Thermodynamics Conceptual

2 H_2(g) + O_2(g) → 2 H₂O_(g)

For the reaction represented above at 25°C, what are the signs of ΔH°, ΔS°, and ΔG°?

• +ΔH° +ΔS° +ΔG°

• +ΔH° -ΔS° -ΔG°

• -ΔH° -ΔS° -ΔG°

• -ΔH° -ΔS° +ΔG°

2 H_2(g) + O_2(g) → 2 H₂O_(g)

When H_2(g) and O_2(g) are mixed together in a rigid reaction vessel at 25°C, no reaction occurs. When the mixture is sparked, however, the gases react vigorously according to the equation above, releasing heat. Which of the following statements correctly explains why the spark is needed for the reaction to occur when the gases are originally at 25°C?

• The reaction is not thermodynamically favorable at 25°C.

• ΔH° for the reaction has a large positive value at 25°C.

• ΔS° for the reaction has a large negative value at 25°C.

• The reaction has a large activation energy at 25°C.

A cube of ice is added to some hot water in a rigid, insulated container, which is then sealed. There is no heat exchange with the surroundings. What has happened to the total energy and the total entropy when the system reaches equilibrium?

• Energy - Remains constant

  Entropy - Remains constant

• Energy - Remains constant

  Entropy - Decreases

• Energy - Remains constant

  Entropy - Increases

• Energy - Decreases

  Entropy - Increases

Two molecules of the amino acid glycine join through the formation of a peptide bond, as shown above. The thermodynamic data for the reaction are listed in the following table.
ΔG° = + 15 kJ/mol
ΔH° = + 12 kJ/mol
ΔS° = - 10 J/K mol

Based on the thermodynamic data, which of the following is true at 298 K?

• Keq = 0

• 0 < Keq < 1

• Keq = 1

• Keq > 1

Two molecules of the amino acid glycine join through the formation of a peptide bond, as shown above. The thermodynamic data for the reaction are listed in the following table.

ΔG° = + 15 kJ/mol

ΔH° = + 12 kJ/mol

ΔS° = - 10 J/K mol

Under which of the following temperature conditions is the reaction thermodynamically favored?

• It is only favored at high temperatures.

• It is only favored at low temperatures.

• It is favored at all temperatures.

• It is not favored at any temperature.

K_(s) + 1/2 Cl_2(g) → KCl_(s) ΔH = -437 kJ/mol

The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table to answer the questions that follow.

Cl_2(g) + 2e- → 2Cl^-_(g)

Which of the following expressions is equivalent to ΔH^o for the reaction represented above?

• x+y

• x-y

• x + 2y

• (x/2) - y

CO_(g) + 2 H_2(g) ⇄ CH₃OH_(g)

The synthesis of CH₃OH_(g) from CO_(g) and H_2(g) is represented by the equation above. The value of K_c for the reaction at 483 K is 14.5.

Which of the following statements is true about bond energies in this reaction?

• The energy absorbed as the bonds in the reactants are broken is greater than the energy released as the bonds in the product are formed.

• The energy released as the bonds in the reactants are broken is greater than the energy absorbed as the bonds in the product are formed.

• The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed.

• The energy released as the bonds in the reactants are broken is less than the energy absorbed as the bonds in the product are formed.

The combustion of C₂H₅OH is represented by the equation above and the standard entropy and enthalpy changes for the reaction are provided. When the reactants are combined at 25^oC, essentially no CO_2(g) or H₂O_(g) is produced after a few hours. Which of the diagrams above could best help explain the low yield of the reaction under these conditions, and why?

• Diagram 1, because it represents a reaction that is not thermodynamically favorable with , regardless of its reaction rate.

• Diagram 1, because it represents a reaction that reaches equilibrium quickly after a very small amount of the reactants is consumed.

• Diagram 2, because it represents a reaction with a high activation energy barrier for molecules to overcome and a very slow reaction rate, even if it is thermodynamically favorable with ΔG < 0

• Diagram 2, because it represents a reaction that is thermodynamically favorable with ΔH < 0 the products formed are unstable and quickly revert to form reactants.

The chemical equation above represents the exothermic reaction of CH_{4 (g)} with O_2(g). Which of the following best helps to explain why the reaction is thermodynamically favored (ΔG < 0) at 2000 K and 1 atm?

• The total number of gaseous product molecules is less than the total number of gaseous reactant molecules, thus ΔS < 0

• The total number of gaseous product molecules is greater than the total number of gaseous reactant molecules, thus ΔS > 0.

• The amount of energy released when the product bonds form is much less than the amount of energy needed to break the reactant bonds.

• The amount of energy released when the product bonds form is much greater than the amount of energy needed to break the reactant bonds.

2 POCl_3(g) ↔ 2 PCl_3(g) + O_2(g) ΔG = +490 kJ/mol

A sample of POCl_3(g) is placed in a closed, rigid container at 298 K and allowed to reach equilibrium according to the equation above. Based on the value for ΔG which of the following is true?

• K << 1 and at equilibrium POCl_3(g) >> PCl_3(g)

• K > 1 and at equilibrium O_2(g) > POCl_3(g)

• K < 1 and at equilibrium POCl_3(g) < PCl_3(g)

• K >> 1 and at equilibrium O_2(g) >> POCl_3(g)