Chapter 19: Practice
Is lighting spontaneous?
yes
A particular constant-pressure reaction is barely spontaneous at 390 K . The enthalpy change for the reaction is +23.7kJ. Estimate ΔS for the reaction.
.0608 kJ/K
A certain reaction has ΔH= -35.4 kJ and ΔS= -85.5 J/K. Use this information to answer the questions below: 1.) Is the reaction exothermic or endothermic? 2.) Does the reaction lead to an increase or decrease in the disorder of the system? 3.) Calculate ΔG for the reaction at 298 K. 4.) Is the reaction spontaneous at 298 K under standard conditions?
1. exothermic 2. decreases c. -9.921 d. yes
Calculate delta S standard values for the following reaction and account of the sign (why?) a. 2CH3OH(g) + 3O2(g) = 2CO2(g) + 4H2O(g)
92.32 + because more moles of molecules
Predict the sign of the entropy change of the system for each of the following reactions. (a) N2(g)+3H2(g) = 2NH3(g) (b) CaCO3(s) = CaO(s)+CO2(g) (c) 3C2H2(g) = C6H6(g) (d) Al2O3(s)+3H2(g) = 2Al(s)+3H2O(g)
A. - b. + c. - d. no change
a) For a process that occurs at constant temperature, does the change in Gibbs free energy depend on changes in the enthalpy and entropy of the system? (b) For a certain process that occurs at constant T and P, the value of ΔG is positive. Is the process spontaneous? (c) If ΔG for a process is large, is the rate at which it occurs fast?
A. YES (BOTH) B. no C. not necessarily
Calculate the ΔG of the reactions and indicate whether each reaction is spontaneous at 298K under standard conditions. a. 2SO2(g) + O2(g) = 2SO3(g) b. NO2(g) + N2O(g) = 3NO(g)
a. -140 kJ/mol - spontaneous b. 104.7 kJ/mol - non-spontaneous
Use data from Appendix CC to calculate the equilibrium constant, K,K, and ΔG∘ΔG∘ at 298 K for each of the following reactions: (a) H2(g)+I2(g)⇌2HI(g) (b) C2H5OH(g)⇌C2H4(g)+H2O(g)
a. -16.77 kJ, K = 870 b. 8.04 kJ, K = .0389
Octane (C8H18) is a liquid hydrocarbon at room temperature that is the primary constituent of gasoline. (a) Write a balanced equation for the combustion of C8H18(l) to form CO2(g) and H2O(l). (b) Without using thermochemical data, predict whether ΔG° for this reaction is more negative or less negative than ΔH°.
a. 2 C8H18(l) + 25O2(g) = 16CO2(g) + 18H2O(l) b. more
For a particular reaction, ΔH = -32 kJ and ΔS = -98J/K. Assume that ΔH and ΔS do not vary with temperature. a. At what temperature will the reaction have ΔG = 0? b. If T is increased from that in part (a), will the reaction be spontaneous or nonspontaneous?
a. 330 K b. nonspontaneous
Predict which member of each of the following pairs has the greater standard entropy at 25∘C: a. Sc(s) or Sc(g) b. O2(g) or O3(g)
a. Sc(g) b. O3(g)
Consider the vaporization of liquid water to steam at a pressure of 1 atm. (a) Is this process endothermic or exothermic? (b) In what temperature range is it a spontaneous process? (c) In what temperature range is it a nonspontaneous process? (d) At what temperature are the two phases in equilibrium?
a. endothermic b. above 100C c. below 100 C d. 100 C
Consider a system consisting of an ice cube. (a) Under what conditions can the ice cube melt reversibly? (b) If an ice cube melts reversibly, is delta E zero for the process? Briefly explain.
a. if the solid and liquid are at equilibrium and the temp is raised by a tiny amount b. no, process is endothermic
Classify each of the following reactions as one of the four possible types: Type 1: spontaneous at all temperatures, Type 2: nonspontaneous at all temperatures, Type 3: spontaneous at low T, nonspontaneous at high T, Type 4: spontaneous at high T, nonspontaneous at low T a. N22(g) + 3F22(g) →2NF33(g); ΔΔHoo= - 249 kJ; ΔSo= - 278 J/K b. N22(g) + 3Cl22(g) → 2NCl33(g); ΔΔHoo = 460 kJ; ΔSo= -275 J/K c. N22F44(g) → 2NF22(g); ΔΔHoo = 85 kJ; ΔSo= 198 J/K
a. iii b. ii c. iv
The normal boiling point of Br2(l) is 58.8 ∘C, and its molar enthalpy of vaporization is ΔHvap = 29.6 kJ/mol. a) When Br2(l) boils at its normal boiling point, does its entropy increase or decrease? b) Calculate the value of ΔS when 1.00 mol of Br2(l) is vaporized at 58.8 ∘C.
a. increase b. 89.17 J/K
Indicate whether each statement is true or false. (a) A reaction that is spontaneous in one direction will be nonspontaneous in the reverse direction under the same reaction conditions. (b) All spontaneous processes are fast. (c) Most spontaneous processes are reversible. (d) An isothermal process is one in which the system loses no heat. (e) The maximum amount of work can be accomplished by an irreversible process rather than a reversible one.
a. true b. false c. false d. true e. false
Indicate whether each statement is true or false. (a) ΔS is a state function. ( b) If a system undergoes a reversible change, the entropy of the universe increases. (c) If a system undergoes a reversible process, the change in entropy of the system is exactly matched by an equal and oppositechange in the entropy of the surroundings. (d) If a system undergoes a reversible process, the entropy change of the system must be zero.
a. true b. false c. true d. false (because the overall enthalpy change is zero on the enthalpy of the system or surroundings)
For the isothermal expansion of a gas into a vacuum, ΔE=0, q=0 and w=0. (a) Is this a spontaneous process? (b) Explain why no work is done by the system during this process. (c) What is the "driving force" for the expansion of the gas: enthalpy or entropy?
a. yes b. nothing to push back on (no work to be done) c. entropy
a. Using figure 19.12 as a model, sketch how the entropy of water changes as it is heated from -50C to 110 C at sea level. Show the temperatures at which there are vertical increases in entropy. b. Which process has the larger entropy change: melting ice or boiling water? Explain
b. boiling water Bonds are completely broken between molecules so the molecules are more disordered and have more freedom of movement.
Does the system loose heat during an isothermal process?
no (if phase change - all heat used to accomplish phase change - break or form bonds/why temperature does not change)
Go to page 839, 19.10 answer the true false questions
true false false false true
