domingo, 11 de marzo de 2007

¿Cómo se produce la llama de una vela?

Traducido de ask a scientist.

¡La combustión química es muy complicada! Muchas, muchas reacciones tienen lugar secuencial y simultáneamente. Científicos e ingenieros no entienden completamente la química y los mecanismos de la llama de una vela (u otros tipos de llama), pero si se conoce bastante.

En general, el destino de las moléculas de cera es este: el calor de la llama de la vela primero funde la cera y ésta sube por la mecha por capilaridad. En la parte superior de la mecha, la gran cantidad de calor vaporiza las moléculas de cera, que pasan a la zona que rodea la mecha. El calor de la llama y las moléculas reactivas (radicales libres) en la llama rompen las moléculas de cera, en particular arrancando átomos de hidrógeno de la cadena carbonada. Algunas cadenas carbonadas se fragmentan en carbono gaseoso (C2) y en pequeñas moléculas y fragmentos moleculares (normalmente de dos átomos de carbono). Los átomos de hidrógeno arrancados de las moléculas de cera finalmente se combinan con los átomos de oxígeno del aire para formar moléculas de agua. Los átomos de carbono se combinan con oxígeno para formar monóxido y dióxido de carbono, pero, primero, muchos de ellos se unen para formar grandes agrupaciones de material sólido, rico en carbono, llamado hollín. Parte de este hollín se quema para convertirse en dióxido de carbono en la llama, y algunas veces parte de éste escapa de la llama.

Muchas zonas de la llama de una vela pueden apreciarse a simple vista. Al fondo de la llama está una región que desprende luz azulada. Esta luz es, de hecho, la emisión molecular de carbono gaseoso. Mas arriba, la llama tiene una región opaca que desprende una luz amarilla. Esta es conocida como la región incandescente y es donde las partículas calientes de hollín se ponen incandescentes, y desprenden luz como el filamento de una bombilla. La parte interior de la llama, cerca de la mecha, es deficiente en oxígeno, y la mayor parte de las reacciones que ocurren aquí son rupturas y reagrupamientos de moléculas inducidos por el calor. En las regiones más externas, donde el oxígeno puede acceder del aire que rodea la llama, los fragmentos se combinan con oxígeno, formando finalmente agua y dióxido de carbono.

Muchos factores afectan a la combustión de una vela. Muchos de ellos son difíciles e modificar como la presión del aire, la concentración de oxígeno, la conductividad térmica del aire y la fuerza ascensional de los gases calientes. Un factor que es fácil de modificar, sin embargo, es el viento. Una buena brisa, o incluso la respiración de una persona, puede soplar sobre los gases calientes de la llama y alejarlos de la fuente (la vela y la mecha) interrumpiendo el proceso y apagando la llama.

Richard E. Barrans Jr. Ph.D. Director of Academic Programs. PG Research Foundation, Darien, Illinois

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