Chemistry of Acid and Bases

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Acids- a word derived from Latin word "acere" or French word "acide" means "sour"; or you could say everything that tastes "sour" has acid in it. For example, the citric acid in all citrus fruits like orange, lemon juice, kiwi, strawberries, the tartaric acid in grape juice and wine, or the acetic acid in vinegar.

However, this is not the only property that characterizes an acid.

An acid is corrosive in nature, which makes it widely used as a cleaning agent. It changes the colors of indicators, turning blue litmus red, and even conducts electric current. The electrolyte in your car battery is also acid. Most of the food we eat are also acidic. The chart shows the acidity of most foods we consume. Acids have a pH lower than 7 on the pH scale.

 

Bases have a characteristic bitter taste and a soapy feel. They change the colour of red litmus to blue and have a pH greater than 7. Soluble bases are called alkalies. The alkali you may be most familiar with is sodium hydroxide also called 'caustic soda'. It is a common lab reagent and is also used as household lye. Although it has a soapy feel( which you can feel only in very dilute solutions), it cannot be tasted for a bitter taste. It is so corrosive that it causes deep, slow healing burns! However, mild bases find abundant daily uses. Baking soda( alkaline sodium carbonate), for example, is a basic kitchen staple. On getting mixed up with the acid of buttermilk or vinegar in food, it releases CO2 gas which renders the dish a lovely foamy texture. Some other uses of bases are listed below:

While the properties of acids and bases were known since long, the first attempt to describe it's chemistry was made by a Swedish  chemist Svante Arhenius in the year 1887. According to his theory, acids are substances that furnish H+ ions in aqueous solutions while bases are those that furnish OH- ions in aqueous solutions. In water, NaOH and Ca(OH)2 dissociate to give OH- ions while HCl and HNO3 give H+ ions.

Thus NaOH and Ca(OH)2 are bases, and HCl and HNO3 are acids. All acid and base combine to form salt and water. This reaction is called a neutralization reaction, which is illustrated below.

However, this theory couldn't explain the acidic or basic nature in non-aqueous solvents like ammonia. It also fails to explain the acidity of CO2, SO2, NO2, SO3 and basicity of CaO, MgO.

In 1923, a Danish chemist Johannes Nicolaus Brønsted and a British chemist Thomas Martin Lowry independently put forward a theory, widening the concept of acids and bases. According to their theory, any substance that donates a proton or H+ is an acid, while any substance that accepts a proton is a base. This brought in the concept of conjugate acid and conjugate base. Consider an aqueous ammonia molecule.

It splits water molecule to give an ammonium ion and a hydroxide ion, thus acting as a base. Now this ammonium ion is able to release the proton, acting as an acid.

Thus NH3 is a base and NH4+ is its conjugate acid. Similarly, H2O can form 2 acid base pairs as shown.

Here water acts both as an acid and a base. This is called the ' amphiprotic' nature of water.

Thus from the theory, acidic or basic nature of all proton containing ions or molecules such as H3O+, NH4+, HSO4- and  Cl-,Br-, NO3-,  etc can be explained.

However, a loophole still exists. We don't get an explanation for the acidity of CO2, SO2, or NO2, neither for the basicity of CaO and MgO.

 

In 1923, an American chemist G.N. Lewis endeavoured to broaden this concept further. He postulated that any substance accepting a lone pair of electron is an acid, while the one donating the electron pair is a base. This eliminates the need of the acid or base to be an ion. They could be both ions, or both neutral molecules as well.  Let's look into some of the examples. All the electron rich neutral molecules like NH3, H2O, R-O-H, etc and all the negatively charged ions as F-, CN-, HSO4-, etc can act as a base. While, all the electron deficient neutral molecules like BF3, AlCl3( all with an incomplete octet) and all positively charged ions as Fe3+, Al3+, Ag+, H+ etc can act as an acid. All these are defined as Lewis acids and bases. 

An example of Lewis acid and base reacting is:

The reaction between ammonia and boron trifluoride gives a Lewis acid-base adduct. Here ammonia is a base while BF3 is and an electron deficient Lewis acid. However, you must have noticed that all the Bronsted- Lowry acids and bases are Lewis Acids and bases, but not the other way around.

Till date, Lewis theory is the most acceptable theory, but the use of Bronsted-Lowry theory is still prevalent.

Posted by: Jagriti Sharma. in Science | Date: 27/01/2016

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