Endothermic

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In the physical sciences, the word endothermic, in contrast to the word exothermic, describes a process or reaction that absorbs energy in the form of heat. The concept is frequently applied to chemical reactions, where chemical bond energy is converted to thermal energy (heat). If the system undergoes a transformation which is both endothermic and adiabatic, i.e. adiabatic meaning that heat is generated internally but no heat is given off to the surroundings, its temperature decreases.

Contents

1 Overview
2 Key points
3 Endothermic materials in passive fire protection
4 See also
5 References
6 External links

Overview

Endothermic materials absorb energy in the form of heat. Its etymology stems from the Greek suffix –thermic, meaning “to heat”, and the Greek prefix endo-, meaning “inside”. It refers to a transformation in which a system receives heat from the surroundings: Q > 0. When the transformation occurs at constant pressure: ∆H > 0; and constant volume: ∆U > 0. If the surroundings do not supply heat, an endothermic transformation leads to a drop in the temperature of the system.

Key points

Energy is absorbed.
Energy is a reactant of the reaction.
Reaction vessel becomes cooler.
Temperature inside reaction vessel decreases.
Energy of the reactants is less than the energy of the products.
∆H = H_(products) - H_(reactants) = a positive number

Endothermic materials in passive fire protection

Endothermic substances, both natural, e.g. gypsum, and synthetic, e.g. resin-based intumescents, are popular for use in heatshielding, ablation, materials in space physics, fireproofing, e.g. fire-resistive coatings for LPG vessels, and compartmentalisation of fire in buildings, which is the cornerstone of passive fire protection. Typically, the technological basis is the conversion of hydrates, or chemically bound water into vapour, or steam.

Steam, at a 100°C, is considered cold in fire protection. The aim in passive fire protection is typically to maintain the item or the side to be protected at or below either 140°C (for walls, floors and electrical circuits required to have a fire-resistance rating) or ca. 540°C, which is considered the critical temperature for structural steel, above which, it is in jeopardy of losing its strength, leading to collapse. Fire testing involves live fire exposures upwards of 1100°C, depending on the fire-resistance rating and duration one is after. So long as the protective endothermic layer still contains hydrates, the temperature on the unexposed side cannot climb above the boiling point of water. As soon as all the water is spent in fire-resistance testing, the temperature on the unexposed side of fire test specimens, of conventional design, typically increases rapidly.

Common endothermic building materials include concrete and gypsum wallboard. During fire testing of concrete floor slabs, water can be seen to literally boil out of a slab. Gypsum wall board typically loses all its strength during a fire, underlining the need for stringent bounding. The use of endothermic materials is established and proven to be sound engineering practice. Too much water can be a problem, however. Concrete slabs that are too wet, will literally explode in a fire, which is why test laboratories insist on measuring water content of concrete and mortar in fire test specimens, before running any fire tests.

References

External links

[Endothermic Definition] - MSDS Hyper-Glossary