Brønsted-Lowry Acids and Bases

Brønsted-Lowry theory of acids and bases

According to Arrhenius:

• An acid is a substance that produces hydrogen ions in solution.
• A base is a substance that produces hydroxide ions in solution.

But Brønsted and Lowry both thought that this definition was too narrow. Take the reaction between aqueous ammonia and hydrochloric acid, shown below.

${\mathrm{NH}}_3\left(\mathrm{aq}\right)+\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)$

You’ll probably agree that this is indeed an acid-base reaction. Hydrochloric acid dissociates in solution to form hydrogen ions and chloride ions, and ammonia reacts with water to form ammonium ions and hydroxide ions. By Arrhenius’ definition, they are therefore acids and bases respectively.

$\mathrm{HCl}\to {\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}$

${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {{\mathrm{NH}}_4}^{+}+{\mathrm{OH}}^{-}$

However, if we instead combined the two reactants in gaseous form, the exact same reaction producing the exact same product wouldn’t count as an acid-base reaction! This is because it isn’t in solution. Brønsted and Lowry instead focused on how acids and bases react with other molecules.

According to the Brønsted-Lowry theory:

This means that an acid is any species that reacts by releasing a proton, while a base is a species that reacts by taking up a proton. This still fits in with Arrhenius’ theory - for example, in solution an acid reacts with water by giving a proton to it.

Examples of Brønsted-Lowry acids and bases

Some examples of common Brønsted-Lowry acids and bases are given below:

Reactions of Brønsted-Lowry acids and bases

The Brønsted-Lowry theory gives a general equation for reactions between acids and bases:

acid + base ⇌ conjugate acid + conjugate base

A Brønsted-Lowry acid always reacts with a Brønsted-Lowry base to form a conjugate acid and a conjugate base. This means that acids and bases must go around in pairs. One substance donates a proton and the other accepts it. You’ll never find a hydrogen ion, which you’ll remember is a proton, by itself. This means you can never find just an acid by itself - it will always be reacting with some sort of base.

Conjugate acids and bases

As you can see from the above equation, when an acid-base pair reacts, it produces substances known as conjugate acids and conjugate bases. According to the Brønsted-Lowry theory:

Let’s look at this in more detail.

Take the general equation for the reaction of an acid with water. We represent the acid using HX:

$\mathrm{HX}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{X}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

In the forward reaction, the acid donates a proton to the water molecule, which is therefore acting as a base. This forms a negative X^- ion and a positive H₃O⁺ ion, shown below.

$\mathrm{HX}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{X}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

But you’ll notice that the reaction is reversible. What happens in the backward reaction?

${\mathrm{X}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}\to \mathrm{HX}+{\mathrm{H}}_2\mathrm{O}$

This time, the positive H₃O⁺ ion donates a proton to the negative X^- ion. The H₃O⁺ ion acts as an acid and the X^- ion acts as a base. By definition, the H₃O⁺ ion is a conjugate acid - it was formed when a base gained a proton. Likewise, the X^- ion is a conjugate base - it was formed when an acid lost a proton.

To summarise, our species that initially behaved as an acid turned into a base, and our basic species turned into an acid. These acid-base combinations are called conjugate pairs. Every acid has a conjugate base, and every base has a conjugate acid.

In summary:

The reaction between an acid and a base forms a conjugate base and a conjugate acid. Vaia Original

You could also look at this reaction from back to front. This way, H₃O⁺ is our original acid that donates a proton to form H₂O, our conjugate base, and Cl^- is a base that gains a proton to form a conjugate acid.

Conjugate acids and bases behave just like any other acid or base. Vaia Original

Look at the following example, the reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl). Here, hydrochloric acid acts as an acid by donating a proton, which sodium hydroxide accepts. This means that sodium hydroxide is a base. We form sodium chloride (NaCl) and water (H₂O).

$\mathrm{HCl}\left(\mathrm{aq}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right)\to \mathrm{NaCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

However, if this reaction reverses, then water donates a proton which sodium chloride accepts. This makes water an acid and sodium chloride a base. Therefore, we have formed two conjugate pairs:

The reaction between hydrochloric acid and sodium hydroxide, and the conjugate acid and base they form. Vaia Original

In general: The stronger an acid or base, the weaker its conjugate partner. This works the other way round too.

Examples of Brønsted-Lowry acid and base reactions

Now that we know what Brønsted-Lowry acids and bases are, we can move on to look at some reactions between common acids and bases. Any reaction between an acid and a base is known as a neutralisation reaction, and they all produce a salt. Most also produce water.

Neutralisation reactions include:

• Acid + hydroxide.
• Acid + carbonate.
• Acid + ammonia.

Acid + hydroxide

Hydroxides are a special type of base known as an alkali.

Reacting an acid with a hydroxide gives a salt and water. For example, hydrochloric acid and sodium hydroxide react to give sodium chloride and water. We looked at this reaction earlier in the article:

$\mathrm{HCl}+\mathrm{NaOH}\to \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$

Acid + carbonate

Acids react with carbonates to give a salt, water and carbon dioxide. For example, if you react sulfuric acid (H₂SO₄) with magnesium carbonate (MgCO₃), you produce the salt magnesium sulfate (MgSO₄):

${\mathrm{MgCO}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{MgSO}}_4+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

Acid + ammonia

Reacting an acid with ammonia (NH₃) gives an ammonium salt. For example, we can react ethanoic acid (CH₃COOH) with ammonia to produce ammonium ethanoate (CH₃COO^-NH₄⁺):

${\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{NH}}_3\to {\mathrm{CH}}_3{\mathrm{COO}}^{-}{{\mathrm{NH}}_4}^{+}$

You may have noticed that this doesn’t look like a typical neutralisation reaction - where is the water? However, if we take a closer look at the reaction, we can see that water is actually produced.

In solution, ammonia molecules react with water to form ammonium hydroxide (NH₄OH). If we then add acid to the solution, the ammonium hydroxide ions react with the acid to produce an ammonium salt and - you guessed it - water.

Take a look at the following equation for the reaction between ammonia and hydrochloric acid. It has two steps:

${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NH}}_4\mathrm{OH}$

${\mathrm{NH}}_4\mathrm{OH}+\mathrm{HCl}\to {\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$

The second step produces water, as you can clearly see. If we combine the two equations, the water molecules cancel out, and we get the following:

${\mathrm{NH}}_3+\mathrm{HCl}\to {\mathrm{NH}}_4\mathrm{Cl}$

The same thing happens with ethanoic acid instead of hydrochloric acid.