Chapter 5: Problem 113
Construct a table with the headings \(q, w, \Delta U,\) and \(\Delta H\). For each of the following processes, deduce whether each of the quantities listed is positive \((+),\) negative (-), or zero (0): (a) freezing of benzene, (b) reaction of sodium with water, (c) boiling of liquid ammonia, (d) melting of ice, (e) expansion of a gas at constant temperature.
Short Answer
Step by step solution
Understanding the Exercise
Thermodynamic Definitions
Processing Each Process
Process (a): Freezing of Benzene
Process (b): Reaction of Sodium with Water
Process (c): Boiling of Liquid Ammonia
Process (d): Melting of Ice
Process (e): Expansion of a Gas at Constant Temperature
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Thermal Energy Exchange
For instance:
- When a substance like benzene freezes, it releases heat, making \(q < 0\).
- Conversely, when ice melts or ammonia boils, the system absorbs heat, resulting in \(q > 0\).
- During the isothermal expansion of a gas, heat is typically added to maintain constant temperature, hence \(q > 0\).
Phase Change
- Freezing of Benzene: A phase change from liquid to solid, releasing energy.
- Boiling of Liquid Ammonia: The transformation from liquid to gas, requiring the absorption of heat.
- Melting of Ice: Solid to liquid conversion, which absorbs energy.
Enthalpy
- Freezing of Benzene: Since heat is released, \(\Delta H < 0\).
- Boiling of Ammonia: Here, energy is absorbed, making \(\Delta H > 0\).
- Melting of Ice: Energy requirement for melting results in \(\Delta H > 0\).
Internal Energy
- Exothermic Reactions (e.g., Sodium with Water): Result in \(\Delta U < 0\) as energy is released.
- Endothermic Processes (e.g., Melting of Ice): Lead to \(\Delta U > 0\) since energy is absorbed.
- Isothermal Expansion: \(\Delta U = 0\) because there is no change in temperature thus no net energy change.
Work Done
- Work Done by System: In the expansion of gases, like in sodium reacting with water, the system does work, thus \(w < 0\).
- Phase Changes: Like freezing or melting, have no volume change because of constant pressure, hence \(w = 0\).
- Isothermal Gas Expansion: In this, work is done by the expanding gas, making \(w < 0\).