Brønsted–Lowry Acids & Bases (DP IB Chemistry): Revision Note
Brønsted–Lowry acids & bases
What are Brønsted-Lowry acids and bases?
The Brønsted-Lowry Theory defines acids and bases in terms of proton transfer between chemical compounds
A Brønsted-Lowry acid is a species that gives away a proton (H+)
A Brønsted-Lowry base is a species that accepts a proton (H+) using its lone pair of electrons
Equilibrium of a Brønsted-Lowry acid and base
The Brønsted-Lowry Theory is not limited to aqueous solutions only and can also be applied to reactions that occur in the gas phase
The reaction between hydrogen chloride gas and water is as follows:
HCl (g) + H2O (l) → H3O+ (aq) + Cl- (aq)
The formation of the ions is known as dissociation or ionisation
H2O is acting as a base (proton acceptor)
HCl is acting as an acid (proton donor)
Diagram to show how hydrochloric acid acts as a Brønsted-Lowry acid and ammonia acts as a Brønsted-Lowry base
Worked Example
Identify the correct role of the species in the following reaction:
H2PO4−(aq) + H2O(l) → HPO42−(aq) + H3O+(aq)
| Brønsted-Lowry acid | Brønsted-Lowry base |
|---|---|---|
A | H2PO4– | H2O |
B | H2PO42– | H2PO4– |
C | H2PO4– | H3O+ |
D | H2O | H2PO4– |
Answer:
The correct option is A.
H2PO4− is donating a proton to H2O
So, H2PO4− must be an acid and H2O must be a base
Examiner Tips and Tricks
An atom of hydrogen contains 1 proton, 1 electron and 0 neutrons
When hydrogen loses an electron to become H+ only a proton remains, which is why a H+ ion is also called a proton.
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