Arrhenius and Bronsted-Lowry Theory of Acids and Bases
In Year 11 Module 3: Reactive Chemistry, you learned how to write balanced chemical equations for acid/base and acid/carbonate reactions. Students were given physical properties of acids and bases, such as acids taste sour and bases have a slippery touch, but this did not categorize acids and bases in terms of their chemical behaviour.
In Year 12 Module 6: Acid/Base Reactions, we give you two theories about the chemical behaviours of acids and bases.
The Arrhenius Theory
Svante Arrhenius
The Arrhenius theory defines an acid as ‘a substance that produces hydrogen ions in aqueous solution’ and a base as ‘a substance that produces hydroxide ions in aqueous solution.’
HA(aq) → H⁺(aq) + A⁻(aq)
BOH(aq) → OH⁻(aq) + B⁺(aq)
I put the definition in quotations to emphasise that the Arrhenius theory defined acids and bases in aqueous solution only. This is one of the many disadvantages of the theory - that it is limited to one particular state of matter. It does not explain why HCl(g), for example, is as much of an acid as HCl(aq) is. Another disadvantage of the Arrhenius theory is it does not explain the acidity of non-metal oxides and alkalinity of metal oxides that, respectively, do not have a hydrogen ion or hydroxide ion in its chemical structure that can be produced in aqueous solution. Thirdly, the Arrhenius theory does not explain why ammonia (NH₃) is a base as it does not have a hydroxide ion in its formula.
The Arrhenius theory does have its advantages. One advantage is that it effectively explains the production of water during a neutralisation reaction. The hydrogen ion from the acid and the hydroxide ion from the base synthesise to produce a water molecule according to the following equation.
H⁺(aq) + OH⁻(aq) → H₂O(l)
The Bronsted-Lowry Theory
Johannes Nicolaus Bronsted (left)
Thomas Martin Lowry (right)
Bronsted and Lowry are two scientists who independently theorised the following definitions. An acid is a substance that donates a proton to a base, and a base is a substance that accepts a proton from an acid. Put simply, an acid is a proton donor and a base is a proton acceptor. A hydrogen ion is a proton. A neutral hydrogen atom contains one proton and one electron; losing the electron to become an ion means it is composed of a single proton.
If the Bronsted-Lowry acid or base is present in water in the form of an aqueous solution, the proton is donated to or accepted from the water molecule. Below are the dissociation equations for an acid and base in water.
HA(aq) + H₂O(l) → A⁻(aq) + H₃O⁺(aq)
B(aq) + H₂O(l) → BH⁺(aq) + OH⁻(aq)
One key advantage of the Bronsted-Lowry theory over the Arrhenius theory is that it relates the acid and base to each other by the process of proton transfer. Also, there is no state of matter specified so as long as the chemical formula is the same, the substance is an acid or a base regardless of whether it is a solid, liquid or gas. This means HCl(g) and HCl(aq) are both Bronsted-Lowry acids. Furthermore, because a base is only required to accept a proton and not have anything in its chemical formula, the Bronsted-Lowry theory can explain the basicity of metal oxides. The Bronsted-Lowry theory is also beneficial because it can explain why ammonia is a base. Ammonia can accept a proton from an acid as seen in the following equation with water.
NH₃(aq) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq)
However, like the Arrhenius theory, a Bronsted-Lowry acid needs to have a hydrogen ion in its formula that it can donate (to a base). This means the Bronsted-Lowry theory also cannot explain the acidity of non-metal oxides.
Bonus: The Lewis Theory
The Lewis theory is outside the scope of the HSC syllabus but it offers the most comprehensive definition of an acid and base. A Lewis acid is a substance that can accept a pair of electrons from a Lewis base. Defining an acid and base in terms of its electronic configuration does not limit it to molecules that contain particular elements. This means the Lewis theory overcomes the main disadvantage of the previous Bronsted-Lowry theory. The Lewis theory is able to explain why non-metal oxides are acidic.
Question: Which of the following equations shows the first species in the reaction acting as a Bronsted-Lowry base but not an Arrhenius base?
A. HCl(aq) + H₂O(l) → Cl⁻(aq) + H₃O⁺(aq)
B. OH⁻(aq) + H₃O⁺(aq) → 2H₂O(l)
C. CH₂C₆H₅⁻(aq) + NH₃(aq) → CH₃C₆H₅(aq) + NH₂⁻(aq)
D. HCl(aq) + NH₃(aq) → NH₄⁺(aq) + Cl⁻(aq)
Answer: C
A Bronsted-Lowry base accepts a proton from a Bronsted-Lowry acid. An Arrhenius base produces a hydroxide ion (OH⁻) in aqueous solution.
A and D are incorrect as the first species, HCl, is seen donating a proton to the second species, making it a Bronsted-Lowry acid, not base.
B is incorrect as the OH⁻ ion is both an Bronsted-Lowry base and Arrhenius base. The question is looking for a Bronsted-Lowry base only.
C is the answer as CH₂C₆H₅⁻ turns into CH₃C₆H₅, indicating it accepted a proton like a Bronsted-Lowry base, but it does not have an OH in its formula so it cannot be an Arrhenius base.
