Ch. 20 Pt. 1 Smartbook

The formation of water from its elements is exothermic (ΔHsys = -572 kJ/mol) and spontaneous although the reaction results in a large decrease in the entropy of the system: 2H2(g) + O2(g) → 2H2O(ℓ) ΔSsys = -327 J/K Which of the following is the best explanation how this process is able to obey the second law of thermodynamics that all spontaneous processes increase the entropy of the universe?

The reaction is highly exothermic and the surroundings increase in entropy.

Calculate ΔSuniv and identify the process CO2 (g) → CO2 (aq) as spontaneous, nonspontaneous, or at equilibrium at 25C. CO2 = 213.6 S(J/molxK) -393.5 H(kJ/mol) CO2 = 121.3 S(J/molxK) -412.9 H(kJ/mol)

The reaction is nonspontaneous at this temperature. ΔSuniv = -27.2 J/mol⋅K ΔSsys = 121.3 - 213.6 = -92.3 J/mol⋅K ΔSsurr = - (-412.9-(-393.5)/298) = -(-19.4/298) = +0.0651 kJ/mol⋅K = +65.1 J/mol⋅K ΔSuniv = +65.1 - 92.3 = -27.2 J/mol⋅K

The combustion of glucose, represented by the equation C6H12O6 (s) + 6O2 (g) → 6CO2 (g) + 6H2O (g), is an exothermic reaction. Does this reaction cause an increase or decrease in entropy for the system and its surroundings?

Increase in ΔSsys Increase in ΔSsurr

Which statements correctly describe the entropy changes that occur when an ionic solid dissolves in water?

The dissociation of an ionic solute in water causes the entropy of the system to increase. The hydration of highly charged ions generally results in a decrease in the entropy of the system. The mobility of H2O molecules in such a solution is reduced since they are arranged in a specific way around the dissolved ions.

In an exothermic reaction, heat is _____ the surroundings. The particles of the surroundings have _____ freedom of motion and ΔSsurr therefore _____.

released to; greater; increases

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

Which of the conditions described below will result in a spontaneous reaction?

ΔSsys = -20 J/K; ΔSsurr = 25 J/K ΔSsys = 30 J/K; ΔSsurr = 10 J/K (ΔSuniv = ΔSsys + ΔSsurr must be > 0 for a spontaneous reaction)

In which of the following options does the first species of each pair have a greater standard entropy?

A gas at 250°C vs. the same gas at 100°C A more complex substance vs. a simpler substance in the same phase if their molar masses are similar

Which of the following is an application of the second law of thermodynamics?

A gas expands because matter tends to spread out.

Which of the following factors increase the likelihood that a reaction will be spontaneous?

A liquid is generated from a solid. A gas is generated from a liquid. Energy is released in the form of heat.

Which of the following statements correctly reflect the relationship between the spontaneity of a reaction and the sign of ΔH?

It is possible for an exothermic process to be nonspontaneous. An endothermic process may be spontaneous under one set of conditions but nonspontaneous at a different temperature and pressure.

Calculate the standard entropy change for the reaction 2A + 3B → 2C. A = 1 J/mol x K B = 4 J/mol x K C = 3 J/mol x K

-8 J/mol⋅K ΔSo = Sproducts - Sreactants = 2(3) - [3(4) + 2(1)] = -8 J/mol⋅K

Select the definition of entropy in terms of the number of microstates over which the energy of a system can be dispersed.

S = k ln W

A system is in a state of ___when ΔSuniv = 0.

equilibrium

Which of the following reactions are predicted to have a positive value for ΔSrxn?

H2CO3 (aq) → H2O (l) + CO2 (g) N2O4 (g) → 2NO2 (g)

For a chemical reaction, ∆S°rxn = ΣnS°(___) − ΣnS°(___).

products, reactants

Calculate the entropy change of the surroundings in J/mol⋅K when 30 kJ of heat is released by the system at 27C.

