MadSci Network: Chemistry

Re: Why do acids dissociate when dissolved in water ?

Date: Thu Aug 24 10:45:21 2000
Posted By: Lon Brouse, Faculty, Chemistry, Challenge Charter School
Area of science: Chemistry
ID: 966857333.Ch

I do not know how old you are or what level you are in shcool but to ask 
about hydrochloric acid dissolution into ions puts you at least in Jr. 
High School.

There are two major types of chemical bonds: covalent and ionic.  A 
covalent bond forms when the atoms involved have attractions for the 
bonding electron(s) that are about the same strength.  The electron(s) end 
up being about equally shared between the atoms and a strong bond is 
formed.  Compounds like these (sugar, oxygen, carbon dioxide, etc.) do not 
split into ions when they are dissolved in water.  The second type of 
chemical bond is ionic.  These bonds form between atoms that have more 
dissimlar attractions for the bonding electron(s).  The more 
electronegative atom (the non-metal) will unequally take the bonding 
electrons.  So long as the ionic compound is not dissolved in water the 
ions will probably stay together.  Examples of ionic compounds are table 
salt, hydrogen chloride (HCl), etc.  

Your question asked why hydrochloric acid dissolves into H+ and Cl- ions 
when dissolved in water.  Water is a covalent compound and therefore 
exhibits very strong bonds between the Hydrogen and Oxygen atoms.  In 
addition, the water molecule is bent, that is, the atoms are not in a 
straight line, but make an angle of approximately 109 degrees.  The Oxygen 
is still the negative part of the water molecule, so the electrons spend a 
little more time in orbit near the oxygen end of the molecule.  This 
leaves the hydrogens with a slight deficiency of negative charge from the 
orbiting electrons.  The molecule ends up with what is called a dipole.  
Part of the molecule has a partial negative charge while the other end has 
a partial positive charge.  These dipoles on every water moleclue makes 
them very powerful agents for taking apart ionic compounds.  Hydrochloric 
acid is no exception.  The partial negative oxygen attracts and takes the 
H+ portion of HCl away while the partial positive Hydrogen end of the 
water molecule takes the Cl- ion away.  Each of these ions are then 
surrounded by water molecules with their polar ends pointing toward each 
of these separated H+ and Cl- ions.  This completely separates them from 
each other and at this point the ions are said to be dissolved. 
Hydrochloric acid is almost 100% dissociated into ions in water but other 
ionic bonded compounds may not be 100% separated.  The more completely 
dissociated HCl is said to be a strong acid, at low concentrations, while 
carbonic acid H2CO3 (the acid formed when carbon dioxide CO2, is dissolved 
in water) is said to be a weak acid, because the hydrogens are only partly 
separated from their molecule.

I hope this helps.

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