Competitive Exams: Chemistry MCQs (Practice-Test 4 of 31)

  1. 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

    5. unable to determine unless G of vaporization is known.

  2. Which statement is TRUE for an operating electrolytic cell?

    1. G > 0 and Ecell < 0

    2. G = 0 and Ecell > 0

    3. G < 0 and Ecell = 0

    4. G = 0 and Ecell = 0

    5. G > 0 and Ecell > 0

  3. For which ONE of the following reactions does the entropy of the system DECREASE?

    1. NH4NO3 (s) → NH4 + (aq) + NO3- (aq)

    2. 2 H2O2 (l) → 2 H2O (l) + O2 (g)

    3. Ba (OH) 2.8H2O (s) + 2 NH4SCN (s) → Ba (SCN) 2 (aq) + 2 NH3 (aq) + 10 H2O (l)

    4. NO (g) + NO2 (g) → N2O3 (g)

    5. CO2 (s) → CO2 (g)

  4. For a particular chemical reaction at 25oC, H = -432 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.

    5. There is insufficient information provided to answer this question.

  5. For a particular chemical reaction, Ho = + 60.0 kJ and So = + 121 J/K. At what temperature (in K) would this reaction become spontaneous?

  6. Consider the gas-phase hydrogenation of ethylene, C2H2, to produce ethane, C2H6, C2H2 (g) + 2 H2 (g) → C2H6 (g) and the following thermodynamic data: Substance Hfo, kJ mol-1So, J mol-1 K − 1 C2H2 (g) 227.0201. 0 H2 (g) 0131.0 C2H6 (g) -84.70229. 5 Under standard conditions, which of the following statements is TRUE?

    1. The reaction is spontaneous; Ho is favorable while So is not.

    2. The reaction is spontaneous; So is favorable while Ho is not.

    3. The reaction is not spontaneous; So is favorable while Ho is not.

    4. The reaction is not spontaneous; Ho is favorable while So is not.

    5. The reaction is not spontaneous; neither Ho nor So are favorable.

  7. Consider the gas-phase reaction of hydrogen, H2, and oxygen, O2, to produce water, 2 H2 (g) + O2 (g) → 2 H2O (g) and the following thermodynamic data: Substance Gfo, kJ/mol H2 (g) 0 O2 (g) 0 H2O (g) -228.6 Calculate the value of the equilibrium constant for this reaction under standard conditions.

  8. Consider the gas-phase reaction of hydrogen, H2, with carbon monoxide, CO, to form formaldehyde, H2CO, in a closed system at constant pressure, H2 (g) + CO (g) → H2CO (g) and the following thermodynamic data, substance Hfo, kJ mol-1So, J mol-1 K − 1, H2 (g) 0131, CO (g) -110198, H2CO (g) -116219 If the initial concentration of CO (g) is 1.5 M and the initial concentration of H2 (g) is 2.3 M, calculate the concentration (in M) of H2CO (g) at equilibrium, at 298 K.

    1. N/A

    2. N/A

    3. N/A

    4. N/A

  9. For a certain process, S (system) > 0 and S (surroundings) > 0. The process:

    1. is spontaneous.

    2. is exothermic.

    3. decreases the entropy of the universe.

    4. is endothermic.

    5. is at equilibrium.

  10. 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

    5. H < 0 and S > 0

  11. Consider the following reaction: H2O (g) + Cl2O (g) ⇔ 2 HOCl (g) For this reaction, Keq = 0.090 at 25oC. Calculate the value (in kJ/mol) of Go for this reaction.

  12. For a particular process at 500 K, G = -3.0 kJ and H = -23.0 kJ. If the process is carried out reversibly, caluclate the amount (in kJ) of useful work that can be performed.

    1. N/A

    2. N/A

    3. N/A

    4. N/A

  13. 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. Initially both G and Go are equal to zero. The value of Go changes to a value determined by the equilibrium constant; the value of G peaks and then decreases to zero at equilibrium.

    4. 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.

    5. Go remains constant while G changes and becomes equal to zero at equilibrium.

  14. The enthalpy of vaporization of ethanol is 38.7 kJ/mol at its boiling point of 78oC. Calculate the change in entropy (in J/mol) of the surroundings when 1.00 mole of ethanol is vaporized at 78oC and 1.00 atm of pressure.

    1. N/A

    2. N/A

    3. N/A

    4. N/A

  15. A certain chemical reaction carried out at constant temperature and pressure has S > 0. What can you conclude about the spontaneity of this reaction?

    1. The reaction is not spontaneous if it is endothermic and the temperature is sufficiently high.

    2. The reaction is not spontaneous at any temperature.

    3. The reaction is not spontaneous if it is exothermic and the temperature is sufficiently low.

    4. The reaction is only spontaneous at high temperatures.

    5. The reaction is spontaneous if it is exothermic and the temperature is sufficiently high.

  16. Consider the following reaction: 2 C (graphite) + H2 (g) → C2H2 (g) Go = 209.2 kJ at 25oC, P (H2) = 100 atm, P (C2H2) = 0.10 atm Calculate the value (in kJ) of G for this reaction.

    1. N/A

    2. N/A

    3. N/A

    4. N/A

  17. We observed that when two white crystalline substances were mixed the beaker froze to the wooden board it was sitting on. The reaction is as follows: Ba (OH) 2: 8H2O (s) + 2 NH4NO3 (s) → Ba (NO3) 2 (aq) + 2 NH3 (g) + 10 H2O (l) For this reaction, Ho = 170.44 kJ and So = 657.4 J/K. Which of the following statements must be TRUE about the process?

    1. Enthalpy is favorable and entropy is unfavorable.

    2. Neither entropy nor entropy are favorable.

    3. Both enthalpy and entropy are favorable.

    4. Enthalpy is unfavorable and entropy is favorable.

    5. The process is not spontaneous because it had to be stirred.