Question

Define these terms: (a) molar heat of vaporization, (b) molar heat of fusion, (c) molar heat of sublimation. What are their units?

Short answer

Molar heat of vaporization is the heat required to vaporize a mole of a substance. Molar heat of fusion is the heat absorbed by a mole of a solid substance to turn into a liquid. Molar heat of sublimation is the energy required to directly convert a mole of substance from solid to gas. Common units are kilojoules per mole (kJ/mol) or calories per mole (cal/mol).

Step-by-step solution

Define molar heat of vaporization

Molar heat of vaporization is the amount of heat necessary to vaporize a mole of substance, without changing the temperature. It occurs when a substance changes from a liquid to a gas.

Define molar heat of fusion

Molar heat of fusion refers to the heat absorbed by one mole of a solid substance to melt into a liquid at a constant temperature. Essentially, it is the heat required for a substance to change from a solid to a liquid state.

Define molar heat of sublimation

Molar heat of sublimation describes the energy required to convert one mole of a substance directly from a solid state to a gaseous state, bypassing the liquid phase. It is the sum of the molar heat of fusion and molar heat of vaporization.

Provide the units

The units of these quantities are generally kilojoules per mole (kJ/mol) or calories per mole (cal/mol). These units indicate the energy required per mole of substance for a phase change to occur.

Key concepts

Molar Heat of Vaporization

Understanding the molar heat of vaporization is crucial when studying the thermal properties of substances. It measures the amount of heat needed to turn one mole of a liquid into vapor without changing its temperature. This concept is pivotal in fields such as meteorology, where evaporation plays a major role in weather patterns, and in industries that rely on distillation processes. For instance, the production of distilled spirits involves the precise application of the molar heat of vaporization.

Molar heat of vaporization is not just a fixed value; it can vary depending on the substance and external pressure. Water, for example, has a high molar heat of vaporization, which means it requires substantial energy to transition from liquid to gas. This is why sweating is an effective cooling mechanism for the body since the heat taken from the skin to evaporate sweat substantially cools the surface.

Molar Heat of Fusion

Equally important to the study of phase changes is the molar heat of fusion. This term describes the heat absorbed when one mole of a solid turns into a liquid at a constant temperature. This phase change is readily observed in everyday phenomena, such as ice melting into water. It's a cornerstone concept in understanding how matter responds to temperature changes and is integral to various applications, from designing efficient cooling systems to understanding natural phenomena like the melting of polar ice caps.

Substances differ in their molar heat of fusion values; for example, metals typically have lower molar heat of fusion compared to water, suggesting that less energy is required to melt a mole of metal than to melt a mole of ice. This is vital knowledge for industries that rely on melting and casting processes, such as metallurgy and manufacturing.

Molar Heat of Sublimation

The molar heat of sublimation is rather fascinating as it involves a direct transition from solid to gas. This quantity represents the energy needed to sublimate one mole of a substance, thereby skipping the liquid phase entirely. Dry ice, which is solid carbon dioxide, provides a common demonstration of sublimation as it transitions directly into the gaseous state when exposed to room temperature.

Sublimation requires more energy than either fusion or vaporization alone because the substance must undergo both phase changes consecutively. This process is exploited in lyophilization, or freeze-drying, where the molar heat of sublimation comes into play to preserve food and pharmaceuticals by removing water content without liquid formation.

Phase Change

A phase change is a transition between different states of matter: solid, liquid, and gas. These changes occur when energy, in the form of heat, is absorbed or released, allowing molecules within a substance to move closer together or further apart. Common phase changes include melting, freezing, vaporization, condensation, and sublimation.

Understanding how different substances undergo phase changes is critical. For example, when designing a refrigeration system, engineers must understand how the refrigerant transitions between liquid and gaseous phases to efficiently transfer heat. Meteorologists study phase changes to predict weather phenomena like snow, rain, and fog.

Thermochemistry

Thermochemistry is the branch of chemistry that focuses on the energy changes during chemical reactions and phase changes. It involves concepts such as enthalpy (heat content of a substance), entropy (measure of disorder), and Gibbs free energy (energy associated with chemical reactions).

In thermochemistry, the precise calculation of molar heats of phase changes is essential. It allows scientists to predict how much energy will be absorbed or released during a reaction. This information is paramount in various sectors like energy production, environmental science, and food processing. For example, understanding the thermochemistry of fossil fuels enables us to optimize combustion for energy generation while minimizing environmental impact.