In this tutorial, you will compare and contrast different acid base theories. You will be introduced to the Arrhenius and Bronsted-Lowry theories, and the idea of an Arrhenius acid and a Bronsted-Lowry acid (Bronsted acid).
Make sure to check out the Lewis Acids & Bases tutorial to learn about the Lewis theory!
There are 3 different acid base theories in chemistry; each of these theories have their own definition of what qualifies as an acid- base reaction. Let’s learn the different qualifications of each theory below!
This theory defines both the acid and base as a compound that is able to dissociate in water, meaning it breaks up into its ion counterparts (i.e. HBr → H + + Br – ).
An Arrhenius acid is a compound that yields H+ ions in solution. Nitric acid, HNO3, is an example of an Arrhenius acid because it will dissociate and create H + and NO3 – ions. The hydrogen ions then combine with water to form H3O + ; this is because the oxygen’s lone pair attracts the hydrogen, which is too weak to exist alone in solution.
On the other hand, an Arrhenius base is a compound that yields OH – ions in solution. Potassium hydroxide, KOH, is an Arrhenius base because it will dissociate and create K + and OH – ions.
Furthermore, neutralization occurs within the solution due to the H + and OH – ions also reacting together to form water.
The Arrhenius theory has the most specificity out of the three theories because it states that either the H + or OH – ion must be present to be labeled an acid or base; however, this is not true in other theories. Let’s move on to a more general theory that includes more compounds – the concept of a Bronsted-Lowry acid.
This definition of acids and bases can be used to analyze solutions both containing and not containing water; it has to do with how well a compound can accept or donate protons.
A Bronsted-Lowry acid is a substance that donates a proton (a hydrogen nucleus) to another substance. This process is called proton transfer, and it results in the formation of a new chemical species called a conjugate base. For example, when hydrochloric acid (HCl) donates a proton to water (H2O), it forms the conjugate base chloride ion (Cl-) and a hydronium ion (H3O + ), which is a positively charged species formed by the combination of a proton and a water molecule.
The Bronsted-Lowry definition of an acid is based on the concept of proton transfer, and it is a more general and inclusive definition than the traditional Arrhenius definition, which only considers acids as substances that produce hydrogen ions (H + ) in water. The Bronsted-Lowry concept is widely used in chemistry to explain the behavior of acids and bases in aqueous solutions.
Like we mentioned above, a Bronsted-Lowry acid, aka Bronsted Acid, is a proton donor, meaning it can release a proton; when an acid dissociates in solution, it increases the H+ yield. Again, nitric acid, HNO3, can be labeled as an acid because it will dissociate and create H+ and NO3 – ions.
A Bronsted-Lowry base, aka Bronsted base, is a proton acceptor; when a base dissociates, it takes a proton from water to generate OH – ions in solution. So is NH3 an acid or base? Ammonia, NH3, is an example of a base because it takes a proton from water to form the products of NH4 + and OH – .
In this theory, water can be either an acid or a base; this is because it can accept a proton to form H3O + or donate a proton to form OH – ; when a compound can act as both a Bronsted-Lowry acid or base, it is said to be amphoteric.
The Bronsted-Lowry theory adds onto the Arrhenius theory. The general idea of yielding H + and OH – ions is the same, but this theory is more general, which allows more compounds to be labeled as acids or bases; for example, bases do not have to contain OH – (they have to in the Arrhenius theory) because taking a proton from water forms the hydroxide ion.
This theory is has the least specificity, meaning it allows for a broad selection of acids and bases. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. There is no mention of protons, or hydrogens, which creates a chance for more compounds to fit this category. The Lewis theory states that a lewis base donates a pair of electrons that is either shared with or used by a lewis acid. Read our Lewis Acids & Bases tutorial to go into more detail!