note: Key to the questions and updates, if any, can be downloaded from http://groups.google.com/group/adichemadi Prepared by V. Aditya vardhan [email protected]

CHEMICAL THERMODYNAMICS (PART-II) IIT-JEE

1)

ENTROPY The Kelvin’s statement of second law of thermodynamics is 1) The entropy of universe increases in a spontaneous process. 2) It is impossible to take heat from a hotter reservoir and completely convert into work by a cyclic process with out transferring a part of heat to a cooler reservoir. 3) It is impossible for a cyclic process to transfer heat from a body at lower temperature to one at a higher temperature with out converting some work to heat. 4) All.

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Note: 3rd statement is known as Clausius statement of second law of thermodynamics.

2)

Choose the incorrect statement. 1) Entropy is a measure of the degree of spreading and sharing of thermal energy within a system. 2) The total change in entropy of system and surroundings is zero in a reversible process. 3) Change in entropy is inversely proportional to the temperature at which the process occurs. 4) Entropy of a perfectly-ordered solid at 0°C is zero. Note: 1) According to third law of thermodynamics, the entropy of a perfectly ordered crystal is zero at absolute zero temperature. 2) Absolute entropy value of the system is proportional to temperature. But the change in entropy is inversely related to temperature.

3)

The thermodynamical expression for change in entropy is given by

qrev q w q w 2) irrev 3) 4) irrev T T T T The unit of molar entropy is 1) J.mol-1 2) J.K-1.mol-1 3) J.K-1 4) J.K.mol-1 Which of the following is true. 1) Entropy is an intensive property but a state function. 2) Entropy is an extensive property and not a state function. 3) Molar entropy is an extensive property but not a state function. 4) Entropy is an extensive property and a state function. The enthalpy of vaporisation of a substance is 72 J mol-1 and its boiling point is -73oC. The entropy of vaporization is 1) 72 J mol-1 K-1 2) 0.36 J mol-1 K-1 3) 0.986 J mol-1 K-1 4) 72 J mol K For a process at 350 K and constant pressure,  Ssurr is -315 J K-1. The quantity of heat (in kJ) absorbed by the system is 1) 72 2) 3.15 3) 99.2 4) 3.5 Consider the following statements about entropy. a) An ideal gas occupying larger volume has higher entropy than when it occupies smaller volume under given conditions. b) The gases with higher atomic or molecular weights possess greater entropy values. c) The change in entropy of an ideal gas in isothermal free expansion is always negative. d) The molar entropy of a dilute solution is greater than that of concentrated solution. e) Solids possess very high entropy values than liquids and gases. The incorrect statments are 1) c & e 2) a,c & d 3) a,b & c 4) b,c & e Which of the following contain higher entropy? 1) 1 mole acetylene 2) 1 mole ethane 3) 1 mole ethylene 4) 1 mole methane

1) 4) 5)

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Prepared by V. Aditya vardhan [email protected] Hint: Entropy will be higher for those molecules with more number of atoms and bonds.

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10) Which of the following produces an increase in entropy of the system ? 1) H2O(l)  H2O(s) 2) O2(g) + 2SO2(g)  2SO3(g) 3) 2CH3OH(g) + 3O2(g)  2CO2(g) + 4H2O(l) 4) I2(s)  I2(l) 11) Which of the following is the significance of Third law of thermodynamics? 1) The absolute entropy of a substance decreases with increasing temperature. 2) The change in entropy of the universe must be positive for a spontaneous process. 3) The absolute value of entropy can be measured for some very pure substances. 4) The entropy of the universe is constant. 12) For which of the following processes does the entropy of the system decrease? 1) Precipitation of silver chloride from a solution containing silver ions and chloride ions. 2) Dissolving table salt in water. 3) Decomposition of liquid hydrogen peroxide to produce liquid water and oxygen gas.. 4) Melting of ice. 13) For which one of the following reactions does the entropy of the system increase? 1) NH3(g) + HCl(g)  NH4Cl(s) 2) 2H2(g) + O2(g)  2H2O(g) + 3) NH3(g) + H2O(l)  NH4 (aq) + OH (aq) 4) 2H2O2(l)  2H2O(l) + O2(g) 14) Which of the following statements best describes the spontaneity of a reaction at 25oC for which  Ssys is negative and  Ssurr is positive? 1) The reaction cannot be spontaneous at 25oC. 2) The reaction will be spontaneous at all temperatures. 3) The reaction will be spontaneous only if  Ssys is greater in magnitude than  Ssurr. 4) The reaction will be spontaneous only if  Ssurr is greater in magnitude than  Ssys. Note: For a spontaneous process, S universe  S sys  S surr  0

