Sublimation is a phase transition process wherein a substance changes from a solid directly to a gas without forming an intermediate liquid state. It’s an exceptional case of vaporization. It occurs when the pressure and temperature of a substance are below its triple point in its phase diagram.
The triple point of a substance is the temperature at which it can exist in all three states i.e., solid, liquid, and gas in equilibrium.
In this article, we’ll provide you with a detailed explanation about sublimation and whether it’s endothermic or exothermic?
So, is sublimation endothermic or exothermic? It’s endothermic. This is because sublimation requires energy in some form to convert a solid to a gas. Usually, heat energy is supplied to conduct sublimation to bring about the physical change from a solid to a gas. Sublimation occurs by absorbing heat that provides adequate energy to break the intermolecular force of attraction between the molecules of a solid substance and transform it into a gas.
In an endothermic process, the system absorbs energy from its surroundings to reach its completion. While in an exothermic process, the system releases energy into its surroundings to reach its completion.
Why is Sublimation an Endothermic Process?
The molecules of a solid are tightly packed. There exists a strong intermolecular force of attraction between them.
To break this rigid structure, we require energy to be supplied to conduct its sublimation.
Also, a solid exists at a lower energy level than a gas. Any phase transition that occurs from a lower energy level to a higher energy level requires energy.
For instance, solid to liquid, liquid to gas, and solid to gas. Hence, sublimation is an endothermic process.
When Does Sublimation Occur?
As temperature rises, the vapor pressure of a substance increases.
Sublimation occurs when the vapor pressure of the substance is greater than the total atmospheric pressure. And when the temperature hasn’t reached the melting point of the substance.
Sublimation occurs when the heat absorbed by the substance from its surrounding is adequate to break the intermolecular force of attraction between its molecules to conduct a phase change from solid to gas.
The enthalpy of sublimation (heat of sublimation) is calculated by adding the enthalpy of vaporization with the enthalpy of fusion.
What factors determine the rate of Sublimation?
The rate of sublimation is determined by the following factors:
The rate of sublimation increases with an increase in temperature. This occurs because, with an increase in temperature, the particles acquire enough kinetic energy to transform from a solid to a gas or vapor.
The rate of sublimation increases with an increase in the surface area of the solid.
As more area of the solid substance gets in contact with its environment, more energy gets absorbed by its molecules. Thus, speeding up its sublimation process.
Humidity is defined as the amount of water vapor present in the atmosphere. The rate of sublimation of a substance is inversely proportional to humidity. Thus, the rate of sublimation increases when humidity decreases.
The sublimation rate is directly proportional to wind speed. Hence, with an increase in wind speed, we witness an increase in the sublimation rate.
This is because wind speed helps in decreasing humidity by moving away vapor particles present in the atmosphere.
Time of Exposure
The sublimation rate of a substance is directly proportional to its time of exposure to the atmosphere. Hence, the longer the exposure, the faster the rate of sublimation will be.
Latent heat is defined as the heat required by a substance to convert itself from a solid to a liquid or a gas without changing the temperature.
The rate of sublimation increases with an increase in latent heat.
Difference Between Endothermic and Exothermic Processes?
In an endothermic process, the system absorbs heat from its surroundings to conduct the reaction. While in an exothermic process, the system releases heat into its surroundings to complete the process.
For endothermic reactions, since heat is absorbed, enthalpy is positive. But for endothermic reactions, since heat is released, enthalpy is negative.
Energy gets absorbed in the form of heat in endothermic reactions. On the contrary, energy is released in the form of sound, electricity, or light in exothermic reactions.
To differentiate between the two processes, it is crucial to keep tabs on the change in temperature by placing a thermometer in your reaction sample.
And calculate the enthalpy change (del H) of the reaction. If del H is positive, the reaction is endothermic. If del H is negative, the reaction is exothermic.
