Chem 211 Ch.15
A reaction taking place at 100°C has an entropy change of 200 J/K and an enthalpy change of -20 kJ. Which of the following expressions would be used to calculate ΔG under these conditions?
-20,000 - (373)(200)
Select all the options in which the first species of each pair has a higher standard entropy.
-A more complex substance vs. a simpler substance in the same phase, if their molar masses are similar -A gas at 250oC vs. the same gas at 100oC
Which of the following statements correctly reflect the relationship between the spontaneity of a reaction and the sign of ΔH?
-A negative value of ΔH favors spontaneity. -The sign of ΔH alone is not sufficient evidence to determine the spontaneity of a process.
Which of the following changes would result in an increase in the entropy of the system?
-An ice cube melts at 273 K. -A crystal of sodium chloride is heated from 275 K to 290 K. -A tire is punctured, releasing the gas trapped inside.
Select all the options that describe the behavior of a reversible reaction for which both ΔH and ΔS < 0.
-At low temperatures, ΔGrxn < 0. -The reaction will become less spontaneous as the temperature increases. -The formation of the reactants will become increasingly favored as the temperature rises.
Which of the following statements correctly describe general trends in standard entropy?
-For two monatomic species, the one with the larger molar mass will have a higher standard entropy. -For elements with two allotropic forms, the one that is more mobile will have a greater value for standard entropy
Select the statements that correctly describe the microstates of a system, W.
-Increasing the temperature of a system will increase W. -W is the number of energetically equivalent ways the particles of a system may be arranged. -Entropy increases with the number of microstates.
Which of the following statements correctly explain the increase in entropy that occurs when substance changes from solid to liquid, or liquid to gas?
-The energy of the particles increases during these phase changes. -The number of possible arrangements of the particles increases.
What effect does increasing the temperature of a gas have on its entropy, and why?
-The kinetic energy of the particles in the system increases. -There is an increase in the entropy of the system as temperature increases.
Select all the options that correctly describe the free-energy change and spontaneity of the reaction CuO (s) → Cu (s) + 1212O2 (g) at 375 K if ΔH = 168 kJ/mol and ΔS = -9.63 J/K⋅mol.
-The reaction is not spontaneous at this temperature. -ΔG = +172 kJ
Which of the following conditions are specified by standard-state conditions?
-The standard state of an element is its most stable allotropic form at standard-state temperature and pressure. -A solution will have a concentration of 1 M. -Gases are at 1 atm pressure.
Which of the following options correctly describe standard entropy?
-The units of standard entropy are Jmol/mol⋅K. -The standard entropy of a substance is its absolute entropy at 1 atm.
Which phase change will have a more dramatic increase in entropy?
-Vaporization of a substance -The mobility of the particles increases much more when this phase change occurs.
Which of the following statements correctly describe the standard free-energy of formation (ΔG°f) for a substance?
-ΔG°f for any element in its most stable allotropic form at 1 atm is equal to zero. -ΔG°f is the free energy change for the synthesis of 1 mole of a compound from its constituent elements in their standard states.
Which of the following must be known in order to assess the spontaneity of a chemical reaction or physical process at a particular set of conditions?
1) change in enthalpy 2) change in entropy
Rank the following compounds in order of decreasing standard entropies. (Place the compound with the highest entropy at the top of the list).
1. C3H8 (g) 2. CH4 (g) 3. C3H8 (l) 4. CH4 (l)
If we know that the entropy change for the surroundings is x J/K, and the system absorbed y kJ of heat during that reaction, what is the temperature at which this occurred (in K)?
1000y/x
A chemist finds that the sublimation of a compound has ΔG = -2.5 kJ at 350oC, and ΔG = +0.5 kJ at 140oC. From this information, what is a reasonable value for the temperature of sublimation?
175^oC
For a particular reaction ΔH° = -5. kJ and ΔS° = -100 J/K. What is the temperature (in K) at which this reaction favors the products?
50 K
Which of the following is an application of the second law of thermodynamics?
A gas expands because matter tends to spread out.
