Why did element with higher oxidation states more acidic?
Answers
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Answer:
The oxide in the higher oxidation state of the element is more acidic than that of a lower oxidation state. Why?
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Generally, a compound of any element with higher oxidation state is more acidic than the compound of that element with lower oxidation state, viz., Fe3+ salts are more acidic than Fe2+, on the same line we may say that; oxide in the higher oxidation state of the element is more acidic than of a lower oxidation state. This is because, as the oxidation state increase the carge density of that atom also increases, consequently such atom strongly pulls the electron density towards itself from other specie that comes closer or in contact with it!
Now, question arises why pulling the electron density towards itself by the atom increase the acidity of that atom? Let us try to understand this by taking an example of reaction of such atom (in high and low oxidation state) with water. Fe2+ as well as Fe3+ in water forms hydrated ions viz., [Fe(H2O)6]2+ and [Fe(H2O)6]3+ respectively, these hydrated ions suffers hydrolysis of which former suffers less hydrolysis than later, as former is less acidic than later. Now, both the cations Fe2+ and Fe3+ are coordinated to six water molecules (ligands) but as the relative charge density of Fe3+ is more (higher charge and smaller size) than that of Fe2+ (lower charge and bigger size) the lone-pair of water molecules coordinated to Fe3+ will get more strongly pulled towards the metal ion, remember, that, it is the oxygen and not hydrogen of water molecule that is coordinated with the metal, so, this will cause a net drift of electron density from hydrogen to oxygen (of water molecule) making hydrogen atoms of coordinated molecule more positively charged, consequently, these hydrogen atoms may leave as H+ ions and thus can act as acid
Explanation:
We see this trend of acidic-basic property of oxides in most cases: Mn2O7 is acidic, MnO is basic, the intermediate oxides are progressively less acidic and more basic, as the oxidation state of Mn decreases.
What is the reason of this general trend in almost all cases?
I have seen many explanations of this, most of them involving the proton releasing capacity of E−O−H bond, in the hydrolysed oxide. But most oxides of transition metals do not hydrolyze in water. So, I am seeking an alternative explanation without having to involve the hydrolysis of oxide. Also, most explanations on the internet lack any reference, so I prefer a reference
In the bonds Mz+−O−H, where M can be Mn or Cr, the atom M is positively charged. When this charge z is large, M repells the H atom stronger than if z is small. So the molecule containing these bonds is a stronger acid. For example, Z = 6 in H2CrO4 and Z=7 in HMnO4. They both contain at least one M−O−H bond. As Z is high in these molecules, they are strong acids. Molecules containing H,O and the same atoms Mn and Cr at a lower oxidation state exist. But they are not acidic in water. Examples : Mn2+ makes Mn(OH)2 which is insoluble in water and not acidic. Cr3+ and Cr2+ both produces Cr(OH)3 and Cr(OH)2 which are also insoluble and not acidic.