Unit 5 Physical Chemistry

Acids, Bases and Buffers

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Acid - a substance containing a replaceable hydrogen atom. When it is added to water it ionsies or dissociates releasing hydrogen ions (H ). Also a substance that can donate a proton. In aqueous solutions acids can ionise and donate protons to the water molecules.

 


Base - a substance that neutralises acids to form salt and water. A soluble base is called an alkali. Also a substance that can accept a proton. In aqueous solutions bases accept protons from water molecules.

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Strong acid - fully ionised in solution or fully dissociates e.g.

 

 

Weak acid - partially ionised in solution or partially dissociates e.g.

 

In the forward reaction, HA donates a proton to water leaving A  . In the reverse reation A  accepts a proton from H O and reforms HA. HA is acting as an acid and A  is acting as a base.

The two species are described as a conjugate pair where A  is called the conjugate base of the acid HA.

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Strong base - fully ionsied in solution or fully dissociates e.g.

 

 

Weak base - partially ionised in solution or partially dissociates e.g.ammonia and the homologous series of amines.

 

 

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pH calculations for strong acids

For a strong monoprotic acid the H  is equal to the concentration of the acid.

 

 

 

Sulphuric acid is a diprotic acid so each acid molecule produces two hydrogen ions in solution.

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The value of Ka depends upon the extent of dissociaion of the acid. The weaker the acid the smaller the value of Ka. The dissociation of an acid is an endothermic process and as temeprature increases the dissociation will increase. This will increase the value of the acid dissociation constant.

 

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pH calculations for weak acids

We cannot use the same method as we used for strong acids because the weak acid is only partially ionised in solution and therefore the hydrogen ion concetration of the acid is not the same as the concentration of the acid. HA  is much greater than H .

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A buffer is a solution whose pH is able to resist change on the addition of relatively small quantities of acid or base. They are found in shampoos and in our blood.

Acid buffers - contain a weak acid and its salt formed with a strong base.

 

 


Basic buffers - contains a weak base and its salt form with a strong acid.

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pH = -log10[H+]

[H+] = 10-(pH)

Partial dissociation of water: H2O    H+(aq) + OH-(aq

The ionic product of water: Kw = [H+(aq)][OH-(aq)]

At 298K it is equal to 1.0 x 10 mol  dm

Ka= [H+(aq)][A-(aq)]     mol dm 

         [HA(aq)]

pKa = -log10Ka

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pH calculations for strong bases

The ionic product of water provides a mothod of calculation the pH for a solution of a strong base using:

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Buffer calculations

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