The shortcomings (problems) of the Arrhenius theory lead scientists to seek other explanations for the nature of acids and bases. Two groups of scientists in the same year (1923) independently proposed a new theory . Their theory explained the behaviour of all of the acids and bases that Arrhenius did but also was able to resolve some of the problems with his theory. That is they were able to explain why some salts are acidic and basic and why no free protons are found in solutions of some acids.
To understand their theory examine the following reaction between vinegar and ammonia solutions. If we test each solution for conductivity we see that neither is a conductor in dilute solution. They therefore can be represented as molecules. When equal volumes are mixed the resulting solution conducts electricity readily. The reaction that occurs can be written as seen below.
CH3COOH ( aq) + NH3 (aq) 
NH4 + (aq) + CH3COO - (aq)
When Bronsted and Lowry examined this reaction, they defined vinegar as an acid because it donated a hydrogen ion to the ammonia. They defined ammonia as a base because it accepted this hydrogen ion.
 | Acid | substance that donates hydrogen ions (protons) in solution. |
| Base | substance that accepts hydrogen ions (protons) in solution. |
 | Click on the flash icon on the left to watch a flash presentation on how Bronsted and Lowry viewed and acid and base |
Bronsted- Lowry solutions to Arrhenius problems:
- Bronsted and Lowry stated that when acids dissolve in water they donate a proton to water to form H3O + ions. H3O + ions were later proved to exist through independent experimentation.
HCl (g) + H2O (l) H3O + (aq) + Cl - (aq)
- Bronsted and Lowry could also explain why some salts were acidic, while other were basic by saying that the ions formed when they dissolve react with water . When the solution were acidic they formed H3O + ions, when basic they formed OH- ions.
- Why sodium bicarbonate is basic in solution. The bicarbonate ion reacts with water to form OH- ions. HCO3 - is behaving as a base because they are accepted protons.
HCO3 - (aq) + H2O (l) H2CO3 (aq) + OH - (aq)
- Why sodium bisulfate is acidic in solution. The bisulfate ion reacts with water to form H3O + ions. HSO4 - is an acid because it is donating a proton to the water.
HSO4 - (aq) + H2O (l) SO4 2- ( aq) + H3O + (aq)
Amphoteric Substances
Bronsted and Lowry not only solved the problems with Arrhenius theory, they also explained how some substances can behave as acid under one set of conditions and bases under a different set.
Examine water in the above reactions . In the bicarbonate reaction water is behaving like an acid and is donated a proton. In the bisulfate reaction water is behaving as a base and is accepting a proton. Substances that can behave in this way are said to be Amphoteric.
| | Demonstration of Amphoteric substances
Amphoteric substance may behave as either acids or bases. Certain metal hydroxide will behave as amphoteric substances. Click on the test tube on the right and complete the following demonstration to illustrate this concept
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Unfortunately there is no way to tell by looking at the formula if the substance will behave as acid or base.
Acid / Base Equilibrium
The acid - base reactions that Bronsted and Lowry described are equilibrium. The proton when transferred from acid to base can be transferred back in a reverse reaction. The substances formed when the proton is transferred are termed conjugate acids and bases.
Conjugate Acid: chemical species formed when a reactant base accepts a proton. Chemical species has one more proton than base.
| | Base | Conjugate Acid |
| OH - (aq) | H2O (l) |
Conjugate Base: chemical species formed when a reactant acid donates a proton. Chemical species has one less proton than acid.
| | Acid | Conjugate Base |
| H2SO4 (l) | HSO4 - (aq) |
An overall reaction could be :
| HCO3- (aq) | + | H2O (l) | | H2CO3 (aq) | + | OH - (aq) |
| Base (acceptor) | | Acid (donator) | | Conjugate Acid | | Conjugate Base |
| | Click on the flash icon on the left to watch a presentation on the exchange of protons between acids and bases and conjugate acids and bases |
The assignment above will help you check your understanding of the transfer of a proton from acid to base to form conjugate acids and bases.
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