Bronsted vs Lewis
Acids and bases are two important concepts in chemistry. They have contradictory properties. We normally identify an acid as a proton donor. Acids have a sour taste. Lime juice, vinegar are two acids we come across at our homes. They react with bases producing water, and they also react with metals to form H2, thus increase metal corrosion rate. Acids can be categorized into two, based on their ability to dissociate and produce protons. Strong acids like HCl, HNO3 are completely ionized in a solution to give protons. Weak acids like CH3COOH are partially dissociates and give fewer amounts of protons. Ka is the acid dissociation constant. It gives an indication of the ability to lose a proton of a weak acid. To check whether a substance is an acid or not we can use several indicators like litmus paper or pH paper. In the pH scale, from 1-6 acids are represented. An acid with pH 1 is said to be very strong, and as the pH value increases, acidity is decreased. Moreover, acids turn blue litmus to red.
Bases have a slippery soap like feeling and a bitter taste. They react easily with acids producing water and salt molecules. Caustic soda, ammonia, and baking soda are some of the common bases we come across very often. Bases can be categorized into two, based on their ability to dissociate and produce hydroxide ions. Strong bases like NaOH and KOH are completely ionized in a solution to give ions. Weak bases like NH3 are partially dissociated and give fewer amounts of hydroxide ions. Kb is the base dissociation constant. It gives an indication of the ability to lose hydroxide ions of a weak base. Acids with a higher pKa value (more than 13) are weak acids, but their conjugate bases are considered as strong bases. To check whether a substance is a base or not we can use several indicators like litmus paper or pH paper. Bases show a pH value higher than 7, and it turns red litmus to blue.
Other than the above characteristics we can identify acids and bases based on some other features. Acids and bases are defined in several ways by various scientists like Bronsted, Lewis and Arrhenius.
Bronsted defines a base as a substance that can accept a proton and acid as a substance that can give out a proton. Bronsted put this theory forward in 1923. At the same time, Thomas Lowry independently presented the same theory. Therefore, this definition is known as Bronsted-Lowry definition.
In 1923 Lewis put forward his theory on acids and bases. There, he explains an acid as a species, which accepts an electron pair. A Lewis base is a substance that can donate an electron pair. So according to Lewis, there can be molecules, which don’t possess hydrogen, but can act as an acid. For example, BCl3 is a Lewis acid, because it can accept an electron pair. And also molecules, which don’t possess hydroxides, can act as a base. For example, NH3 is a Lewis base, because it can donate the electron pair on nitrogen.
What is the difference between Bronsted and Lewis?
• Bronsted defines an acid as a proton donor whereas Lewis defines an acid as an electron pair acceptor.
• According to the Bronsted theory, a base is a proton acceptor. According to the Lewis theory a base is an electron pair donor.
• Therefore, some molecules, which don’t possess protons, can be acids according to the Lewis theory.