Chapter 17

ΔS(surr) =

-ΔH

Which of the following is consistent with a spontaneous process in the forward direction? A) ΔSuniv > 0, ΔG < 0, TΔSuniv > 0 D) ΔSuniv < 0, ΔG < 0, TΔSuniv < 0 B) ΔSuniv < 0, ΔG > 0, TΔSuniv < 0 E) ΔSuniv > 0, ΔG > 0, TΔSuniv > 0 C) ΔSuniv > 0, ΔG < 0, TΔSuniv < 0

A) ΔSuniv > 0, ΔG < 0, TΔSuniv > 0

Which of the following is consistent with a spontaneous endothermic reaction? A) DH > 0, DS < 0, DG < 0 D) DH < 0, DS > 0, DG > 0 B) DH > 0, DS > 0, DG < 0 E) DH > 0, DS < 0, DG > 0 C) DH < 0, DS < 0, DG < 0

B) DH > 0, DS > 0, DG < 0

The normal melting point sulfur is 113 °C. If a sample of solid sulfur is at 95 °C, Predict the signs of ΔH, ΔS, and ΔG for the melting process at this temperature. A) DH > 0, DS > 0, DG < 0 D) DH < 0, DS > 0, DG > 0 B) DH > 0, DS > 0, DG > 0 E) DH < 0, DS < 0, DG > 0 C) DH > 0, DS < 0, DG < 0

B) ΔH > 0, ΔS > 0, ΔG > 0

Which of the following is consistent with a nonspontaneous process in the forward direction? A) ΔSuniv > 0, ΔG < 0, TΔSuniv > 0 D) ΔSuniv < 0, ΔG < 0, TΔSuniv < 0 B) ΔSuniv < 0, ΔG > 0, TΔSuniv < 0 E) ΔSuniv > 0, ΔG > 0, TΔSuniv > 0 C) ΔSuniv > 0, ΔG < 0, TΔSuniv < 0

B) ΔSuniv < 0, ΔG > 0, TΔSuniv < 0

Which of the following is consistent with an exothermic reaction that is spontaneous at all temperatures? A) DH > 0, DS < 0, DG < 0 D) DH < 0, DS > 0, DG < 0 B) DH > 0, DS > 0, DG < 0 E) DH > 0, DS < 0, DG > 0 C) DH < 0, DS < 0, DG < 0

D) DH < 0, DS > 0, DG < 0

With respect to the system only, a reaction with DH > 0 and DS < 0 is predicted to be: A) Spontaneous at all temperatures B) Spontaneous at high temperatures only C) Spontaneous at low temperatures only D) Nonspontaneous at all temperatures

D) Nonspontaneous at all temperatures

Which of the following is consistent with an exothermic reaction that is nonspontaneous at high temperatures? A) DH > 0, DS < 0, DG < 0 D) DH < 0, DS > 0, DG > 0 B) DH > 0, DS > 0, DG < 0 E) DH < 0, DS < 0, DG > 0 C) DH < 0, DS > 0, DG < 0

E) DH < 0, DS < 0, DG > 0

ΔG = 0

Equilibrium

Microstates

How something can be arranged

Which of the following is expected to have zero entropy? I. N2(g) at 273 K II. SiO2(s, amorphous) at 0 K III. NaCl(s) perfectly ordered crystal at 25 K IV. Na(s) perfectly ordered crystal at 0 K

IV only

T/F Spontaneous reactions need to be exothermic.

No, they can be both exo and endothermic.

ΔG > 0

Nonspontaneous (spontaneous in the opposite direction)

Negative ΔH and Negative ΔS

Reaction proceeds spontaneously at low temps. High temps, the reverse reaction becomes spontaneous.

ΔG < 0

Spontaneous

Positive ΔH and Positive ΔS

Spontaneous at high temps, reverse reaction at low temps

Examples of spontaneous reactions

Sugar dissolving in water but not reforming. Rust forming but not disappearing Water running down a waterfall but not going up

Entropy

a measure of how spread out or dispersed the energy of a system is among the different possible ways that system can contain energy

How does the entropy of a system change for each of the following processes? a. A solid melts b. A liquid freezes. c. A liquid boils. d. A vapor is converted to a solid. e. A vapor condenses to a liquid. f. A solid sublimes. g. Urea dissolves in water.

a. A solid melts - increase b. A liquid freezes. - decreases c. A liquid boils. - increases d. A vapor is converted to a solid. - decreases e. A vapor condenses to a liquid. - decreases f. A solid sublimes. - increases g. Urea dissolves in water. - increases

For each pair of substances listed here, choose the one having the larger standard entropy value at 25ºC. The same molar amount is used in the comparison. Explain the basis for your choice. a. Li(s) or Li(l) b. C2H5OH(l) or CH3OCH3(l) c. Ar(g) or Xe(g) d. CO(g) or CO2(g) e. O2(g) or O3(g) f. NO2(g) or N2O4(g)

a. Li(l) - liquid phase molecules have more intermolecular spaces than solid. b. CH3OCH3 (l) has the ability to form hydrogen bonds C2H5OH(l) thus it has a larger standard entropy value c. Xe (g) has more entropy than Ar (g) because 1 mol Ar has less mass than 1 mole of Xe (g). d. CO2 (g) has larger entropy value than CO (g) because it has a larger mass. e. O3 (g) has larger entropy value than O2 (g) because it has more O, therefore more mass. f. N2O4(g) has greater entropy value than NO2(g) since N2O4 is a dimer of NO2.

First Law of thermodynamics

also known as Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system.

Entropy increases as

disorder increases

The third law of thermodynamics

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

Many biological reactions are

nonspontaneous. They are driven by the hydrolysis of ATP, for which ΔG⁰ is negative.

Standard entropy of reaction (ΔS'rxn)

the difference in standard entropies between products and reactants

The greater the dispersal of energy, the greater

the entropy

The second law of thermodynamics

the entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.

Standard free-energy of reaction (ΔGrnx)

the free-energy change for a reaction when it occurs under standard state conditions, when reactants in their standard states are converted to products in their standard states.

Negative ΔH and Positive ΔS

ΔG is always negative. Reaction proceeds spontaneously at all temps

Positive ΔH and Negative ΔS

ΔG is always positive. Reverse reaction always spontaneous at all temps