Acid-Base Reactions

Acid-base titrations

Acid-base reactions involve the transfer of protons from acid to bases. Acids are proton donors and bases are proton acceptors. A ‘strong’ acid refers to an acid that readily donates protons. Some examples of strong acids include HNO3, H2SO4 and HCl. In contrast, a ‘weak acid’ refers to one that does not readily donate protons. Examples include CH3COOH. Similarly, ‘strong’ bases refer to bases that readily accept protons whereas weak bases refer to bases which do not readily accept protons.

In acid-base titrations, indicators are often used to help identify the equivalence point of reactions. Indicators are usually weak acids that exhibit a colour change when there is a change in the hydrogen ion concentration of a solution. Examples of indicators include phenolphthalein and methyl orange.

Conjugate acid-base pairs are pairs of molecules that differ by 1 H+. Examples of conjugate pairs include Cl/HCl and HSO4/H2SO4.


pH is a measure of acidity; the concentration of H3O+ ions in a solution.

pH=-\log _{ 10 }{ [H_{ 3 }O^{ + }] }

(pH is further discussed in unit 4).

For titration curves, a sharp end point is desired to accurately determine when a reaction has reached its equivalence point. Hence, reactions involving strong acids and strong bases are ideal to provide a sharp end point and those involving weak acids/bases need to undergo back titration. Reactions involving weak acids with weak bases produce a very ‘flat’ curve which greatly affects the accuracy of results obtained.

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