Video: Energy Change in the Conversion of Sulfur Dioxide to Sulfur Trioxide

Nathan March

When sulfur dioxide reacts with oxygen gas to form sulfur trioxide, energy is absorbed when bonds are broken and released when bonds are formed. The S−O Bonds in sulfur dioxide have an energy of 522 kJ/mol, while those of sulfur trioxide have an energy of 495 kJ/mol. The O−O double bond in oxygen has an energy of 494 kJ/mol. Determine whether the reaction is endothermic or exothermic, and calculate the total energy change per mole of sulfur dioxide.

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Video Transcript

In this video, we are going to look at the energetics of one of the steps of the contact process. When sulfur dioxide reacts with oxygen gas to form sulfur trioxide, energy is absorbed when bonds are broken and released when bonds are formed. The S-O Bonds in sulfur dioxide have an energy of 522 kilojoules per mole, while those of sulfur trioxide have an energy of 495 kilojoules per mole. The O-O double bond in oxygen has an energy of 494 kilojoules per mole. Determine whether the reaction is endothermic or exothermic, and calculate the total energy change per mole of sulfur dioxide.

I’m gonna break down this question into three steps: the first is to find the balanced reaction equation; the second is to take the bond energies given and calculate the bond energies of the reactants and the products; and then the third step is to compare the energies to see which is larger and determine whether the reaction is endothermic or exothermic, and then calculate the overall energy change.

So onto the first step, balancing the equation. So we can start with the word equation sulfur dioxide plus oxygen reacts to form sulfur trioxide. Then we convert each chemical word into a chemical formula. Sulfur dioxide, SO2, plus oxygen, which is a diatom so it goes around as O2, reacts to form sulfur trioxide, SO3.

But now we have a problem. We have two ways of balancing this equation. In the first way, we just take half an oxygen molecule and add it to one molecule of sulfur dioxide forming sulfur trioxide. The second option, we double everything up and remove all the fractions so we have two sulfur dioxide molecules plus one oxygen molecule forming two sulfur trioxide molecules.

As we’re working with energetics, we go back to the question and remember that the answer has to be per mole of sulfur dioxide. So it’s easier if we only have one equivalent of sulfur dioxide in our balanced formula; otherwise, we’ll have to divide everything by two at the end. So I’m gonna go with the first one, but you could go with the second and divide by two; and that’s perfectly fine.

The next step is to draw the structures. This isn’t 100 percent necessary, but it’s much easier to work out how many bonds you have if you can visualize the molecule. Sulfur dioxide is a bent molecule with sulfur in the middle and two oxygen atoms bonded by double bonds. Oxygen is easy; it’s just two oxygen atoms bonded by a double bond. Sulfur trioxide is an atom of sulfur with three atoms of oxygen all bonded to the sulfur with double bonds.

The next step is to calculate the energies of the reactants and the products, and I’m gonna to start off by labeling each bond with its energy. So that’s two double bonds in sulfur dioxide with an energy of 522; that’s one bond in the oxygen with an energy of 494 — and, remember, we’re going to be taking half of it — and that’s three bonds in sulfur trioxide with energy 495. So just remember that key multiplication through by a half for oxygen; otherwise, you’re gonna end up with a wrong value.

So now we have to work out the total energy for the reactants and for the products. Two times 522 is 1044; a half times 494 is 247; and three times 495 is 1485. Now we sum together the energies of the bonds of the reactants to a total of 1291. Now we can move on to the third step. In step three, we’re going to compare the energies of the reactants and the products and work out where the energy is being released or absorbed.

Now we look back at our definitions of exothermic and endothermic. Exothermic means a reaction releases energy; endothermic means that it absorbs energy. Now if you look at the total energy of the products versus the reactants, we can see the total energy stored in the reactants is less than the energy stored in the products. As it says at the beginning of the question, when bonds are formed, energy is released; therefore, the products have released more energy than what’s required to break up their reactants. Therefore the reaction overall is exothermic.

Now all that remains is to work out the total energy change. So the total energy change is 1485 minus 1291, which equals 194. And remember the units, kilojoules per mole, and it’s per mole of sulfur dioxide. But we’re not quite finished; there is one more thing to do, and that’s observing the sign of the changing energy. Exothermic reactions are negative; endothermic reactions are positive.

So in this case we just need to put a minus sign before the 194, and there we have our answer. Using the bond energies of sulfur dioxide and oxygen and sulfur trioxide, we have worked out that the reaction between sulfur dioxide and oxygen has a change in energy of minus 194 kilojoules per mole of sulfur dioxide. As I said before, if you went with the balanced equation without fractions, then all you have to do is divide your answer by two to get to 194.

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