Acids and Bases  Elmhurst College
Acids Neutralization & Salts Acid-Base Strength  Chemistry Department
Bases pH  Indicators  Virtual ChemBook

 

Acid and Base Strength

Strength of Acids and Bases:

All acids and bases do not ionize or dissociate to the same extent. This leads to the statement that acids and bases are not all of equal strength in producing H+ and OH- ions in solution.

The terms "strong" and "weak" give an indication of the strength of an acid or base. The terms strong and weak describe the ability of acid and base solutions to conduct electricity. If the acid or base conducts electricity strongly, it is a strong acid or base. If the acid or base conducts electricity weakly, it is a weak acid or base.

Demonstration of Acid and Base Conductivity:

The instructor will test the conductivity of various solutions with a light bulb apparatus. The light bulb circuit is incomplete. If the circuit is completed by a solution containing a large number of ions, the light bulb will glow brightly indicating a strong ability to conduct electricity as shown for HCl.

If the circuit is completed by a solution containing large numbers of molecules and either no ions or few ions, the solution does not conduct or conducts very weakly as shown for acetic acid.

Conductivity Principle:

An acid or base which strongly conducts electricity contains a large number of ions and is called a strong acid or base.

An acid or base which conducts electricity only weakly contains only a few ions and is called a weak acid or base.

Conductivity Behavior of Acids and Bases
 Compounds

Appearance of light bulb

 Classification
Weak or Strong

 Inference of Ions or Molecules
 H2O  no light  weak  molecules
 HCl  bright  strong  ions
  HC2H3O2  dim  weak  molecules
 H2SO4  bright    
 H2CO3  dim  
 NaOH  bright    
 KOH  bright    
 NH4OH  dim  

 Bond Strength:

The bond strengths of acids and bases are implied by the relative amounts of molecules and ions present in solution. The bonds are represented as:

 acid

 base

 H-A

 M-OH
where A is a negative ion, and M is a positive ion

Strong acids have mostly ions in solution, therefore the bonds holding H and A together must be weak. Strong acids easily break apart into ions.

Weak acids exist mostly as molecules with only a few ions in solution, therefore the bonds holding H and A together must be strong. Weak acids do not readily break apart as ions but remain bonded together as molecules.

Bond Strength Principle:

Acids or bases with strong bonds exist predominately as molecules in solutions and are called "weak" acids or bases. Acids or bases with weak bonds easily dissociate into ions and are called "strong" acids or bases.

Quiz: Compare the strength of the bonds, (M-OH), in a weak base and a strong base by inference of the amounts of ions and molecules present.  

 


 

Link to animations on water, strong and weak acids.
Link to weak and strong acid animation - Chang text

Link to animations on buffers

Summary List of Characteristics
for Strong and Weak Acids and Bases
 All characteristics of acids and bases are related to whether the predominate forms are molecules and ions.
 Characteristic  Strong Acid or Base  Weak Acid or Base
 Molecules  few  large number
 Ions  large number  small number
 Conductivity  strong  weak
 Bond Strength  weak  strong