AAMC Sample test PS q12

[sanguinee]

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I normally would have gotten this question right, however, in the passage it states that "it can be theorized that if the central atom, A, is an alkali or alkaline earth metal, the compound is basic. But if A is a nonmetal, the compound is acidic," where chemical formulas are AOx(OH)x. Can someone explain why this statement is irrelevant when looking at NO2(OH) and Ni(OH)2 where neither A (N or Ni) are alkali or alkaline earth metals? Thanks!

 

[aldol16]

Can someone explain why this statement is irrelevant when looking at NO2(OH) and Ni(OH)2 where neither A (N or Ni) are alkali or alkaline earth metals? Thanks!

You can approach this passage in one of two ways, both of which will give you the same result. Let's look at it your way first - using the information given in the passage. The passage states that if A is a non-metal, the compound is acidic. In the case of NO2(OH), N is a non-metal, so therefore it must be acidic. Ni(OH)2 is more difficult because it's neither alkali nor alkaline earth metal and thus could act as either acid or base. That is, Ni(OH)2 is a d8 metal and thus could either donate two electrons to become a high- or low-spin d6 metal or accept two electrons to become a d10 metal. The transition metals are quirky in that they can be both acids or bases, depending on the situation. Here, the question tells you that it only dissolves in acidic solution. That means that it must be acting as a base - the protons are "pulling" the hydroxo ligands off the Ni center.

The second way of approaching it requires you to return to fundamental chemical principles but does not require any information from the passage. In this case, NO2(OH) must be acidic. You know that N-O bonds are generally covalent since the electronegativities of nitrogen and oxygen are similar. Now visualize what NO2(OH) looks like. It's just nitric acid! So which bond is more likely to break - a N-O bond or a O-H bond? The O-H bond by far because of the difference in electronegativity and resonance. So it'll be an Arrrhenius acid as opposed to an Arrhenius base. Now with Ni(OH)2, you have to follow the same logic as above, namely why does it dissolve in acid as opposed to base?

 

[sanguinee]

@aldol16 thank you! helped a lot