A chemist needs to know which of these oxoacids of sulphur can act as a strong oxidising agent…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A chemist needs to know which of these oxoacids of sulphur can act as a strong oxidising agent through a peroxo link. The acids are as follows. has one S=O and two OH groups on sulphur. has an S-S bond, and each sulphur has two S=O and one OH group. has an S-O-S bridge, and each sulphur has two S=O and one OH group. has an O-O link between two sulphate groups. has one sulphur atom with two S=O, one OH and one O-OH group. Take oxygen as , but as in an O-O link, and hydrogen as . Find the average oxidation state of sulphur in each acid. Then give the number of oxygen atoms at in .
Need a reminder? The key ideaShow
The basicity of an oxoacid is the number of X-OH groups. Hydrogen atoms bonded to the central atom are not acidic. The oxidation state comes from oxygen at (at in an O-O link) and hydrogen at , shared among the central atoms. Pauling's rule gives the first pKa as about , for non-hydroxyl oxygen atoms, and each next hydrogen is about 5 units weaker.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. For each acid, write the sum: ordinary oxygen , peroxo oxygen , hydrogen and sulphur atoms .
Step 1. . , so .
Step 2. . , so . The S-S bond makes no difference.
Step 3. . , so .
Step 4. . Six ordinary oxygen atoms give and two peroxo oxygen atoms give . With from hydrogen: , so .
Step 5. . Three ordinary oxygen atoms give and two peroxo oxygen atoms give . With from hydrogen: , so .
Answer: , , , and . Two oxygen atoms in are at .
Check. If the peroxo oxygen atoms were wrongly taken as , would give and would give . Both are impossible for sulphur.
What if? and are the oxidising acids, because the O-O link can take electrons.
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