Video Transcript
Oxygen gas is used as a reactant in a
number of steps of the contact process. Assume sulphur is the starting
material. How many molecules of oxygen gas react
per atom of sulphur in the final product?
To answer this question, we’re going to
need to recall what the contact process is and all the steps involved. The contact process is the industrial
synthesis of sulfuric acid. There’s a mnemonic you can use to help
you recall all the steps of the contact process, should dogs travel alone. The S stands for sulphur, or
sulphur-containing mineral. The D stands for dioxide in sulphur
dioxide. The T stands for trioxide in sulphur
trioxide. And the A stands for acid in sulfuric
acid.
In step one, we convert sulphur to
sulphur dioxide. In step two, we convert sulphur dioxide
to sulphur trioxide. And in step three, we convert sulphur
trioxide into sulfuric acid. So, our final product in the question is
sulfuric acid. Now, let’s construct the steps
one-by-one.
The conversion of sulphur into sulphur
dioxide requires the introduction of oxygen. So, in the first step, we add one
molecule of oxygen for each sulphur atom. In the second step, we have sulphur
dioxide being transformed into sulphur trioxide. So, we need more oxygen. However, this equation isn’t balanced, so
we can double up the amount of sulphur dioxide and sulphur trioxide to produce a balanced
equation.
In the last step, we’re converting
sulphur trioxide to sulfuric acid. We can see that our product has two extra
hydrogen atoms and one extra oxygen atom. So, hopefully, that’ll jog your memory
that the last step involves the reaction of sulphur trioxide and water. Now, we can start to figure out how many
molecules of oxygen gas react per atom of sulphur in the final product. We can see for the formula of sulfuric
acid that there’s one atom of sulphur per molecule.
Now, let’s make some sulfuric acid and
see how many oxygen molecules we use along the way. In the first step, we’re burning sulphur
as our starting material. So, we use one oxygen molecule per atom
of sulphur. In the next step, we react sulphur
dioxide with oxygen to produce sulphur trioxide. But in our first reaction, we only
produce one molecule of sulphur dioxide. So, to make sure we get the right amount
for the second, we should either double up the first reaction or halve the second.
I’m going to halve the second reaction,
but you could do it whichever way you like. Our next step is to take the sulphur
trioxide molecule and turn it into sulfuric acid. By halving the second reaction, we only
produce one equivalent of sulphur trioxide. And that’s exactly what we need for the
third one. So, we don’t need to scale that up or
down.
This entire process produces one molecule
of sulfuric acid. In the process, we used one and a half
oxygen molecules to produce one molecule of sulfuric acid. So, how do we work out the number of
oxygen molecules required per atom of sulphur in the final product. We take 1.5 oxygen molecules per sulfuric
acid molecule, and multiply it by one sulfuric acid molecule per sulphur atom. This gives us 1.5 oxygen molecules per
sulphur atom, which we can just write as 1.5 for our answer.
