Ch.19 - Chemical Thermodynamics

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.19 - Chemical Thermodynamics

Problem 23a

Chapter 19, Problem 23a

The normal boiling point of Br₂(l) is 58.8 °C, and its molar enthalpy of vaporization is ΔHvap = 29.6 kJ/mol. (a) When Br₂(l) boils at its normal boiling point, does its entropy increase or decrease?

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Identify the phase change occurring: Boiling is a phase change from liquid to gas.

Understand the nature of the phases: In the liquid phase, molecules are more ordered compared to the gas phase where molecules are more dispersed and have higher randomness.

Relate entropy to randomness: Entropy, a measure of disorder or randomness in a system, increases when a substance transitions from a more ordered state (liquid) to a less ordered state (gas).

Apply this understanding to the boiling of Br₂(l): As Br₂(l) boils and changes from liquid to gas, the entropy of the system increases due to the increased disorder and freedom of movement of the gas molecules compared to the liquid.

Conclude the effect on entropy: Therefore, when Br₂(l) boils at its normal boiling point, its entropy increases.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Entropy

Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it is often associated with the number of ways a system can be arranged. When a substance transitions from a liquid to a gas, such as bromine (Br2) boiling, the molecules move from a more ordered state to a less ordered state, leading to an increase in entropy.

Phase Change

A phase change refers to the transition of a substance from one state of matter to another, such as from liquid to gas during boiling. This process involves energy changes, specifically the enthalpy of vaporization, which is the energy required to convert a unit amount of a liquid into vapor at its boiling point. Understanding phase changes is crucial for analyzing changes in entropy.

Enthalpy of Vaporization

The enthalpy of vaporization (ΔHvap) is the amount of energy required to convert one mole of a liquid into vapor at constant temperature and pressure. For bromine, this value is 29.6 kJ/mol. This energy input not only facilitates the phase change but also contributes to the increase in entropy, as the molecules in the gas phase have greater freedom of movement compared to those in the liquid phase.

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Textbook Question

A system goes from state 1 to state 2 and back to state 1. (c) Suppose the changes in state are reversible processes. Is the work done by the system upon going from state 1 to state 2 the same or different as compared to that upon going from state 2 back to state 1?

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Textbook Question

The normal boiling point of Br₂(𝑙) is 58.8 °C, and its molar enthalpy of vaporization is Δ𝐻_vap=29.6kJ/mol. (b) Calculate the value of Δ𝑆 when 1.00 mol of Br2(𝑙) is vaporized at 58.8 °C.

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Textbook Question

The element gallium (Ga) freezes at 29.8 °C, and its molar enthalpy of fusion is ΔH_fus = 5.59 kJ/mol. (a) When molten gallium solidifies to Ga(s) at its normal melting point, is ΔS positive or negative?

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Textbook Question

Indicate whether each statement is true or false. (a) ΔS is a state function. (b) If a system undergoes a reversible change, the entropy of the universe increases. (c) If a system undergoes a reversible process, the change in entropy of the system is exactly matched by an equal and opposite change in the entropy of the surroundings. (d) If a system undergoes a reversible process, the entropy change of the system must be zero.

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Textbook Question

The element gallium (Ga) freezes at 29.8 °C, and its molar enthalpy of fusion is ΔH_fus = 5.59 kJ/mol. (b) Calculate the value of ΔS when 60.0 g of Ga(l) solidifies at 29.8 °C.

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Textbook Question

Consider a system consisting of an ice cube. (a) Under what conditions can the ice cube melt reversibly?

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The normal boiling point of Br2(l) is 58.8 °C, and its molar enthalpy of vaporization is ΔHvap = 29.6 kJ/mol. (a) When Br2(l) boils at its normal boiling point, does its entropy increase or decrease?

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Key Concepts

Entropy

Phase Change

Enthalpy of Vaporization

The normal boiling point of Br₂(l) is 58.8 °C, and its molar enthalpy of vaporization is ΔHvap = 29.6 kJ/mol. (a) When Br₂(l) boils at its normal boiling point, does its entropy increase or decrease?