Wasa€ˆita€ˆeasya€ˆfora€ˆpeoplea€ˆtoa€ˆaccepta€ˆthea€ˆideaa€ˆthata€ˆmattera€ˆisa€ˆmadea€ˆ
ofa€ˆatomsa€ˆwhena€ˆthisa€ˆideaa€ˆwasa€ˆfirsta€ˆintroduceda€ˆina€ˆthea€ˆearlya€ˆ19tha€ˆ
century?a€ˆHowa€ˆdida€ˆthisa€ˆideaa€ˆappeara€ˆanyway?
The idea that matter is made of very small units that cannot be divided further
first appeared in Greece in the 5th century B.C. Philosophers named Leucippus
and Democritus were the first to propose that matter was made of small
indivisible particles called atoms. But their idea was not studied
scientifically until the early 19th century. In 1805, an English chemist and
physicist named John Dalton did scientific experiments that led him to suggest
that matter was made of atoms. He revived an idea lost through the centuries
and came up with what is known as the atomic theory. It was not widely accepted
at first, and there were serious objections to it from some very learned and
wellrespected scientists. This theory stated the following: » All matter is
made up of tiny atoms'way too small to be seen, but there nonetheless. » All
atoms of a given element are identical in every way'size, mass, and other
properties. » Atoms cannot be created ordestroyed, or even broken into smaller
pieces. » Different atoms combine in various ratios to form compounds. » In a
chemical reaction, atoms are simply separated, combined, and rearranged into
new compounds.
To come up with the atomic theory, Dalton
relied on two laws: (1) the law of conservation of mass, and (2) the law of
definite proportions. These laws may have seemed counterintuitive at the time,
but they are obvious to people who are familiar with the atomic theory. The law
of conservation of mass, formulated in 1789 by a French chemist named Antoine
Lavoisier, states that in a chemical reaction, the
total mass of all the reactants is the same as the total mass of all the
products.
N2(g) 3H2(g)
container, and no atoms were created or destroyed by the reaction. The atoms
that were there in the beginning were simply rearranged into new molecules; so,
of course, the mass would not change. But without the atomic theory, there is
no reason why the mass of a system should stay constant. The same concept
applies to the law of definite proportions. This law, first established in 1799
by the French chemist Joseph Louis Proust, states that if a substance is brokendown
into its constituent elements, then the masses of the constituents always have
the same proportions. That any sample of a given
2NH3(g)
2 atoms N
6 atoms H
2 atoms N and 6 atoms H
Anyone who has ever built a fire in a fireplace or woodburning stove knows that
the ashes left over weigh a lot less than the logs you put in. Imagine being
told that this observation was false and that if the fire was allowed to burn
in a completely sealed-off container, the content of that container would weigh
the same before and after being burned. You would think that was absurd. But
when viewed from the point of view of the atomic theory, the reasoning is
obvious: No atoms entered or left the
Equipo de protección para el manejo
EI equipo de protección personal debera seleccionarse, tomando en
cuenta el trabajo que se va a desarrollar,
entre los que a continuación se enumeran:
* Botas de hule.
* Guantes de hule.
* Mandil de hule.
* Chaqueta y pantalón de hule o de otro material resistente al
acido.
* Gafas de protección contra sustancias químicas.
* Pantallas faciales.
* Mascara con cartucho para vapores organicos y
gases acidos.
* Mascara o capuchón con suministro de aire forzado.
* Cinturones o arneses.
Accidentes por manejo de acidos
CONCENTRACION
Se encuentran en el comercio en distintos grados de
concentración, siendo el mas concentrado el que corresponde a una
solución al 98.5% (66o84), la cual, a la temperatura de 15.5oC (60oF),
tiene una densidad relativa de 1.843 (agua= ,
cuyo punto de ebullición es de 338oC (640oF); para una solución
de 85.66% (64o84) la densidad relativa, también a la temperatura de
15.5oC (60oF), es de 1.795. Al calentar acido sulfúrico puro
entre 30oC y 40oC (86oF y 104oF)comienzan a desprenderse
vapores de anhídrido sulfúrico; su punto de ebullición se
alcanza a la temperatura de 338oC (730oE). El punto
de fusión es de -40oC (-40oF) para una concentración de
65.13% (52o8& ), y -32oC (-25.5oF) para una
concentración de 93.19% (66o8&127;).
