Soluble Salts and Nitrates
Date: Thursday, August 22, 2002
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.
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
(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
X = Cl, Br, or I. But here the precipitation of (Ag-X) is the "driving
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