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Name: Nelly
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002


Question:
In my text book, it is said that soluble salts can be prepared by reacting acid and base, acid and carbonates, acid and metals. I wonder why it cannot be prepared by reacting acid and nitrates. Both carbonates and nitrates are salt. Why we can use carbonate but not nitrates? Thank you.


Replies:
The reason is that the counterion (nitrate) is still around. When you react an acid HX with a metal hydroxide MOH, you get

HX + MOH --> HOH + MX

and the HOH, being water, is just one more solvent molecule. When you react an acid HX with a carbonate MCO3, you get

2 HX + M2CO3 --> 2 MX + H2CO3

This H2CO3 further decomposes in aqueous solution,

H2CO3 --> H2O + CO2,

and the CO2, bein bubbles away. When an acid HX reacts with a metal M, there is a displacement reaction

HX + M --> MX + 1/2 H2

and the H2, being carbon dioxide gas, also bubbles away.

When you combine an acid with a metal nitrate, no new gaseous product is formed to take away unwanted products:

HX + MNO3 <==> HNO3 + MX

Since the H+, X-, M+, and NO3- ions are dissociated rather than tightly paired in aqueous solution, you do not get a soluble salt that can be easily isolated. Insoluble salts are a different matter; since they precipitate from solution, they can be separated from the soluble nitric acid product.

Actually, it IS possible to make soluble metal salts from a metal nitrate and an acid, but it's a bit more work than just mixing them together. You have to add lots more acid than you have nitrate salt, and then boil it all off. In the process, any nitric acid HNO3 that you make is also boiled off. Adding more acid HX and boiling it off several times eventually will lead to fairly pure salt residue HX.

Richard E. Barrans Jr., Ph.D.
Director of Academic Programs
PG Research Foundation, Darien, Illinois


In principle you are correct, but there are some practical considerations that come into play. 1. Most nitrate salts of metals are soluble, so if you react a metal nitrate (M-NO3) with an acid (H-A) that forms a soluble salt product (M-A) you are left with nitric acid (H-NO3) as a product. Both reactants and products are all water soluble, so separation is difficult. 2. When metal carbonates (M-CO3) react with a soluble acid (H-A), CO2 gas is the product. Since this has limited solubility in acidic water, there is a "driving force" to yield the soluble metal salt (M-A).

In cases where the metal salt product is water INSOLUBLE, nitrate salts are frequently used. A classic case is the formation of silver halides (Ag-X), where X = Cl, Br, or I. But here the precipitation of (Ag-X) is the "driving force".

Vince Calder



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