BrØsted – Lowry definition of Bases

 Hello, welcome to today’s lesson. I believe that our introduction to the concept of bases  yesterday was meaningful so today we need to take the concept a bit further. We are going to start dealing with the other definitions of bases. At least, we would major on one definition for the purpose of this lesson.


Let us recall that yesterday, we agreed that a base is a substance that produces hydroxyl ions in solution. This is the definition that was advanced by Arrhenius. We identified the limitation of this definition to be the fact that basic property was limited only to the possession of hydroxyl ions in the formula unit of the compound and being able to dissolve in solution. What if a compound hasn’t both of these?


I am sure that most of us have observed that the compound, sodium carbonate (Na2CO3). Is classified as a base. Though the substance does dissolve in water but it conspicuously lacks the hydroxyl ion. Also, ammonia (NH3) exhibits basic properties but also lacks the hydroxyl ion. How would we reconcile these obvious deviations from the Arrhenius definition?


As always, new experimental evidence would emerge that extends beyond the initial limitations. BrØsted and Lowry defined a base further as a substance that is able to accept hydrogen ions or protons.


Let us try to validate this theory by looking at the behavior of NH3 and Na2CO3 in solution.


In solution ammonia can interact with water as shown below;

NH3(aq) + H2O(l) --> NH4^+(aq) + OH^-(aq)

There is a transfer of a proton from water to ammonia in this case. This transfer is possible because ammonia can interact with a proton via the lone pair of electrons that is localized on its nitrogen atom. Thus water acts as an acid in the BrØsted – Lowry sense by donating a proton(Refer to the BrØsted – Lowry definition






 of acids in previous lessons)  while ammonia acts a base in the BrØsted – Lowry sense by accepting a proton.


In the second case, when we first dissolve the sodium carbonate in water, water would interact with the sodium ions(Na^+) in the compound as well as the carbonate ions (CO3^-) and push them apart. This is called hydration(interaction of an ionic substance with the dipoles in water causing the ionic compound to split into its constituent ions in solution). The reaction occurs as follows;

Na2CO3(s) --> 2Na^+(aq) + CO3^2+(aq)


Next, the carbonate ions in solution can interact with water as shown below;

CO3^2+(aq) + H2O(l) -->HCO3^-(aq) + OH^-(aq)

Again, water donated a proton to the carbonate ion but this time, the bond between H^+ and CO3^2+ is ionic rather than coordinate covalent(as in NH4^+). This is the logic behind the use of Na2CO3 as a base in volumetric analysis.

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