8.1 Theories of Acids and Bases

Syllabus

What does this mean?

What is an acid?

Theory 1

People had been aware that sour tasting substances like lemon juice and vinegar shared some properties – reacted similarly. Even the name comes from the Latin for sour

Alkalis were solutions that clearly neutralised acids. The name comes from the Arabic for ashes because wood ash forms alkaline solutions.

Theory 2

Lavoisier knew that Oxygen made wine and milk sour. He knew that Sulphur burned in Oxygen and dissolved to make Sulphuric Acid. So, he assumed that all acids must contain Oxygen. He even invented the name Oxygen from the Greek for Acid-Maker

But Hydrogen Cyanide solution (HCN) is acidic & Hydrogen Chloride solution (HCl) is Hydrochloric Acid. So the theory was clearly false.

Theory 3

It became clear that all known acids contained Hydrogen – though not all Hydrogen-containing substances were acidic.

· August Arrhenius defined acids as substances that dissociate when dissolving to form Hydrogen ions (H+).

· He defined alkalis as bases that dissociate when dissolving to form Hydroxide ions (OH-).

Neutralisation was the reaction between the two ions:

H+(aq) + OH-(aq)→ H2O(l)

This is still the correct answer if asked for the Ionic Equation for Neutralisation.

Theory 4

Arrhenius’ ideas work in most cases, but not all.

One of the problems was the reaction between Ammonia and HCl.

When dissolved in water there is no problem because Ammonia dissolves to form (some) OH- ions:

NH3(g) + H2O(l) → NH4+(aq) + OH-(aq)

And this can neutralise HCl(aq) to form Ammonium Chloride (NH4Cl)

But Ammonia gas still neutralises HCl(g) to produce Ammonium Chloride

So, the theory needed adapting.

Bronsted and Lowry worked independently but had much the same idea.

They still believed that Acids donated H+ ions – but called them proton donors.

In reality, protons in water form coordinate bonds to water molecules.

H+(aq) + H2O(l) → H3O+(aq)

Draw a dot-cross diagram to show the bonding in a Hydronium ion (H3O+)

Bronsted and Lowry believed that Bases were proton-acceptors

This explained how the two gases neutralised each other without OH- ions.

NH3 + HCl → NH4+ + Cl- (or NH4Cl )

  • Draw a dot-cross diagram to show the bonding in an Ammonium ion, (NH4+).


  • Explain how Ammonia is acting as a base when it becomes ammonium ions.


Exercise

When Limestone dissolves in rainwater containing H2CO3 it reacts as below:

CaCO3(s) + 2H2CO3(aq) --> Ca(HCO3)2(aq) + H2O(l) + CO2(g)

a) Identify the B-L Acid in the reaction. Explain how you know which one it is?

b) Identify the B-L Base in the reaction. Explain how you know which one it is?

CONJUGATE ACID BASE PAIRS

When a weak acid and a weak base neutralise each other an equilibrium is formed.

CH3COOH (aq) + NH3 (aq) ⇌ CH3COO- (aq) + NH4+ (aq)

The Ethanoate ion (CH3COO-) may well pick up an H+ ion from the Ammonium ion (NH4+).

This means that the Ethanoate ion could act as a Base .

And, the Ammonium ion could act as an Acid because it would have to donate the H+ ion.

We say that the Ethanoate ion is the Conjugate-Base of Ethanoic Acid in the reaction.

And, the Ammonium ion is the Conjugate-Acid of the Ammonia (base).

Exercise

Identify the acid-base conjugate pairs in the following equations

I. HF + H2O → F−+ H3O+

II. HSO4- + NH3 →SO42− + NH4+

III. C2H3OO− + HCl → C2H3OOH + Cl−

IV. HNO2 + H2O → H3O+ + NO2-

V. HCN+H2O→H3O+ +CN−

Note: When a strong acid neutralises a base it is too powerful a proton donor to re-accept the H+ ion. So strong Acids make extremely weak Conjugate Bases

Amphiprotic Species

There are substances that can either accept or donate protons depending on which other substances they mix with.

Amino acids have both an acid group (which can donate protons) and an amino group (which can accept them).

· Sulphuric Acid – Hydrogen Sulphate ions.

When Sulphuric acid donates one proton it makes the Hydrogen Sulphate ion.

This may either lose another proton or regain one as shown below.

Which substance is Amphiprotic? How do you know?





Autoionisation of Water

Water molecules may interact with each other to form ions as shown below:

How does this show that water is Amphiprotic?