15) Which of the following processes will result in a decrease in the entropy of the closed system? 1) Zn2+ (aq) + 4 NH3 (aq)  Zn(NH3)42+ (aq) 2) Cl2 (g) (298 K)  Cl2 (g) (450 K) 3) ZnS (s)  Zn2+ (aq) + S2- (aq) 4) NH3 (g) (3 atm, 298 K)  NH3 (g) (1 atm, 298 K) 16) For a certain process  S(system) > 0 and  S(surroundings) > 0. The process: 1) is spontaneous. 2) is exothermic. 3) is endothermic. 4) is at equilibrium. 17) Arrange the following compounds in order of increasing standard molar entropy at 25oC. 1) ZnS(s) < H2O(l) < C3H8(g) < C2H4(g) 2) ZnS(s) < C3H8(g) < C2H4(g) < H2O(g) 3) C3H8(g) < C2H4(g) < H2O(l) < ZnS(s) 4) ZnS(s) < H2O(l) < C2H4(g) < C3H8(g) 18) The significance of the Second Law of Thermodynamics is that for spontaneous processes the: 1) entropy of the universe is increasing. 2) entropy is the driving force of all chemical reactions. 3) entropy of an ideal solid does not change. 4) absolute value for entropy can be calculated. 19) A change of state occurs within a system and produces 64.0 kJ of heat. This heat is transferred to the surroundings at a constant pressure and a constant temperature of 300 K. For this process,  S of the surroundings is : 1) 64.0 J/K 2) -213 J/K 3) 213 J/K 4) 0.00 J/K 20) The normal freezing point of water is 0oC. Predict the signs of  Ssys,  Ssurr and  Suniv when H2O is supercooled and freezes at -10oC and 1atm.

Prepared by V. Aditya vardhan [email protected]

1)  Ssys < 0;  Ssurr < 0;  Suniv < 0 2)  Ssys > 0;  Ssurr < 0;  Suniv = 0 3)  Ssys = 0;  Ssurr < 0;  Suniv > 0 4)  Ssys < 0;  Ssurr > 0;  Suniv > 0 21) The volume of an ideal gas is doubled in an isothermal process. The change in entropy during this process is 1) -5.76 J K-1mol-1 2) +5.76 J K-1mol-1 3) +8.3 J K-1mol-1 4) 0 J K-1mol-1 Formula: For isothermal processes, S  2.303 x nR log

Vf Vi

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22) The pressure of 2 moles of a gas is changed from 1 atm to 10 atm at 355.5 K. The change in the entropy of gas during this process is 1) -38.29 J K-1mol-1 2) +38.29 J K-1 3) -38.29 J K-1 4) +38.29 J K-1mol-1 Formula: For isothermal processes, S  2.303 x nR log

Pi

Pf

23) The absolute entropy of a substance under given conditions cannot be always calculated accurately T

by using the formula ST  C  0

dT . This is because T

1) The value of C is dependent on temperature and is not a constant. 2) The value of C under constant pressure is always not equal to that under constant volume. 3) The temperature range may encompass the phase change and the entropy change during this process must be taken into account. 4) All. 24) For mercury the molar entropy of vaporization is 92.92 J K-1 mol-1 and the molar enthalpy of vaporization is 58.51 kJ mol-1. Therefore, the normal boiling point of mercury in oC is: 1) 273oC 2) 357oC 3) 516oC 4) 670oC Formula: S 

