MadSci Network: Chemistry

Re: Why doesnt NaCl become an acid when dissolved in H2O ?

Date: Fri Sep 23 14:07:15 2005
Posted By: Yazan Abbas, Undergraduate, Biochemistry
Area of science: Chemistry
ID: 1127151695.Ch

To start, let us examine the structure of NaCl. Due to the complete separation of charges (Na+ and Cl-) a crystal lattice is formed. This is in the form of small positive charges in a 'sea' of negative charges.

As for water, the molecules have only a partial separation of charge in which the two hydrogen charges have a partial positive charge while the oxygen molecule has a partial negative charge. This makes the water molecule polar.

When dissolved in water, what actually happens is that slightly postive charge of hydrogen attracts the chlorine atom, and water molecules with their hydrogens orientated towards the chlorine atom surround it. Similarly, the oxygens surround the sodium atoms. This is known as solvation (also called hydration in the case of water.)

              O                         H   H
          O    \     O               H   \ /    H
         / \    H   / \               \   O    /
        H   H  Cl- H   H               O  Na+ O
               H                      /   O    \
                \                    H   / \    H
                 O                      H   H

This helps the NaCl dissolve by separating; and concerning the question, there is simply not enough energy associated with the hydration to cause the hydrogen and oxygen atoms to separate and reform bonds with chlorine and oxygen. The interaction that occurs is in the form of electrostatic interactions based on charge differences.

Chang, Raymond. 'Physical Chemistry' University Science Books, 2000: US.

Admin note: pH is definted as -log[H+] (negative log-base-10 of the concentration of hydrogen ions in solution). The higher the concentration of H+ in a solution, the lower its pH (more acidic). As solvation of NaCl doesn't specifically alter the concentration of H+ in solution (or of OH-), thus the acidicity and basicity do not change.

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