Unit 9 Thou Shalt Not Forget

• I only

• I and III only

• II only

• II and III only

Answer explanation

II is correct because steam condensing to liquid water decreases disorder, leading to a negative \Delta S. I increases disorder (positive \Delta S) and III also increases disorder as solid decomposes into gas and solid.

Tags

NGSS.HS-PS3-4

The endothermic process among the following is

• Natural gas is combusted in a Bunsen burner.

• Water vapor condenses on the outside of a glass of iced tea.

• A cop of hot water cools as a tea bag brews.

• An ice cube melts on a countertop.

Answer explanation

An ice cube melting on a countertop is an endothermic process because it absorbs heat from the surroundings to change from solid to liquid, while the other options either release heat or involve condensation.

• Positive, positive

• Positive, negative

• Negative, positive

• Negative, negative

Answer explanation

The dissociation of ammonium nitrate in water is an endothermic process, leading to a positive \Delta H. The increase in disorder as solid dissolves results in a positive \Delta S, making the correct answer positive, positive.

The 3rd Law of Thermodynamics indicates that

• enthalpy of the universe is a constant

• entropy of the universe increases

• sum of the entropies of the system plus surroundings is zero

• entropy of a perfectly ordered pure crystal is zero at absolute zero

Answer explanation

The 3rd Law of Thermodynamics states that the entropy of a perfectly ordered pure crystal approaches zero as the temperature approaches absolute zero, making this the correct choice.

The 1st Law of Thermodynamics indicates that

• work is force exerted over a distance.

• energy in the universe is finite.

• an increase in disorder is part of any spontaneous process.

• work is a state function.

Answer explanation

The 1st Law of Thermodynamics states that energy cannot be created or destroyed, only transformed, indicating that the total energy in the universe is finite. This makes 'energy in the universe is finite' the correct choice.

Tags

NGSS.HS-PS3-1

NGSS.HS-PS3-2

Entropy of the universe is

• always zero.

• always constant.

• always increasing.

• always decreasing.

Answer explanation

The entropy of the universe is always increasing due to the second law of thermodynamics, which states that in an isolated system, entropy tends to rise over time, leading to greater disorder.

A process which causes a decrease in the entropy is

• condensation of steam to form liquid water.

• vaporization of liquid water to form steam.

• dissociation of solid sodium chloride into aqueous sodium cations and chloride anions

• sublimation of snow to water vapor.

Answer explanation

Condensation of steam to form liquid water decreases entropy as it transitions from a more disordered gas phase to a more ordered liquid phase, resulting in a decrease in randomness.

Answer explanation

The process MgO(s) + CO2(g) → MgCO3(s) involves the formation of a solid from a solid and a gas, leading to a decrease in disorder (entropy), thus ΔS is negative.

Answer explanation

NaCl has the smallest value for S° due to its simple ionic structure and lower lattice energy compared to the other compounds listed, which have more complex structures and higher lattice energies.

Which of the following substances is expected to have the largest standard molar entropy?

Answer explanation

The standard molar entropy increases with the complexity and gaseous state of a substance. Among the choices, H2O2(g) has more atoms and is in the gas phase, leading to the highest entropy compared to the others.