 vap H Tb

25) The enthalpy of vaporization of ethanol is 38.7 kJ/mol at its boiling point of 78oC. The change in entropy of the surroundings when one mole of ethanol is vaporized at 78oC and one atm of pressure will be 1) +110 J K-1mol-1 2) +220 J K-1mol-1 3) -110 J K-1mol-1 4) 0 J K-1mol-1 26) The significance of the Second Law of Thermodynamics is that for spontaneous processes the: 1) entropy of the universe is increasing. 2) entropy of the surroundings must decrease. 3) entropy of an ideal solid does not change. 4) absolute value for entropy can be calculated. 27) For HI the molar entropy of vaporization is 89.0 J/K mol and the molar enthalpy of vaporization is 21.16 kJ/mol. Therefore, the normal boiling point of HI is: 1) -273oC 2) -35.4oC 3) 23.8 K 4) 42oC 28) A change of state occurs within a system and produces 64.0 kJ of heat. This heat is transferred to the surroundings at a constant pressure and a constant temperature of 300 K. For this process,  S of the surroundings is : 1) 64.0 J/K 2) -213 J/K 3) -64.0 J/K 4) 213 J/K 29) Which of the following statements is false? 1) Entropy is a measure of the driving force behind chemical reactions. 2) Entropy is a measure of disorder or randomness.

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3) Entropy can be created and destroyed. 4) Entropy is a measure of the number of ways energy can be distributed among the motions of particles. Note: Strictly speaking, entropy is a measure of spreading and sharing of thermal energy among the microstates possible in a system under given conditions.

30) Boiling point of Bromine is 59.20C. The standard molar enthalpy of it is 1) 28 kJ mol-1 2) 38 kJ mol-1 3) -28 kJ mol-1

4) -38 kJ mol-1

Note: According to Trouton’s rule, the standard molar entropy of vaporisation values of a wide range of liquids are almost same (85 J K-1mol-1) o

o -1

 85 J K mol

-1

Tb

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 vap S 

 vap H

31) The efficiency of engine that is working between a lower temperature 27oC and a higher temperature 327oC is 1) 0.2 2) 0.35 3) 0.5 4) 1 Formula: Efficiency of engine =



TH  TL TH

32) The heat efficiency of an engine will be 100% when the operating lower temperature is 1) 0oC 2) 273 K 3) 0 K 4) 273oC 33) For which one of the following reactions does the entropy of the system decrease? 1) NH4NO3 (s)  NH4+ (aq) + NO3- (aq) 2) Ba(OH)2.8H2O (s) + 2 NH4SCN (s)  Ba(SCN)2 (aq) + 2 NH3 (aq) + 10 H2O (l) 3) NO (g) + NO2 (g)  N2O3 (g) 4) CO2 (s)  CO2 (g) 34) Precipitation of the Group I ions in the qualitative analysis scheme as AgCl, Hg2Cl2 andPbCl2 is spontaneous because 1) Entropy of the system decreases. 2) Entropy of the surroundings decreases 3) Entropy of the universe increases 4) Entropy of the system increases. 35) The standard entropy values (in JK-1 mol-1) of H2(g) = 130.6 Cl2 (g) = 223.0 and HCl (g) = 186.7 at 298 K and 1 atm pressure, then the entropy change for the reaction will be (in JK-1 mol-1) H2(g) + Cl2 (g)  2HCl(g) is : 1) + 540.3 2) +727.3 3) -166.9 4) +19.8 36) If 900 J/g heat is exchanged at boiling point of water then increase in entropy 1) 43.4 J/mol 2) 87.2 J/mol 3) 900 J/mol 4) zero 37) Oxygen and Helium gases are mixed in ratio of 1 : 1 by weight in a container, then in this process : 1) internal energy decreases 2) internal energy increases 3) entropy increases 4) entropy decreases

1)

The term T Stotal 1) Gibbs function

GIBBS FREE ENERGY at constant temperature and pressure is known as 2) Helmholtz function 3) Entropy function

4) All

Note: Gibbs function or Gibbs free energy, G , is equal to T S total at constant temperature and pressure. This is equal to the maximum useful work other than PV work. Where as Helmholtz function or Helmholtz free energy, A , is equal to

T S total at constant temperature and volume.

This quantiy is equal to maximum work, wmax.

2)

Choose the correct equation. 1) T Stotal  H system  T S system 3) Both 1 and 2

2) G  H system  T S system 4) None

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3)

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Which of the following is incorrect for a reaction A   B? 1) The reaction can spontaneously proceed to the right when G  0 . 2) The reaction can spontaneously proceed to the right when G  0 3) The reaction is at equilibrium when G  0 . 4) All The units of Gibbs function can be given by 1) J K 2) J K-1 3) J Which of the following statement is false? 1) Gibbs free energy is a state function and an extensive property.

