Video Transcript
Why is the combustion of aluminum
in air slower than in pure oxygen? (A) The temperature of oxygen in
air is greater than in pure oxygen. (B) The temperature of pure oxygen
is greater than air. (C) The concentration of oxygen in
air is less than in pure oxygen. Or (D) the concentration of oxygen
in air is greater than in pure oxygen.
Combustion or burning is a chemical
reaction that usually involves the reaction of a substance with oxygen. In this case, the substance
reacting with the oxygen is aluminum. In order for a reaction to occur
between the aluminum and the oxygen, the particles will have to successfully collide
with one another. If we can find a way to increase
the total number of collisions, then the chance of a successful collision will
increase. Another way to say this is that
increasing the number of collisions will increase the number of reactions between
particles. Thus, more collisions means a
faster reaction.
The question describes two
different conditions under which aluminum is combusted, in pure oxygen and in
air. Pure oxygen is nearly 100 percent
oxygen. We might describe pure oxygen as
having a high concentration, where concentration is the amount of a substance in a
particular volume. As pure oxygen is highly
concentrated, there will be lots of collisions between the oxygen particles and
aluminum particles, which should result in more reactions.
Air, on the other hand, is only
composed of about 21 percent oxygen. So, we might describe air as having
a low concentration of oxygen. As there are fewer oxygen
particles, there will be fewer collisions, which will result in fewer reactions, so
we can make the connection that decreasing the concentration decreases the number of
collisions, which decreases the number of reactions. In other words, the lower the
concentration, the slower the reaction.
The question asked us to explain
why the combustion of aluminum in air is slower than in pure oxygen. We now know that this is the case
because the concentration of oxygen in air is lower than the concentration in pure
oxygen. The answer choice which correctly
describes this relationship is answer choice (C). Thus, the combustion of aluminum in
air is slower than in pure oxygen because the concentration of oxygen in air is less
than in pure oxygen, or answer choice (C).
