Question

State differences between acids and bases.

Answer 1

Hint: There are three major breakthroughs which have shaped the definition of acids and bases. Describe each one of them in a chronological fashion.

Complete step-by-step answer:
One of the first explanations about acids and bases was given by Arrhenius. He stated the following:
- Acids are substances which when dissolved in water produce ${{H}^{+}}$ ions. Therefore they increase the overall concentration of ${{H}^{+}}$ ions. It is now known that protons do not freely exist in water. Hydronium ions (${{H}_{3}}{{O}^{+}}$) are the entities that represent the population of protons. These ions are formed by the following process:
\[{{H}^{+}}+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}\]
- Bases are substances that produce $O{{H}^{-}}$ ions when dissolved in water. They therefore increase the concentration of hydroxide ions making a solution more basic.
But Arrhenius could not cover all the compounds which showed acidity or basicity through his definitions. The next important breakthrough came when Bronsted and Lowry gave their theories. They are as follows:
- Acids are substances that donate ${{H}^{+}}$ ions into the solution and convert into their conjugate base forms. For example when $HCl$is made to dissolve in water it dissociates into a proton and a chloride ion as follows:
\[HCl\to {{H}^{+}}+C{{l}^{-}}\]
This makes $HCl$ an acid and the chloride ion ($C{{l}^{-}}$) as its conjugate base.
But many compounds still stood out which were acidic or basic in nature, such as those which are not able to dissolve in a polar solvent or those which do not contain hydrogen atoms in their formula (for example metal salts).
The definition which is accepted today comes from the scientist Gilbert N. Lewis. The Lewis concept states the following:
- Acids are compounds which are able to accept an electron pair. For example let us take aluminium chloride ($AlC{{l}_{3}}$). It does not have hydrogen atoms neither it donates or accepts them, but it is still characterised as an acid because it is able to accept an electron pair due to its vacant d-orbitals.
- Lewis bases are compounds which can donate a pair of electrons to others, forming coordinate bonds with them. For example if we take fluoride ion (${{F}^{-}}$) which has a negative charge, implying it has an excess of electrons and is also capable of donating them.
This theory is also applicable to acids which were already classified as one by the previous concept. For example in the example given below:
\[HCl\to {{H}^{+}}+C{{l}^{-}}\]
The proton can accept a pair of electrons while chloride ions can donate a pair of electrons which make them a Lewis acid-base pair.

Note: Not all the compounds which have an excess of electrons or a negative charge on them, are capable of donating and therefore can become Lewis bases. Same applies to Lewis acids, as not every compound which has a vacant space in their orbital can accept electrons. There are various criteria to determine these things.