4) J K mol-1

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2) In G  H system  T S system , the term T S system represents heat that is wasted in the form PV work and is not available to do useful work 3) G which is equal to T Stotal represents minimum useful work. 4) Gibbs free energy is not conserved.

Note:1) Molar Gibbs free energy is intensive. 2) Although Gibbs free energy has the units of energy, it cannot be conserved - which is one of the most important attributes of energy.

6)

1  H 2O (l ) is used in fuel cells to produce an electric current. The reaction H 2 ( g )  O2 ( g )  2 Given the thermodynamic data: Molar entropies in J mol-1 K-1 for O2(g) =205; for H2(g)=130.6; for H2O(l) =70

and  f H o of H2O(l) = –285.9 kJ mol–1.

What is the amount of electrical work the same reaction can perform in the fuel cell when carried out reversibly at 298 K. 1) -237.2 kJ mol-1 2) +285.9 kJ mol-1 3) +237.2 kJ mol-1 1) +2.859 kJ mol-1 Note: The amount of electrical work done by this reaction is equal to the Gibbs free energy change(-237.2kJ mol-1 ). As it is negative, the reaction is spontaneous. This is the maximum useful work. During this process some amount of heat is liberated and is given by o

o

T S   f H  G = –285.9 kJ mol-1

- (-237.2 kJ mol-1 ) = 48.7 kJ mol-1

This is the amount of heat liberated during the compression work done on the system as1.5 moles of gas during the reaction are disappeared. This is equal to PV work and is not useful.

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For a particular chemical reaction,  Ho is positive and  So is negative. Which of the following statements about the spontaneity of the reaction under standard conditions is TRUE? 1) The reaction will be spontaneous only if the magnitude of  Ho is large enough to overcome the unfavorable entropy change. 2) The reaction will be spontaneous only if the magnitude of  So is large enough to overcome the unfavorable enthalpy change. 3) The reaction will be spontaneous regardless of the magnitudes of  Ho and  So. 4) The reaction cannot be spontaneous at all temperatures. Which of the following is TRUE for an operating voltaic cell? (Where Ecell = emf of the cell) 1)  G > 0 ; Ecell = 0 2)  G < 0 ; Ecell < 0 3)  G < 0 ; Ecell > 0 4)  G = 0 ; Ecell = 0 An exothermic reaction which is spontaneous at all temperatures is 1) C(graphite) + O2(g)   CO2(g) 2) 3H2 (g) + N2(g)   2NH3(g)

;

 H° = –393 kJ

;

 H°= –46.2 kJ

3) N2O4(g)   2NO2(g)

;

 H° = + 55.3 kJ

Prepared by V. Aditya vardhan [email protected]

