IB Chemistry Sub-topic R3.1 Notes

Proton transfer

Topic R3.1 focuses on proton transfer reactions, the most common of which are reactions involving acids and bases. However, their properties are rather complex and require an in-depth look to understand the full process. Acids and bases can be described by three theories:

1. Ionic theory - involves the formation of protons.
2. Brønsted-Lowry theory - involves the transfer of protons.
3. Lewis theory - involves the transfer of electrons, so covered in Topic R3.4. 

Ionic Theory 

Let's start with ionic theory, which is very simplistic. It has two key principles:

1. Acids dissolve in water to produce H⁺ at a concentration greater than 1.0 x 10^-7 mol dm^-3.
2. Bases dissolve in water and can neutralize acids.

Brønsted-Lowry Theory

Brønsted-Lowry theory is more complicated. This also has two principles:

1. Acids dissociate to produce H⁺, which is subsequently donated to another molecule.
2. Bases dissociate to produce OH^-, and subsequently accepts a proton.

Conjugate Acids and Bases 

The species formed from these reactions are:

1. Conjugate base - the ion formed by an acid donating a proton. As a result, this ion can now accept a proton and is thus a base.
2. Conjugate acid - the ion formed by a base accepting a proton. As a result, this ion can now donate a proton and is thus an acid.

As you can see, a conjugate acid-base pair thus differs by one proton.

However, some species can act as both both Brønsted-Lowry acids and bases, which are called amphiprotic species. An example is HCO₃^-:

1. It can donate a proton to become CO₃^2-.
2. It can accept a proton to become H₂CO₃.