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PCl5, PCl3 and water


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Sorry, I'm not 100% sure this an inorganic question, but I'm having a little problem understanding this.


Why does PCl5 + H20 --> H3PO4 + 5HCl


but PCl3 + H20 --> H3PO3 + 3HCl


Basically, why does PCl5 form phosphoric acid as opposed to forming phosphonic acid?


Thank you!

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Assume that PCl5 and H2O produce H3PO3 and HCl as the sole products. Try to balance the following equation:


PCl5 + H2O --> H3PO3 + HCl


If you do the same math for PCl3 + H2O --> H3PO3 + HCl then you'll see that this is easily balanced, while for the other one this is impossible. Without any insight in the chemical properties of PCl5 you can see on the basis of plain mathematics that PCl5 simply cannot give H3PO3 and HCl only, when it is added to water. Either different compounds are formed, or additional compounds are formed.


Also, if you do similar math for PCl5 + H2O --> H3PO4 + HCl you also see that this is easily balanced.


In general you can state that if mathematically an equation cannot be balanced, that the reaction simply cannot occur. If an equation can be balanced, then the reaction might occur, but whether it really occurs or not is the subject of chemistry.

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  • 1 year later...

What he said. But also, as chemistry is basicly the pursuit of stability, the compound with a more stable hybridization of orbitals and a higher degree of ionization will form. Phosporic acid has a larger acid dissociation constant than Phosphorous acid. You don't even have to look this value up. The oxygen to hydrogen ratio in phosphoric acid is 4/3 and the oxygen to hydrogen ration in phosphorous acid is 1 to 1 (3/3). This is of course cased on the general rule that the more oxygens you have realative to the number of hydrogens the stronger te acid. In the PCl5 + H20 reaction we know that the more stable compound will generally be thone that forms so we can safely assume that the product will be Phophic acid rather than phosphorous acid. But this is just the reason why this is so. The statement made by woelen is what you should probably focus on as it is quicker and less complicated. However, learning to think like this will greatly help you with reaction prediction.

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