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In 1903, a scientist named Svante Arrhenius became the first Swede to win a Nobel Prize. He received it for his work on electrolytes and ions in aqueous solution, including his theory of acids and bases. In 1923, Johannes Nicolaus Brønsted and Thomas Martin Lowry both independently built on his work to arrive at a new definition of acid and base, named the Brønsted-Lowry theory of acids and bases in their honour.
According to Arrhenius:
But Brønsted and Lowry both thought that this definition was too narrow. Take the reaction between aqueous ammonia and hydrochloric acid, shown below.
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.
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:
An acid is a proton donor, whilst a base is a proton acceptor.
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.
A proton is just the hydrogen-1 nucleus, H+. But in actual fact, when acids dissociate in water, they form a hydronium ion, H3O+, and a negative ion. However, it can be a lot easier to represent the hydronium ion as an aqueous hydrogen ion, H+.
Look at the following two reactions:
You’ll notice that both reactions involve water, H2O. However, water plays two very different roles in the two different reactions.
Water can behave as both an acid and a base. We call these types of substances amphoteric
Some examples of common Brønsted-Lowry acids and bases are given below:
|Name of acid||Formula||Fun fact||Name of base||Formula||Fun fact|
|Hydrochloric acid||HCl||This acid is found in your stomach and is responsible for heartburn and acid reflux.||Sodium hydroxide||NaOH||Sodium hydroxide is a common means of disposing of corpses... Roadkill, obviously.|
|Sulphuric acid||H2SO4||60% of all manufactured sulphuric acid is used in fertilisers.||Potassium hydroxide||KOH||Potassium hydroxide can be used to identify species of fungi.|
|Nitric acid||HNO3||Nitric acid is used to make rocket fuels.||Ammonia||NH3||You can find ammonia on planets such as Jupiter, Mars, and Uranus.|
|Ethanoic acid||CH3COOH||You find this acid in the vinegar you put on your fish and chips.||Sodium bicarbonate||NaHCO3||This base is responsible for the fluffiness of your favourite cakes and pancakes.|
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.
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:
A conjugate acid is a base that has accepted a proton from an acid. It can act just like a normal acid by giving up its proton. On the other hand, a conjugate base is an acid that has donated a proton to a base. It can act just like a normal base by accepting a proton.
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:
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.
But you’ll notice that the reaction is reversible. What happens in the backward reaction?
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.
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.
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).
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:
In general: The stronger an acid or base, the weaker its conjugate partner. This works the other way round too.
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.
A salt is an ionic compound consisting of positive and negative ions held together in a giant lattice.
Neutralisation reactions include:
Hydroxides are a special type of base known as an alkali.
Alkalis are bases that dissolve in water.
All alkalis are bases. However, not all bases are alkalis!
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:
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):
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+):
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:
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:
The same thing happens with ethanoic acid instead of hydrochloric acid.
These neutralisation reactions happen because in solution, acids and bases ionise. Ionisation is the process of losing or gaining electrons to form a charged species. However, ionisation can also involve moving other atoms around, which is what happens here. Take the example of sodium hydroxide and hydrochloric acid. Hydrochloric acid ionises in solution to form hydronium ions (H3O+) and chloride ions (Cl-):
Sodium hydroxide ionises to form hydroxide ions and sodium ions:
The ions then react with each other to form our salt and water:
If we combine the three equations, then one of the water molecules cancels out:
Common bases include NaOH, KOH, and NH3.
A conjugate acid is a base that has accepted a proton from an acid, whilst a conjugate base is an acid that has lost a proton.
Acids and bases react to form conjugate bases and acids respectively. These are known as conjugate pairs.
An amphoteric substance is a species that can act as both an acid and a base.
A neutralisation reaction is a reaction between an acid and a base. It produces a salt, and often water.
A Brønsted-Lowry acid is a proton donor whilst a Brønsted-Lowry base is a proton acceptor.
Brønsted-Lowry acids include hydrochloric acid, sulfuric acid and ethanoic acid. Brønsted-Lowry bases include sodium hydroxide and ammonia.
A conjugate base is an acid that has lost a proton and a conjugate acid is a base that has accepted a proton. All acids form conjugate bases when they react and all bases form conjugate acids. Therefore, acids and bases all come with a paired conjugate base or acid respectively. For example, the conjugate base of the hydrochloric acid is the chloride ion.
A Brønsted-Lowry acid is a proton donor.
You identify Brønsted-Lowry acids and bases by considering their reactions with other species. Brønsted-Lowry acids lose a proton, whilst Brønsted-Lowry bases gain a proton.
What is a Brønsted-Lowry acid?
A proton donor.
What is a Brønsted-Lowry base?
A proton acceptor.
What is a conjugate acid?
A base that has accepted a proton from an acid.
What is a conjugate base?
An acid that has lost a proton.
What is an amphoteric substance?
A substance that can behave both as an acid and as a base.
What is a neutralisation reaction?
A reaction between an acid and a base.
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