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4) All For a particular chemical reaction, both  Ho and  So are negative. Which of the following statements about the spontaneity of the reaction under standard conditions is true? 1) The reaction will be spontaneous at any temperature. 2) The reaction will be spontaneous only at higher temperatures 3) The reaction will be spontaneous regardless of the magnitudes of  Ho and  So. 4) The reaction will be spontaneous only if the magnitude of  Ho is large enough to overcome the unfavorable entropy change. Which of the following reactions is unfavorable at low temperatures but becomes favorable as the temperature increases? 1) 2 CO(g) + O2(g)  CO2(g);  Ho = -566 kJ;  So = -173 J/K 2) 2 H2O(g)  2 H2(g) + O2(g);  Ho = 484 kJ;  So = 90.0 J/K 3) 2 N2O(g)  2 N2(g) + O2(g);  Ho = -164 kJ;  So = 149 J/K 4) PbCl2(s)  Pb2+(aq) + O2(g);  Ho = 23.4 kJ;  So = -12.5 J/K For a particular chemical reaction, the values for  H and  S are both positive. Which of the following statements is TRUE about this reaction? 1) The reaction is spontaneous at any temperature. 2) The reaction is not spontaneous at any temperature. 3) The reaction is spontaneous when the temperature is high enough to overcome  H. 4) The reaction is spontaneous when the temperature is low enough to overcome  H. A spontaneous reaction always occurs when: 1)  Ho < 0 and  So < 0 2)  Ho > 0 and  So < 0 3)  Ho < 0 and  So > 0 4)  Ho > 0 and  So > 0 In which case does the spontaneity of a reaction depend on the temperature? 1)  H = 0 and  S < 0 2)  H > 0 and  S > 0 3)  H < 0 and  S = 0 4)  H > 0 and  S = 0 Which of the following statements is True? 1) A process in which the entropy of the system increases will always be spontaneous. 2) An endothermic process can never be spontaneous. 3) An exothermic process that is accompanied by an increase in the entropy of the system will always be spontaneous. 4) A process in which the entropy of the surroundings increases will always be spontaneous. For a particular chemical reaction at 25oC,  H = -536 kJ. Which of the following statements is TRUE? 1) The reaction will be spontaneous, at any temperature, if  S is positive. 2) The reaction will be spontaneous, at any temperature, if  S is negative. 3) The reaction will be spontaneous only if  G is positive. 4) The reaction can never be spontaneous, at any temperature. All of the gas-phase reactions listed below are endothermic. Which one of these reactions is most likely to be non-spontaneous at all temperatures? 1) COCl2(g)  CO(g) + Cl2(g) 2) 4NO(g) + 6H2O(g)  4NH3(g) + 5O2(g) 3) 2SO3(g)  2SO2(g) + O2(g) 4) N2O4(g)  2NO2(g) For a particular chemical reaction,  Ho = +70 kJ and  So = +210 J/K. The temperature at which this reaction would become spontaneous is 1) below 60.3 oC 2) above 333.3 oC 3) above 60.3 oC 4) below 333.3 oC

Prepared by V. Aditya vardhan [email protected] Note: For reactions with identical signs of H and S there is a unique temperature at which when G = 0 and T S  H . This is the temperature at which G changes its sign. This is also the temperature at which the system reaches equilibrium.

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19) A change in which of the following will also change the value of  G for a chemical reaction? 1) partial pressures of either gaseous reactants or products 2) temperature 3) concentration of either reactants or products in aqueous solution 4) All 20) For a particular process at 450 K,  G = -5.2 kJ and  H = -43.7 kJ. If the process is carried out reversibly, the amount of useful work that can be performed is 1) -48.9 kJ 2) -5.2 kJ 3) -43.7 kJ 4) -38.5 kJ 21) The following thermite reaction is initiated by burning of a magnesium ribbon. Fe2O3(s) + 2 Al(s)  2 Fe(l) + Al2O3(s) For this reaction, the standard enthalpy change is equal to -851.5 kJ and the standard entropy change is equal to -38.58 J/K. This reaction cannot occur at low temperatures because 1) Enthalpy is favorable and entropy is unfavorable. 2) Neither enthalpy nor entropy are favorable. 3) Both enthalpy and entropy are favorable at high temperatures only. 4) The activation energy is very high. 22. Chemical thermodynamics predicts that materials made of wood, metal, paper, plastic, leather and rubber react with oxygen in the atmosphere and decompose at 25oC and 1 atm pressure. Which of the following is responsible for the fact that these reactions are very slow under these conditions? 1) low entropy 2) high enthalpy 3) high activation energy 4) low activation energy 23) Which of the following statements is true? 1) Endothermic reactions are never spontaneous, at any temperature. 2) Exothermic reactions are always spontaneous, at any temperature. 3) Exothermic reactions in which the entropy change for the system is negative are spontaneous at any temperature. 4) None of the above statements is true. 24) A mixture of hydrogen and chlorine gas remains unreacted until exposed to UV light.Then the following reaction occurs very rapidly: H2(g) + Cl2(g)  2 HCl(g) ;  G= -45.54kJ,  H= -44.12kJ,  S = -4.76 J/K Select the statements below which best explains this behaviour. 1) The reactants are thermodynamically less stable than the products. 2) The reaction is thermodynamically spontaneous, but the reactants are kinetically stable. 3) The ultraviolet light raises the temperature of the system and makes the reaction spontaneous. 4) The negative value for  S slows down the reaction. 25) If a reaction is spontaneous at 27oC and has a standard enthalpy change equal to +150 kJ/mol, which of the following must be true about the standard entropy of reaction? 1) Standard entropy change < 0 J/mol K 2) Standard entropy change = 27 J/mol K 3) Standard entropy change > 500 J/mol K 4) Standard entropy change = 150 J/mol K 26) The normal freezing point of water is 0oC. Predict the signs of  H,  S and  G when H2O is supercooled and freezes at -10oC and one atmosphere. 1)  H < 0,  S < 0,  G < 0 2)  H < 0,  S > 0,  G = 0 3)  H > 0,  S = 0,  G > 0 4)  H > 0,  S > 0,  G = 0 27)  S > 0 for a certain chemical reaction carried out at constant temperature and pressure . Which of