Select the statement that correctly describes the relationship between the entropy and spontaneity of a process.
A reaction/process in which ΔSsys > 0 and ΔSsurr > 0 will be spontaneous.
What is the difference between a spontaneous process and a nonspontaneous process?
A spontaneous process is a process that does occur under a given set of conditions while a nonspontaneous process is a process that does not occur under a given set of conditions
For a chemical reaction, ΔSorxn = ΣnSo() - ΣnSo().
For a chemical reaction, ΔSorxn = ΣnSo(products) - ΣnSo(reactants).
Match each state of matter with the appropriate standard state.
Gases =1 atm pressure Solids =Pure Elements =The most stable form at 1 atm and 25°C Solutions =1 M
The combustion of glucose (shown below) is exothermic. C6H12O6 (s) + 6O2 (g) → 6CO2 (g) + 6H2O (g) Does this reaction cause an increase or a decrease in entropy for the system and surroundings?
Increase in the entropy of the surroundings Increase in the entropy of the system
Since entropy is typically tabulated in units of _____ / K and standard enthalpies are typically tabulated in units of _____/ mol, it is usually necessary to perform a unit conversion when calculating Gibbs free energy.
J; kJ
Two unknown reactions (A and B) are analyzed. It is found that reaction A is exothermic, while reaction B is endothermic. Which of these reactions is spontaneous?
Spontaneity cannot be determined from the given information. Reason: Correct - we must also know how the entropy of the system changes to assess spontaneity. Spontaneity is determined by the sign of ΔG, and ΔG is a function of both ΔH and TΔS.
Given the balanced chemical equation CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) select the equation that should be used to calculate the total entropy change. (The physical state of the substances has been omitted to simplify the equation, but remember that the physical state of a substance has a significant effect on its entropy.)
S°[CO2] + 2(S°[H2O]) - S°[CH4] - 2(S°[O2])
Which of the following processes or changes will lead to a decrease in entropy?
Temperature decrease Condensation
Which of the following is a correct statement of the third law of thermodynamics?
The entropy of a perfect crystalline substance is 0 at 0 K.
What is entropy?
The entropy of a system is a measure of the ways in which the energy of the system can be dispersed
If the reaction shown below is to be coupled to a second reaction in order to generate an overall spontaneous process, what must be true regarding this second reaction? Fe2O3 (s) → 2 Fe (s) + 3232 O (g); ΔG = 740.98 kJ Multiple choice question.
The free-energy change must be more negative than -740.98 kJ.
Entropy will generally increase when ______.
a molecular substance dissolves in water a liquid evaporates a solid sublimes
In order to assess the spontaneity of a chemical reaction or physical process both the change in _____ and ______ associated with the reaction or process must be known.
enthalpy; entropy
The second law of thermodynamics states that the total _____ of the universe will _____ for any spontaneous process.
entropy; increase
A system is in a state of _____ when ΔSuniv = 0.
equilibrium
The Gibbs ____-energy change (symbolized by Δ______) is a measure of the spontaneity of a process, and of the useful energy available from it.
free; G
As temperature increases, entropy ______ due to a(n) ______ in the number of available energy states, and thus a(n) ______ in the number of possible arrangements of molecules within those energy states
increases, increase, increase
If a given reversible reaction has positive values for both ΔH and ΔS, the value of ΔG will become _____ negative as temperature increases, and the formation of the _____ will be increasingly favored.
more, products
ΔSsys = nR ln VfinalVinitial n = ? R=? Vfinal=? Vinitial=?
n= number of moles R=ideal gas constant Vfinal= final volume Vinitial= initial volume
In order for the products to be favored in a reaction, the standard free-energy change (ΔG°) must have a ______ sign.
negative
When the Gibbs free-energy change for a reaction is less than zero (negative), that reaction is ______ and the entropy change (ΔS) for the universe is ______.
spontaneous, positive
The ΔG for a reaction that occurs under standard-state conditions is called the ______ Gibbs free energy of reaction. The term "standard state" implies a temperature of _____oC or ______K for the reaction.
standard; 25; 298 or 298.15
In order to determine the spontaneity of a reaction or process, the entropy changes in both the ____ and the _____ must be considered.
surroundings; system
Gibbs free energy is used most prevalently to determine the spontaneity of a reaction because it is only dependent on the ______, making it much more convenient to measure experimentally.
system
Define translational, vibrational, rotational.
translational: the entire molecule moves through space vibrational: atoms in the molecule move relative to one another rotational: Atoms spin about an axis running through the center of mass of the molecule.