+100 ΔSsurr = - ΔHsys/T = - (-30,000J)/300K = +100 J/K

Calculate the standard entropy change for the following reaction at 25oC. Use the values provided in the table below. 2CO (g) + O2 (g) → 2CO2 (g) CO = 197.9 J/mol x K O2 = 205.0 J/mol x K CO2 = 213.6 J/mol x K

-173.6 J/mol⋅K ΔSrxn = 2[213.6] - [2(197.9) + 205.0] = -173.6

If we know that the entropy change for a reaction 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

What is a spontaneous process?

A process that occurs under specified conditions without a continuous input of energy

What effect does increasing the temperature of a gas have on its entropy, and why?

As the temperature of a system is increased, all types of kinetic energies increase. The entropy of a system increases as the temperature increases.

Rank the following compounds in order of decreasing standard molar entropies. Place the compound with the highest entropy at the top of the list.

C3H8 (g) CH4 (g) CH4 (l)

Which of the following correctly describes the first law of thermodynamics?

Energy can be converted from one form to another, but it cannot be created or destroyed.

Which of the following options correctly describe entropy?

Entropy is a measure of the energy dispersal of a system. The greater the freedom of motion of particles in a system, the greater the entropy of the system.

Which of the following statements correctly describe the general trends in standard entropy?

For elements with two allotropic forms, the one that is more mobile will have a greater value for standard entropy. For two monatomic species, the one with the larger molar mass will have a higher standard entropy.

H2O (l) → H2O (s); ΔSsys = -22 J/K The process shown above is spontaneous at temperatures below 0oC, even though the entropy of the system is decreasing. Which of the following is the best explanation for the way in which this process obeys the second law of thermodynamics?

Freezing is exothermic; the surroundings increase in entropy.

Given the balanced chemical equation CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g), which of the following equations should be used to calculate the total entropy change for this reaction?

S[CO2] + 2(S[H2O]) - S[CH4] - 2(S[O2])

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.

Which of the following statements correctly explain the increase in entropy that occurs when a substance changes from a solid to a liquid, or from a liquid to a gas?

The energy of the particles increases during these phase changes. The freedom of movement of the particles increases in the change solid → liquid → gas.

Which of the following is a correct description of the third law of thermodynamics?

The entropy of a perfect crystalline substance is 0 at 0 K.

Select which phase change, melting or vaporization, will have a greater increase in entropy for a given substance.

The mobility of the particles increases much more when this phase change occurs. Vaporization of a substance

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. Gases are at a pressure of 1 atm. Solutions are at a concentration of 1 M.

The third law of thermodynamics states that a perfect ___ of a pure solid substance has ___ entropy at a temperature of 0 K.

crystal, zero

The ___ of a system, symbolized by the letter S, is related to the number of different possible ways the energy of a system can be dispersed.

entropy

The second law of thermodynamics states that the total _____ of the universe will _____ for any spontaneous process.

entropy; increase

A key factor that influences reaction spontaneity is changes in the ___ of motion of the particles in the system, i.e. the ___ of the energy of motion of the particles.

freedom, dispersion

The dissolution of a molecular solute in a polar solvent (e.g., sugar dissolving in water) typically results in a(n) _____ in entropy because both the molecules in the system and their energies are dispersed in a _____ volume.

increase; larger

As the temperature of a system increases, the 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

How is the number of microstates in a system, W, related to the entropy of the system, S?

ln W ∝ S

The different possible ways for arranging the particles of a system are called _____. The greater the number of these states, the _____ the entropy of the system.

microstates; larger

Entropy is a ___ function, and the change in entropy for a process therefore depends only on the ___ between the final and initial states, not on the path taken for the process.

state, difference

When heat is transferred from the system to the surroundings, the amount of energy that is dispersed is greater when the temperature is low. Which of the following reflect this relationship (at constant pressure)?

ΔSsurr = - ΔHsys/T ΔSsurr = - qsys/T

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 reflect the relationships between ΔSsurr, ΔHsys, and T (at constant pressure so qsys = ΔHsys)?

ΔSsurr is inversely proportional to temperature. If ΔHsys is negative, ΔSsurr will be positive.