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29) 30)

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the following will be true about spontaneity of the reaction? 1) The reaction is spontaneous if it is endothermic and the temperature is sufficiently high. 2) The reaction is spontaneous, if it is exothermic, at any temperature. 3) The reaction is not spontaneous if it is exothermic and the temperature is sufficiently low. 4) Both 1 and 2. Chloroform, CH3Cl, is a common organic solvent. Consider the following process at 70oC, in a closed system: CH3Cl(l) -> CH3Cl (g) This process is endothermic. Which of the following statements is most likely true regarding the spontaneity of this process at 70oC? 1) The process will be spontaneous because  Ssys is negative. 2) The process will be spontaneous because  Ssurr is positive. 3) The process will be spontaneous if the boiling point of CH3Cl is less than 70oC. 4) The process will be spontaneous if the boiling point of CH3Cl is greater than 70oC. Which of the following thermodynamic properties cannot be experimentally determined? 1)  So 2)  Go 3) Go 4) So An ideal solution is formed spontaneously by mixing equal volumes of benzene and touene. For this process  Hsoln is almost zero. Which of the following will be responsible for the spontaneity of this process? 1) positive  Ssoln 2) negative  Ssoln 3) positive  Vsoln 4) positive  Gsoln Consider the gas-phase reaction of hydrogen, H2, and chlorine, Cl2, to produce hydrogen chloride, H2(g) + Cl2(g)   2HCl(g) and the following thermodynamic data : substance  fGo, kJ/mol H2(g) 0 Cl2(g) 0 HCl(g) -95.30 o The value of  H (in kJ) for this reaction at 25oC if the standard state entropy change is equal to 20.0 J/K. 1) 184.6 kJ 2) 101.26 kJ 3) 196.56 kJ 4) 51.3 kJ o Note: H of the given reaction is to be calculated and not  f H

o

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32) At elevated temperatures, hydrogen iodide, HI, decomposes to produce hydrogen, H2, and iodine, I2, as shown in the equation below: 2 HI (g)  H2 (g) + I2 (g) Using the thermodynamic data below, calculate the standard free energy change (in kJ) for this reaction. Compound So, J mol-1 K-1  fHo, kJ mol-1 HI(g) 25.9 206.3 H2(g) 0 131.0 I2(g) 62.4 260.6 1) 62.4 kJ 2) 16.9 kJ 3) 32.8 kJ 4) 8.45 kJ 33) Consider the following gas-phase reaction and average bond enthalpies, 2NOCl(g)   2NO(g) + Cl2(g) bond average bond enthalpy, kJ mol-1 N-Cl 201 N=O 716 Cl-Cl 243 Which of the following statements about the spontaneity of this reaction is true at 25oC?

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34)