True or false: The state with the largest number of possible arrangements has the greatest entropy.
true! reason: The most probable state is the one with the largest number of energetically equivalent possible arrangements: S = k ln W.
Which of the following types of energy are forms of motional energy?
vibrational rotational translational
The sign of which quantity indicates whether a reaction or process will occur spontaneously?
ΔG
Match the value of the Gibbs free-energy change for a reaction with its implication.
ΔG < 0 spontaneous process ΔG > 0 nonspontaneous process ΔG = 0 process at equilibrium
Which of the following is the correct form of the change in the Gibbs equation for a process occurring at constant temperature?
ΔGsys = ΔHsys - TΔSsys
Match each thermodynamic quantity with the information it provides about a given reaction.
ΔG° < 0 Products are favored at equilibrium. ΔG° > 0 Reactants are favored at equilibrium. ΔG < 0 Reaction will occur spontaneously. ΔG > 0 Reaction will be nonspontaneous.
Match the sign of ΔG° with the direction of reaction that is favored for a system at equilibrium.
ΔG° > 0 Favors reactants ΔG° < 0 Favors products
At a certain temperature, the change in entropy of the system is calculated to be ΔSsys. If the system is at equilibrium, what is the value of ΔSsurr under these conditions?
ΔSsurr = -ΔSsys
A particular process results in a decrease in the entropy of the system. If this process is spontaneous, what must be true about the entropy change of the surroundings?
ΔSsurr > -ΔSsys
Which of the following options correctly show how to calculate the entropy change of a system?
ΔSsys= nR ln VfinalV/initial ΔSsys = k ln Wfinal - k ln Winitial
In each of the following pairs, which species will have a greater value for standard entropy? (i) F (g) vs. H2O (g) (ii) H2O2 (l) vs. H2O2 (g)
(i) H2O (g) (ii) H2O2 (g)
Calculate the entropy change of the surroundings in J/mol⋅K when 30 kJ of heat is released by the system at 27°C.
+100
Which of the following changes are favored in a spontaneous process?
An increase in entropy An overall decrease in energy
In which cases do the substance(s) on the left have a higher entropy than the substance(s) on the right?
C3H8 (g) vs. C2H6 (g) 2NO2 (g) vs. N2O4 (g)
What is the entropy of a system in which there is only one microstate?
S =0
What does each symbol mean in the equation S=k ln W
S: entropy k: boltzmann constant ln: natural logarithm W: number of possible arrangements
True or false: If a process occurs spontaneously in one direction under a certain set of conditions, it does not occur spontaneously in the opposite direction under the same conditions.
True! Reason: If a process occurs spontaneously in one direction under a certain set of conditions, it is not possible for the process to also occur spontaneously in the opposite direction under the same set of conditions.
W represents the number of possible arrangements, where mathematically W = XN. X represents the number of ______ containing N ________.
cells; molecules
The third law of thermodynamics states that a perfect _____ of a pure solid substance has _____ entropy at 0 K.
crystal; 0
The ______ entropy of a substance is its absolute entropy at 1 atm.
standard
True or false: Many nonspontaneous biochemical reactions couple with other reactions which supply enough free energy to drive them all.
true
Which of the following values must be known in order to calculate the change in Gibbs free energy using the Gibbs equation?
ΔHsys T ΔSsys
The reaction 4Fe (s) + 3O2 (g) + 6H2O (l) → 4Fe(OH)3 (s) is spontaneous at temperatures below 1950°C. If ΔH = -1582 kJ, calculate the entropy change for this reaction at 1950°C.
ΔS = -712 J/K