Question

Heat of transition is the heat evolved or absorbed when a substance is converted from (1) solid to liquid (2) solid to vapour (3) liquid to vapour (4) onc allotropic form to another allotropic form

Short answer

Heat of transition refers to heat of fusion, sublimation, vaporization, or transformation.

Step-by-step solution

Identify the Process - Solid to Liquid

When a substance is converted from a solid to a liquid, the heat involved is called the heat of fusion.

Identify the Process - Solid to Vapour

When a substance is converted from a solid directly to a vapour without passing through the liquid phase, the heat involved is called the heat of sublimation.

Identify the Process - Liquid to Vapour

When a substance is converted from a liquid to a vapour, the heat involved is called the heat of vaporization.

Identify the Process - Allotropic Form Conversion

When a substance changes from one allotropic form to another, the heat involved is known as the heat of transformation.

Key concepts

Heat of Fusion

The heat of fusion is the amount of heat energy required to change a substance from a solid to a liquid at its melting point. This energy overcomes the forces holding the solid's molecules in a fixed position. For example, to melt ice into water, energy must be supplied to break the hydrogen bonds between the water molecules in the ice.

During this process, the temperature of the substance does not change until the entire solid has melted. This is because the energy input goes into changing the state rather than increasing the temperature. The heat of fusion is a critical concept in understanding the melting process of various materials.

Heat of Sublimation

The heat of sublimation is the energy required for a substance to transition from a solid directly to a vapour without passing through the liquid state. This process occurs in substances that have strong intermolecular bonds, which need significant energy to be overcome.

A common example of this is dry ice (solid carbon dioxide). When dry ice absorbs heat, it turns directly into carbon dioxide gas through sublimation. This process is utilized in various applications like fog machines and preserving frozen items without leaving any liquid residue. The heat of sublimation combines the energy needed for both fusion and vaporization.

Heat of Vaporization

The heat of vaporization is the energy required to change a substance from a liquid to a vapour at its boiling point. This energy is used to break the intermolecular forces in the liquid, allowing the molecules to move freely as gas.

For instance, when water is heated to its boiling point, the heat of vaporization must be supplied to convert the water into steam. This energy input does not raise the temperature of the liquid but instead is used to change the state. Understanding this concept is essential in fields like meteorology and various industrial processes.

Heat of Transformation

The heat of transformation, or enthalpy of transformation, is the energy involved in changing a substance from one allotropic form to another. Allotropes are different structural forms of the same element, where these forms have different physical properties.

A classic example is the transition between diamond and graphite, both of which are forms of carbon. The energy required for this transformation is significant because it involves breaking and reforming bonds in a solid, leading to different structures. This concept is crucial in understanding materials' properties and their behavior under different conditions.