Redox Reactions: Ionic vs Covalent Compounds

[gsingh2011]

Hi, I just learned about oxidation and reduction reactions and I have some questions.
Are all ionic reactions redox reactions?
Do elements that make up covalent compounds always have an oxidation number of zero because they only share electrons, not give or take? That is what I was thinking, but according to my book, in the molecule H2S, H has an oxidation of +1 and S has one of -2, which doesn't make sense if its a covalent compound...
Thanks.

 

[pzona]

No, not all ionic reactions are redox. Let's say you react NaNO3 and HCl. There is no change in oxidation numbers for the reactants, so it isn't redox. A lot of reactions involving ionic compounds are redox, especially ones involving transition metals, but not all of them are.

And no, elements in covalent compounds do not always have oxidation states of zero. There are certain rules regarding oxidation states for each common element, which should be in your book. Read a little more about oxidation states and this will make more sense.

Hope this helps, feel free to ask more questions if you have them

 

[Borek]

Perhaps a better example would be

Ag⁺ + Cl^- -> AgCl

It is obvious that reaction takes place, it takes place between ions, and it is not a redox reaction.