One step of the contact process involves a transition metal catalyst. At what temperature is this reaction typically performed?
This question is asking us about the contact process. You may remember that this is the industrial manufacturing process for sulphuric acid. The contact process has three main steps. The first of these steps is to make sulphur dioxide, SO₂. The second step is a reversible reaction where SO₂ is converted to SO₃, sulphur trioxide. In the final step, SO₃, sulphur trioxide, is converted into sulphuric acid. This question is asking us specifically about one of these three steps. But it doesn’t say which one. It does, however, give us a clue. The step it’s asking us about involves a transition metal catalyst. So let’s revise what we know about each of these three steps to work out which one this question is asking us about.
In the first step, we’re aiming to produce SO₂. And the most common way to do this is by burning sulphur in the presence of oxygen, which luckily is a component of air. This burning of sulphur in air produces SO₂ gas. SO₂ can also be produced by heating sulphide ores. But this is the most common. Production of SO₂ in this manner is quite efficient. And no catalyst is needed. This means that this first step is not the step that our question is asking us about. So let’s move on to look at step two.
In this second step, the SO₂ gas from step one is further reacted with oxygen in the air. This reacts reversibly to form SO₃, sulphur trioxide. Because this reaction is reversible, we need to make sure that the conditions under which it’s carried out are ideal. Both in making sure that we get a good percentage of our product, the desired product, SO₃, but also that the rate of the reaction is fast enough to make this economically viable. This reaction in industry is carried out at 450 degrees C, one to two atmospheres of pressure, and in the presence of a V₂O₅ catalyst.
Interestingly, the enthalpy change for the forward reaction is negative 196 kilojoules per mole. This means that the forward reaction is exothermic. This in turn means that the backward reaction is endothermic. Le Chatelier’s principle tells us that if we want to favour the forward reaction, the exothermic reaction, we want the temperature as low as possible. But here we’re told that the temperature this reaction is carried out at is 450 degrees, which is definitely not a low temperature.
So why do we do this if it’s going to favour the backward reaction? The answer is simply because of the rate of reaction. We want to produce SO₃. But we also want to produce it quickly enough that it’s economically viable. This is the same reason that we use a V₂O₅ catalyst in order to increase the rate. V₂O₅ is also called vanadium oxide. If we look up vanadium on the periodic table, we can see that it’s in the first row of the transition metals. Remember that this question is asking us about a step in the contact process which involves a transition metal catalyst. Step two fits that criteria. But let’s check step three just to be safe.
In step three, we used the sulphur trioxide in order to make sulphuric acid. However, it’s not as simple as just dissolving the SO₃ straight into water. This is a really dangerous reaction. So, to make this safer, we first dissolve the SO₃ into already concentrated sulphuric acid. This produces fuming sulphuric acid, or oleum, H₂S₂O₇. This fuming sulphuric acid, or oleum, can then be safely added to water to produce sulphuric acid. We can see that, in step three, again we don’t need a catalyst. This means that the step in the contact process involving a transition metal catalyst is step two.
Now we can focus purely on this second step. The question asks us, at what temperature is this reaction typically performed? Luckily, we’ve already answered this question. The temperature this is typically performed at is 450 degrees C. This temperature is chosen as a compromise between having a rate that’s fast enough but also not favouring the backward reaction too much. So our answer is 450 degrees Celsius.