1) The reaction will be spontaneous only if the magnitude of  H is large enough to overcome the unfavorable entropy change. 2) The reaction will be spontaneous only if the magnitude of  S is large enough to overcome the unfavorable enthalpy change. 3) The reaction will be spontaneous if both  H and  S are negative. 4) The reaction cannot be spontaneous at any temperature. Solid ammonium nitrate, NH4NO3 (s), dissolves spontaneously in water at 25oC, NH4Cl (s)  NH4+ (aq) + Cl- (aq) If this process is endothermic, which of the following is most likely true ? 1)  G > 0 2)  G = 0 3)  S < 0 4)  S > 0 o The normal freezing point of NH3 is -78 C at one atmosphere. Predict the signs of  H,  S, and  G for NH3 when NH3 supercools and then freezes at -80oC and one atmosphere. 1)  H < 0,  S < 0,  G = 0 2)  H < 0,  S > 0,  G < 0 3)  H < 0,  S < 0,  G < 0 4)  H > 0,  S > 0,  G = 0 Consider the following gas-phase reaction H2(g) + CO(g) <==> H2CO(g) and the following thermodynamic data, substance So, J mol-1 K-1  Hfo, kJ mol-1 H2(g) 0 131 CO(g) -110 198 H2CO(g) -116 219 Which of the following best explains why this reaction is not spontaneous under standard conditions? 1) Both  Ho and  So oppose spontaneity.. 2) Both  Ho and  So favor spontaneity.. 3)  Ho favors spontaneity but  So opposes spontaneity.. 4)  Ho opposes spontaneity but  So favors spontaneity.. Adsorption of gases on solid surfaces is generally exothermic because: 1) Entropy of the gas and solid increases. 2) Entropy of the gas and solid decreases. 3) Free energy increases. 4) The total entropy of the system and surroundings decreases. The correct relation between change in free energy in a reaction and the corresponding equilibrium constant Kc is given by:

1) G  RT ln K c 2) G  RT ln K c 3) G o  RT ln K c 4) G o  RT ln K c 39) When a reaction is at equilibrium, which of the following statements is true? 1)  G =  Go 2)  G = 0 3) ln Keq = 0 4)  Go = 0 40) For the boiling of methanol, CH3OH(l)  CH3OH(g) ;  Ho = +38.0 kJ and  So = +113.0 J/K. The temperature at which the value of  G becomes equal to zero for this process will be 1) Greater than its boiling point 2) Less than its boiling point. 3) Equal to its boiling point. 4) Can not be calculated unless the  G value is given. 41) The following reaction is spontaneous as written: Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) Which of the following statements is true? 1) Keq > 1 and  Go = 0 2) Keq > 1 and  Go < 0 3) Keq < 1 and  Go < 0 4) Keq > 1 and  Go > 0

Prepared by V. Aditya vardhan [email protected]

42) The equilibrium constant for a chemical reaction will be equal to one when 1)  Ho < 0 and  So = 0 2)  Ho = 0 and  So = 0 3)  Ho < 0 and  So > 0 4)  Ho > 0 and  So < 0 43) The standard free energy change for the following reaction if the complex ion formation equilibrium constant, Kf , is equal to 2.1 x 1013.  Cu(NH ) 2+ (aq) Cu2+ (aq) + 4 NH3 (aq)  3 4

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1) -150 kJ 2) -76 kJ 3) -6.4 kJ 4) 19 kJ 44) Consider the following gas-phase reaction, 2 H2 (g) + O2 (g)  2 H2O (g) and the corresponding thermodynamic data: substance  Gfo, kJ/mol H2(g) 0 O2(g) 0 H2O(g) -228.6 The value of the equilibrium constant for this reaction under standard conditions will be 1) 1.4 x 1040 2) 1.4 x 1020 3) 1.4 x 1080 4) 1.4 x 1010 45) Which of the following statements concerning the change in  Go and  G during a chemical reaction is most correct? 1)  Go remains constant while  G changes and becomes equal to  Go at equilibrium. 2) Both  Go and  G remain constant during a chemical reaction. 3)  Go remains constant if the reaction is carried out under standard conditions;  G remains constant if the reaction is carried out under non-standard conditions. 4)  Go remains constant while  G changes and becomes equal to zero at equilibrium. 46). Calculate the value of Ka for hydrofluoric acid (HF) at 25oC if the standard free energy change for the following reaction is equal to +17.9 kJ/mol:  H O+ (aq) + F- (aq) HF (g) + H2O (l)  3 -38 -4 1) 4.0 x 10 2) 7.4 x 10 3) 9.2 x 10-1 4) 1.4 x 103 47) Hydrogen peroxide decomposes according to the following reaction: H2O2(l)  H2O(l) + 1/2 O2(g) For this reaction  H = -98.2 kJ and  S = 70.1 J/K. The approximate value of Kp for this reaction at 25oC is 1) -1 x 1013 2) -3 x 102 3) 5 x 10-2 4) 7 x 1020 Note: Key to the questions and updates, if any, can be downloaded from http://groups.google.com/group/adichemadi