Problem 71 - Chemistry and Chemical Reactivity

Question

Determine whether energy as heat is evolved or required, and whether work was done on the system or whether the system does work on the surroundings, in the following processes at constant pressure: (a) Liquid water at \(100^{\circ} \mathrm{C}\) is converted to steam at \(100^{\circ} \mathrm{C}\) (b) Dry ice, \(\mathrm{CO}_{2}(\mathrm{s}),\) sublimes to give \(\mathrm{CO}_{2}(\mathrm{g})\)

Step-by-Step Solution

Answer

In both processes (a) and (b), heat is required and the system does work on the surroundings.

Step 1: Analyze the Process for Part (a)

The conversion of liquid water at \(100^{\circ} \mathrm{C}\) to steam at \(100^{\circ} \mathrm{C}\) involves a phase change from liquid to gas. This is an endothermic process, meaning it requires energy in the form of heat to overcome the intermolecular forces holding the liquid together. Therefore, energy as heat is required.

Step 2: Determine Work for Part (a)

Because the liquid water expands into steam (a gas), the system does work on the surroundings as it pushes against external atmospheric pressure to expand.

Step 3: Summarize Part (a)

In part (a), energy as heat is required for the phase change, and the system does work on the surroundings.

Step 4: Analyze the Process for Part (b)

Dry ice subliming from \(\mathrm{CO}_{2}( ext{solid})\) to \(\mathrm{CO}_{2}( ext{gas})\) involves a phase change from solid to gas. This is also an endothermic process, needing energy as heat to break the molecular forces within the solid structure. Hence, heat is required.

Step 5: Determine Work for Part (b)

Just like the conversion of water to steam, when dry ice sublimes, it expands from a solid to a gas, resulting in the system doing work on the surroundings as it pushes against atmospheric pressure.

Step 6: Summarize Part (b)

In part (b), energy as heat is required for sublimation, and the system does work on the surroundings.

Key Concepts

Phase Change

A phase change occurs when a substance transforms from one state of matter to another, like from solid to liquid, liquid to gas, or directly from solid to gas. Each of these transitions requires a change in energy. During a phase change, the temperature of the substance remains constant despite the energy input or release. This is because the energy is used to alter the bonding forces between molecules, rather than changing the temperature.

• Melting: Solid to liquid
• Freezing: Liquid to solid
• Evaporation/Boiling: Liquid to gas
• Condensation: Gas to liquid
• Sublimation: Solid to gas directly
• Deposition: Gas to solid directly

Understanding phase changes is essential for studying thermodynamic processes and their impact on energy and work in different systems.

Endothermic Process

An endothermic process is one where a system absorbs energy from its surroundings in the form of heat. These processes require an input of energy to proceed since they involve breaking bonds or overcoming forces among particles. Examples of endothermic processes include melting, boiling, and sublimation.
For instance, when water boils at its boiling point, it absorbs heat, which allows molecules to break free from the liquid state and enter the gaseous state. Similarly, during sublimation, energy is absorbed to transition directly from a solid to a gas.

• Heat absorption is necessary.
• Results in an increase in the system's internal energy.
• Typical in phase changes where a state transition occurs without temperature rise.

An understanding of endothermic processes assists in predicting whether a substance will require energy input to change its state under specific conditions.

Work and Energy

In thermodynamics, 'work' and 'energy' describe how systems interact with their surroundings. When a system does work, it transfers energy to its surroundings, usually by applying a force over a distance. Conversely, when work is done on a system, energy is transferred into the system from its surroundings.
During phase changes like the conversion of liquid water to steam or the sublimation of dry ice, work is done by the system as it expands (e.g., forming gas). This expansion means pushing against external forces, like atmospheric pressure, which requires energy.

• Work done by the system: Energy flows out.
• Work done on the system: Energy flows in.
• Phase changes often involve external energy input to counteract environmental forces.

Understanding work in the context of energy transitions helps clarify why energy input is crucial in phase changes, particularly in endothermic processes.

Sublimation

Sublimation is a fascinating phase change where a substance transitions directly from a solid to a gas, without passing through the liquid state. This process is endothermic, meaning it requires heat energy input to occur. Not all substances undergo sublimation easily; carbon dioxide in its solid form (dry ice) sublimates under standard atmospheric pressure, illustrating this process.

• Sublimation implies absorption of heat.
• Requires breaking molecular forces within a solid structure.
• Results in the system doing work on the surroundings by expanding as gas.

Understanding sublimation is essential because it explains behaviors in materials that bypass liquid phases, impacting industrial and natural processes significantly.