Lesson Video: The Contact Process Chemistry

In this video, we will learn how to describe the production of sulfuric acid in the Contact Process and explain the choices of reaction conditions.

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Video Transcript

In this video, we will look at the contact process, the production of sulfuric acid from sulphur, oxygen, and water. Annual production of sulfuric acid is on the order of 200 million metric tons a year. And the majority of it is produced by the contact process.

The contact process consists of multiple steps. In step one, sulphur or sulphur-containing minerals are reacted with oxygen to produce sulphur dioxide. This sulphur dioxide is channeled into step two. In step two, sulphur dioxide is reacted further with oxygen to produce sulphur trioxide. And in step three, sulphur dioxide is reacted indirectly with water to produce sulfuric acid. So, there’s lots of information here. Let’s go through it step-by-step.

One of the most straightforward ways to produce sulphur dioxide in high purity is burning sulphur. You might see sulphur written S₈, which is the conventional molecular form of sulphur. Sulphur in solid or liquid form is reacted with oxygen from the air to produce sulphur dioxide. Sulphur dioxide can also be generated from certain sulphur-containing minerals. For example, iron disulfide can be roasted in the presence of air to produce iron(III) oxide and sulphur dioxide.

The sulphur dioxide produced has to be purified, washed, and dried before being transferred to step two. If this is not done, there’s a risk the contaminants would poison the catalyst in step two. In step two, sulphur dioxide is converted to sulphur trioxide.

The reaction between sulphur dioxide and oxygen is reversible. So, in a closed system, this reversible reaction reaches equilibrium. The forward reaction is exothermic having a negative change in enthalpy. Certain conditions are chosen in order to accelerate reaction and produce the greatest equilibrium yield.

A temperature of 450 degrees Celsius is chosen so that equilibrium is established quickly. However, this does reduce the equilibrium yield a little. A higher pressure, between one and two atmospheres, is used to increase the rate and increase the percentage yield. Higher pressures aren’t used in order to avoid the health and the safety issues with dealing with such dangerous gases at high pressure.

A catalyst, vanadium oxide, is used to decrease the time it takes to reach equilibrium. As the catalyst accelerates both the forward and reverse reactions equally, it has no effect on the equilibrium yield. And lastly, the introduction of cheap excess oxygen accelerates the reaction rate and increases the yield.

Combined, all these factors produce a practical yield of over 95 percent, which is pretty good for a gas phase reaction. These conditions are chosen to maximize the rate of production of sulphur trioxide while keeping costs down. Now that we’ve got our sulphur trioxide, we can move on to step three.

In step three, sulphur trioxide is indirectly reacted with water. The reaction with pure water is too exothermic to be safe, producing clouds of hot sulphuric acid mist. It is safer to break this reaction down into two steps. In step three a, sulphur trioxide is mixed with concentrated sulphuric acid to form a substance called oleum. Oleum is otherwise known as fuming sulfuric acid, and can contain disulfuric acid H₂S₂O₇.

In step three b, water is slowly added to the oleum, generating our sulfuric acid. This is what we get when we sum the two reactions together. If we cancel common terms, we end up with the overall equation, where we’re adding water to sulphur trioxide. Now that we’ve learnt about the contact process and all the steps involved, let’s have some practice.

What is the end product of the contact process?

The contact process is the industrial synthesis of sulfuric acid. It starts with the reaction of sulphur or sulphur-containing minerals with oxygen to produce sulphur dioxide. The sulphur dioxide is further reacted with oxygen to produce sulphur trioxide. And in the last stage, sulphur trioxide is reacted with water to produce sulfuric acid. So, the end product of the contact process is sulfuric acid.

Now that we’ve looked at a simple question, let’s do an exercise in balancing.

Give a balanced equation for the reaction used to convert elemental sulphur S₈ to a sulphur-containing gas for use as a reactant in the contact process.

The contact process is a way of making sulfuric acid. What we need to do is find a step of the contact process that involves sulphur that generates a sulfur-containing gas. The only step of the contact process that involves elemental sulphur is the first one, where sulphur can be converted into sulphur dioxide. This is done by burning sulphur using oxygen from the air.

So, we know our reactants are sulphur and oxygen, and that our product is sulphur dioxide. We know this is the right answer because sulphur dioxide is used as a reactant in the second step of the contact process. So, our answer starts off with S₈. We react it with oxygen. And we produce sulphur dioxide.

The next step is to check that this equation is balanced. As you can see, we have enough oxygen on both sides, but we’re lacking sulphur in the products. If we increase the number of sulphur dioxide molecules to eight, we get eight sulphur atoms on both sides. However, it means that we bring the oxygens out of balance. So, to finish off, we just need to add seven more oxygen molecules to the reactants, bringing the number of oxygen atoms to 16 on both sides.

So, the balanced equation for the reaction that’s used to convert elemental sulphur to a sulphur-containing gas for use as a reactant in the contact process is S₈ plus 8O₂ react to form 8SO₂.

Now, that we’ve had a look at a simple question applied to the contact process, let’s look at one that requires us to remember the entire thing.

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.

Now that we’ve had a look at a few examples, let’s review the key points of the contact process. The contact process is the industrial synthesis of sulfuric acid. In step one, sulphur or sulphur-containing minerals are burned to make sulphur dioxide. In step two, sulphur dioxide is reacted with excess oxygen, at high temperature, raised pressure, in the presence of a catalyst, to make sulphur trioxide. Finally, sulphur trioxide is reacted with water, via absorption in concentrated sulfuric acid, to make sulfuric acid.

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