IB syllabus > acids & bases (sl) > 8.1 

These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

8.1 - Theories of acids and bases


8.1.1: Define acids and bases according to the Brønsted Lowry and Lewis theories. TOK: Discuss the values of using different theories to explain the same phenomenon. What is the relationship between depth and simplicity


Arrhenius theory

The Arrhenius theory of acids is based on the idea of the behaviour of hydrogen ions in aqueous solution.

Brønsted-Lowry

This was extended by Brønsted and Lowry to include hydrogen ions - protons in any system aqueous or otherwise. They defined acid behaviour as the donation of a proton (hydrogen ion) and base behaviour as acceptance of a proton. This does not exclude Arrhenius it merely extends the idea of acidity to cover non - aquous systems.

Applying Brønsted - Lowry's theory to normal acid base reactions we see that an acid is the specieds donating the hydrogen ion and the base is the precies accepting the hydrogen ion in the aqueous system. Hence Arrhenius' theory of aqueous acidity is encompassed by that of Brønsted and Lowry.

CH3COOH + OH- CH3COO- + H2O

In this example the ethanoic acid is donating the proton (hydrogen ion) and the hydroxide ion (base) is accepting it. Ethoic acid is behaving as an acid according to both Arrhenius and Brønsted-Lowry.

Although this is a rather trivial example it serves to highlight the fact that the Brønsted-Lowry theory is just and extension of Arrhenius theory. Where it is useful is in its application to non aqueous systems where Arrhenius' cannot be applied.

Example: Ammonia + hydrogen chloride

In the reaction between ammonia gas and hydrogen chloride the hydrogen chloride transfers a hydrogen ion to the ammonia making an ammonium ion. In this reaction the ammonia is behaving as a Brønsted Lowry base by accepting a proton (hydrogen ion). The hydrogen chloride is a Brønsted-Lowry acid for providing (donating) that proton (hydrogen ion).

HCl + NH3 NH4Cl

Any system where hydrogen ions are transferred can be considered according to the Brønsted Lowry definition of acids and bases.

In order to consider the possibility of a substance behaving as a Brønsted-Lowry acid or base you must consider the possibility of it accepting or donating hydrogen ions.

Brønsted-Lowry acids

Must have hydrogen atoms capable of being detached as hydrogen ions.

NH4+ can behave as a Brønsted-Lowry acid as it can lose an H+ and become NH3

Brønsted-Lowry bases

These must be capable of accepting a hydrogen ion into the structure

In the reaction between sulphuric acid and nitric acid the sulphuric acid donates a proton to one of the oxygens of the nitric acid and the first stage of formation of the nitronium ion NO2+

In this reaction sulphuric acid is behaving as a Brønsted-Lowry acid and the nitric acid is a Brønsted-Lowry base

H2SO4 + HNO3 [H2NO3]+ + HSO4-

8.1.2: Deduce whether or not a species could act as a Brønsted Lowry and/or a Lewis acid or base


Lewis theory of acids and bases

Lewis' theory extends that of Bronsted-Lowry by considering not the hydrogen ions, but rather the electrons pairs involved. In Bronsted Lowry and Arrhenius the hydrogen ion is attracted to a pair of electrons in the neutralisation process. Lewis rationalised that the electron pair itself was the essential characteristic of a base.

He said that the base was an electron pair donor, and the said the electron pair acceptor.

This allows the discussion of acidity and basicity in systems that do not even involve hydrogen ions.


8.1.3: Deduce the formula of the conjugate acid or base of any Brønsted Lowry base (or acid). Students should make clear the location of the proton transferred, for example CH3COOH/CH2COO- rather than C2H4O2/C2H3O2-


Conjugate acid - base pairs

The idea of conjugate acid - base pairs comes from the idea that all reactions are fundamentally reversible (if not in practice at least in theory). When a proton is transferred to another species that product is then capable in turn of transferring the proton back - in other words it is itself capable of behaving as an acid.

As this acid was created by accepting the proton in the first place it is called the conjugate (paired) acid of the original base. We say that the two species form an acid - conjugate base pair.

Example

Finding the conjugate partner in a reaction involves considering the reverse reaction or considering where the proton (hydrogen ion) goes to. In the following example consider the hydrogen ion and remember that the hydrogen ion donator is the acid.

CH3COOH + H2SO4 CH3COOH2+ + HSO4-

On the left hand side it is the sulphuric acid that donates the proton. It is behaving as an acid. Its conjugate base is the species on the other side of the equation that would accept the proton to go in the reverse direction.

On the right hand side it would be the hydrogen sulphate ion (HSO4- ) that accepts the proton and therefore it is the conjugate base of the sulphuric acid.

On the left hand side of the equation the ethanoic acid accepts a proton and is therefore the Brønsted-Lowry base. It has a conjugate acid on the right hand side that can release a proton - this is the CH3COOH2+ ion.

Look at the following equations as examples of conjugate acid - base pairs. Remember that the conjugate partners are found by considering the reverse reaction.

acid base conjugate base conjugate acid
H2SO4 + CH3COOH HSO4- + CH3COOH2+
             
CH3COOH + NH3 CH3COO- + NH4+
             
H2O + H2O H3O+ + OH-
             
HCl + H2O Cl- + H3O+
             
proton donor proton acceptor proton acceptor proton donor

Logically, if the reactions involve the transfer of a proton (hydrogen ion) from the acid to a base the structures of the acid and its conjugate base must differ in structure by that hydrogen ion.

Look at the acid and conjugate base in the above equations and it may be seen that the acid conjugate base pairs differ by one hydrogen ion as do the base - conjugate acid pairs.

The final part of the syllabus statement states that the position of the hydrogen ion should be indicated. The hydrogen ion being transferrred to the base is accepted by the base. It must be accepted in a location where there is a lone pair of electrons to hold it.

In the first equation above, it is the lone pair on the oxygen atom of the ethanoic acid that accepts the proton. It is therefore correct to write the formula of the conjugate acid of the ethanoic acid (base) as CH3COOH2+ as this indicates that the proton is accepted by the oxygen (or one of them) of the carboxyl group. It would be incorrect to write the formula as CH4COOH+ as this would suggest that the hydrogen has been accepted by the wrong part of the structure.

Note: the example in the syllabus statement is using the ethanoic acid as an acid and so the acid/conjugate base pair would be CH3COOH / CH3COO-


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