Examples of Sublimation
A few examples of sublimation are as follows:
• Dry Ice: Solidified carbon dioxide is known as dry ice. Dry ice transforms from solid into gaseous carbon dioxide when it comes in direct contact with air. The released gaseous carbon dioxide appears like fog.
• Iodine Crystals: When iodine crystals are placed in a container and heated around 100 degrees, a purple-colored gas is evolved.
• Arsenic: Arsenic undergoes sublimation when heated at around 615 degrees.
• Naphthalene Mothballs: The sublimation of naphthalene is similar to that of dry ice. It sublimes when exposed to air.
• Zinc Compounds: At low pressures, zinc compounds undergo sublimation.
• Sulfur: Sulfur sublimates into hazardous gases at temperatures around 25 to 50 degrees.
• Polar Evaporation: Frozen water near the Antarctic and Arctic poles sublimate at 0 degrees. This process is known as “Polar Evaporation”.
• Ice Crystals: Packets of frozen food come with ice crystals that sublimate and release gas when they come in contact with air.
• Snow under Specific Conditions: Under extreme low pressures, snow sublimates to gas.
• Room Fresheners: Fresheners used for bathrooms, bedrooms, and cars sublimate when exposed to air. The fragrance is carried around by the movement of air particles in the atmosphere.
Applications of Sublimation
Following are the applications of the sublimation process:
• Erosion and sublimation result in ablation. Ablation is the process that facilitates the elimination of snow and ice from glaciers and icebergs through processes like evaporation and melting.
• Iodine sublimation can help extract latent fingerprints from paper. Latent fingerprints are the impression of the friction ridges of our skin, specifically fingers. These fingerprints can be transferred from one surface to another.
• In organic chemistry, sublimation is used for the purification of compounds.
• Dry ice is used for creating fog effects. It is also used for the transportation of goods that require low temperatures.
Which processes are endothermic?
Apart from sublimation, processes like fusion, vaporization, evaporation, melting, and boiling are also endothermic.
Fusion is the process when two atoms collide and fuse to form a bigger atom.
Vaporization is the process wherein a solid or a liquid converts into a gas.
Melting is the process wherein a solid transforms into its liquid state when it’s heated to its melting point. While boiling is a process wherein a liquid is converted into its gaseous state when it’s heated to its boiling point.
Which processes are exothermic?
Fission, freezing, condensation, and combustion are exothermic processes.
Fission is the process when a neutron collides with an atom and excites it to form two smaller atoms. Freezing is a process wherein a liquid is converted into its solid state when the temperature meets its freezing point.
Condensation is a process wherein gas is converted into its liquid state. While combustion is a process wherein a substance reacts with oxygen to release heat.
Is sublimation of camphor endothermic or exothermic?
Since heat is required to conduct the sublimation of camphor, it is an endothermic process.
Is sublimation of dry ice endothermic or exothermic?
Sublimation of dry ice is an endothermic process. This is because heat is absorbed by the system to take the reaction forward.
Is sublimation a reversible process?
No, sublimation is not a reversible process. This is because we cannot obtain a solid back from its gaseous state. Hence, sublimation is an irreversible process.
Is sublimation a chemical change or a physical change?
Sublimation is a physical change. A solid gets converted into a gas. There is no change in the chemical composition of the substance.
Is sublimation of iodine a chemical change?
The sublimation of iodine crystals at 100 degrees is a physical change. However, when iodine sublimation occurs in the presence of oxygen, then it’s a chemical change.
At times, iodine sublimation is called a physio-chemical process since we observe a physical as well as a chemical change.
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Sublimation is a phase transition process wherein a substance converts from a solid to a gas without forming an intermediate liquid state. It is an endothermic process because the energy in the form of heat is required to break the intermolecular forces of attraction present between the molecules of the solid to convert it into a gas.
Sublimation is a physical change whose rate depends on factors like temperature, surface area, humidity, wind speed, time of exposure, and latent heat. It’s an irreversible process with several applications in our daily life.