Explain using various acid-base theories (Bronsted Lowry, Lewis, Arrhenius) why the solutions of aluminum and zinc nitrate are acidic.
I know why an acid is an acid is in general, but I'm not sure if there is supposed to be a special condition when NO3 is present or something.
Let's hope the subscripts and superscripts come out right. If not, I'll repost using the usual formating.
Al^+3 exists as the hexahydrate; i.e., as
Al(H2O)63+. Zn 2+ ion is a hydrate, also, but with 4 H2O molecules instead of 6.
Al(H2O)63+ ==> Al(H2O)5(OH)2+ + H+.
Aluminum hexahydrate donated a proton; therefore, it is an acid
To understand why solutions of aluminum and zinc nitrate are acidic, we will need to consider the different acid-base theories: Arrhenius, Bronsted-Lowry, and Lewis.
1. Arrhenius Theory:
According to the Arrhenius theory, an acid is a substance that releases hydrogen ions (H+) when dissolved in water. In the case of aluminum nitrate (Al(NO3)3) and zinc nitrate (Zn(NO3)2), both compounds dissociate in water to form their respective ions: Al3+ and Zn2+. However, these metal ions themselves are not acidic in nature.
2. Bronsted-Lowry Theory:
According to the Bronsted-Lowry theory, an acid is a substance that donates a proton (H+) to another species. In this case, water molecules act as bases and accept protons donated by the metal cations (Al3+ and Zn2+). The following reactions illustrate these acid-base interactions:
a. Al(NO3)3 + H2O ⇌ Al(OH)2+ + HNO3
b. Zn(NO3)2 + H2O ⇌ Zn(OH)+ + HNO3
In both reactions, the metal cations donate protons to water, forming oxide hydroxide complexes (Al(OH)2+ and Zn(OH)+) and nitric acid (HNO3) as a byproduct. The presence of nitric acid results in these solutions being acidic.
3. Lewis Theory:
According to the Lewis theory, an acid is a substance that accepts an electron pair, while a base donates an electron pair. In the case of aluminum and zinc nitrate, the nitrate ion (NO3-) acts as a Lewis base, and the metal cations (Al3+ and Zn2+) act as Lewis acids. The Lewis acid-base interaction can be explained as follows:
a. Al3+ + NO3- → Al-NO3+
b. Zn2+ + NO3- → Zn-NO3+
Here, the nitrate ion donates its electron pair to the metal cations, forming coordination complexes - Al-NO3+ and Zn-NO3+. These complex ions can further react with water to release H+ ions, resulting in acidic solutions.
To summarize, the solutions of aluminum and zinc nitrate are acidic due to the ability of the metal cations to donate protons (Bronsted-Lowry theory) and form coordination complexes with the nitrate ions (Lewis theory). The dissociation of nitric acid from these reactions contributes to the overall acidity of the solutions.