Physical Chemistry : Enthalpy and Entropy

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Enthalpy And Entropy

Which of the following is true regarding enthalpy and entropy?

Possible Answers:

The enthalpy of liquid water is nonzero whereas enthalpy of hydrogen gas is zero

Both of these are true

None of these are true

The entropy of liquid water is higher than the entropy of hydrogen gas

Correct answer:

The enthalpy of liquid water is nonzero whereas enthalpy of hydrogen gas is zero

Explanation:

Enthalpy is the amount of internal energy contained in a compound whereas entropy is the amount of intrinsic disorder within the compound. Enthalpy is zero for elemental compounds such hydrogen gas and oxygen gas; therefore, enthalpy is nonzero for water (regardless of phase). Entropy, or the amount of disorder, is always highest for gases and lowest for solids. This is because gas molecules are widely spread out and, therefore, are more disordered than solids and liquids. Hydrogen gas will have a higher entropy than liquid water.

Example Question #1 : Enthalpy And Entropy

According to the law of thermodynamics, which of the following statement(s) is/are true?

I. Enthalpy of a system is always increasing

II. Entropy of a system is always increasing

III. Absolute entropy can never be negative

Possible Answers:

II and III

II only

III only

I and II

Correct answer:

III only

Explanation:

The first law of thermodynamics states that the energy of the universe is always constant, which implies that energy cannot be created or destroyed. The energy lost by a system is gained by surroundings and vice versa; however, the total energy of the universe is always constant. The second law of thermodynamics states that the entropy, or the amount of disorder in the universe, is always increasing. This suggests that the universe is always going towards a more disordered state. Based on these two laws, we can determine that statement I and statement II are false. Note that these two statements are talking about the system, rather than the universe. The energy (in the form of enthalpy) and entropy can increase or decrease in a system. The surroundings will compensate accordingly to keep the energy of universe constant and increase the entropy. Absolute entropy of a system, surroundings or the universe can never be negative because it isn’t possible to have negative disorder (this is due to the definition of entropy; just remember that entropy can never be negative). Note that the change in entropy can, however, be negative.

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