A lot of you must have heard of acids and bases. In this article, we will look into what acids and bases really are and how they work.
So, what are acids and bases? Over the years, acids and bases have been defined in a number of different ways, each definition a more progressive version of the previous one. In this article, we will be defining acids and bases in three ways:
⦁ Arrhenius Concept
⦁ Bronsted – Lowry Concept
⦁ Lewis Concept
Arrhenius Concept
The Arrhenius Theory was the first modern definition of acids and bases in molecular terms. According to the Arrhenius Concept, an Acid is a substance that donates Hydrogen ions in water and a Base is a substance that donates Hydroxyl ions in water. This can be shown as:
Acid
HCl→H+ + Cl-
Base
NaOH→Na+ + OH-
What about water? Water dissociates partially into Hydrogen and Hydroxyl ions, by itself. This behavior of water is known as auto-dissociation. It can be shown as:
H2O ⇌ H+ + OH-
As you can see, water donates Hydrogen ions as well as Hydroxyl ions, upon dissociation. Therefore, water can be classified as an acid as well as a base. Substances that show both acidic and basic behavior are known as amphiprotic substances.
Bronsted – Lowry Concept
In the Arrhenius theory, we saw that a substance is classified as an acid or a base depending on whether it donates a hydrogen or hydroxyl ion when dissolved in water. However, what if the reaction was taking place in the absence of water? Would the substance not be classified as an acid or a base? This is where Bronsted – Lowry improved the acid – base definition:
According to the Bronsted – Lowry Theory, an Acid is a substance that donates Hydrogen ions to another species, while a Base is a substance that can accept Hydrogen ions from another species. So, an acid can be shown as:
HCl → H+ + Cl-
Here HCl acts as an acid by donating the Hydrogen ion
A base can be shown as: NH3 + H2O ⇌NH4+ + OH-
NH3 acts as base by accepting a Hydrogen ion from water
As you can see, the Bronsted –
Lowry theory eliminates the need for a solvent at all. Therefore, the presence of water as a solvent is not required, according to the Bronsted – Lowry Concept.
Lewis Concept
Now, how does the Lewis Concept differ from the Bronsted – Lowry Concept? As we saw before in the Bronsted –
Lowry theory, the substance must be able to donate a Hydrogen ion in order to be classified as an Acid and it must be able to accept a Hydrogen ion to be classified as Base. But what if the substance in question does not contain any Hydrogens? Will it not be able to act as an acid or base?
This is where the Lewis Concept comes in. In the Lewis Theory, Acids are defined as substances that can accept a pair of electrons while Bases are substances that can donate a pair of electrons. This eliminates the need for the substance in question to contain a Hydrogen, making it easier and more generalised to define acids and bases.
Therefore, a Lewis acid – base pair can be shown as:
Boron Trifluoride acts as a Lewis acid by accepting an electron pair from the Fluoride ion
BF3 + F- → BF4 –
Fluoride ion acts as a Lewis base by donating a pair of electrons
Note that in Bronsted –
Lowry, the acid was donating a Hydrogen ion, while in the Lewis theory, it accepts an electron. In both cases, the substance acting as an acid is gaining a negative charge (hydrogen ion has a positive charge, so losing the positive charge will make the charge on the acid negative). On the other hand, both Bronsted – Lowry and Lewis bases gain positive charges (by accepting a Hydrogen ion or by losing an electron). Therefore, it can be seen that the Lewis theory complements the Bronsted – Lowry theory, and also adds more to it, so that the definition encompasses more substances!
How can we tell whether a substance is an acid or a base? Acids and bases give characteristic colours with specific substances. Some common tests for acids and bases are:
1. Litmus paper: Litmus paper turns Red in acid and Blue in base
2. Indicators: There are certain chemicals that give characteristic colours in acids and bases. Two of the most common indicators are – (i) Phenolphthalein – Colourless in acid and Pink in base
(ii) Methyl Orange – Red in acid and Yellow in base
3. Universal Indicator: Universal Indicator is an indicator composed of a number of indicators, that tells us whether the substance is an acid or a base. Universal indicators also allow us to determine how strong an acid or a base is.
So, how can we tell just how acidic or basic a substance is? The strength of an acid or a base can be determined by measuring its pH. The pH scale has a range of 1 to 14, with 1 being the strongest acid and 14 being the strongest base. A pH of 7 tells us that the substance is neutral. That is, it is neither an acid nor a base. Water has a pH of 7.
pH of a substance can be measured directly by using a pH meter. pH can also be experimentally measured by using a method called “Titration”, where the reaction between a measured amount of acid and base will allow us to calculate the pH.
However, that is for another article! Hope this was helpful!
-Mehrin,Dhaka University,Chemistry