CORROSIVIDAD
Es un liquido altamente corrosivo, particularmente en concentraciones abajo de
77.67% (60o84); corroe los metales, con excepción del oro,
iridio y rodio, dando lugar al desprendimiento de hidrógeno.
REACTIVIDAD
Ademas de atacar a muchos metales, el acido concentrado es
fuerte agente oxidante y puede dar lugar a la ignición al entrar en
contacto con materia organica y compuestos tales como
nitratos, carburos, cloratos, etc. También reacciona
exotérmicamente con el agua; tiene mayor desprendimiento
de calor cuando la proporción es de dos moléculas gramo
de agua por molécula gramo de acido sulfúrico,
alcanzando una temperatura de 158oC (316of). Presenta una gran afinidad
por el agua, debido a lo cual, produce
deshidratación de los compuestos organicos a veces tan fuerte que
Llega a carbonizarlos.
El acido sulfúrico puede contener ciertas cantidades de
anhídrido sulfúrico libre y en estas condiciones se conoce como
óleum, el cual presenta un aspecto nebuloso; sus vapores son irritantes,
de color penetrante y tóxicos. El óleum es mas
pesado que el agua, su densidad es muy variable, dependiendo fundamentalmente del por ciento de
anhídrido sulfúrico libre, como
se puede observar en la tabla anterior.
El acido sulfúrico, cuando llega a estar en contacto con
la piel o con los ojos, provoca fuertes
quemaduras; cuando es ingerido causa daños; e incluso lamuerte; la
inhalación de sus vapores provoca graves daños a los pulmones. El
contacto repetido con soluciones diluidas puede
causar dermatitis.
Almacenamiento de los mismos
Todo recipiente que contenga acido sulfúrico debe llevar en lugar
visible una identificación que cumpla con las Normas Oficiales
Mexicanas correspondientes, por lo anterior, se identificara el contenido de
este acido en:
a) tuberías,
b) tanques de almacenamiento,
c) pipas y carros-tanque,
d) recipientes como botellas, garrafones y tambores.
a) Tuberías
La tubería por donde fluya acido sulfúrico en cualquier
concentración debera identificarse de acuerdo a
nuestro código interno para identificación de
tuberías.
Para tuberías con diametros, de
2' a 6' el espaciamiento sera de acuerdo al croquis siguiente.
En el centro se colocaren los letreros con mayúsculas del tamaño
de 1 1/2' (pulgadas).
Para tuberías con diametro de
8' a 18' el espaciamiento sera de
acuerdo al croquis siguiente. En el cen
www.acs.org/chemmatters
compound is made up of the same elements in the same ratio of masses seems
obvious to people who know that watera€™s formula is H2O or that the structure of
table salt is NaCl. If it takes two atoms of hydrogen and one atom of oxygen to
make a molecule of water, then it would take 200 atoms of hydrogen and 100
atoms of oxygen to make 100 molecules of water. So, their combining masses
would always be in a fixed ratio. At the time, compounds wereconsidered like
mixtures. Various elements could be put together
2 Chemmatters, APRIL 2010
+
a†’
to form compounds in the same way that various ingredients, such as flour,
sugar, butter, and eggs could be put together to form cookies. So, imagine that
the recipe also calls for 2 cups of flour and 1 cup of sugar. You decide to
make a batch of these cookies using all the same proportions, but with 2 cups
of flour and 1 A½ cups of sugar. After letting them bake for 10 minutes, you
open the oven door, expecting to find sweeter cookies. Imagine your surprise
when, instead, you find a batch of cookies and a half-cup of sugar piled in a
corner of the cookie sheet. Preposterous, right? How
can the ingredients themselves possibly know the recipe that they are supposed
to follow? Yet, that is exactly what the law of definite proportions claims for
chemical elements. Indeed, it would be nearly impossible to explain either of
these two laws without using words such as a€œatomsa€ or a€œmolecules.a€ The atomic
theory was, and still is, huge. It completely reshaped our perspective on the
world and every change that has taken place within it.
A
