A Proton Transfer Reaction from Acids to Bases

3 A Proton Transfer Reaction from Acids to Bases 

The take-home lessons from the behaviors of acids and bases can be summarized as follows  

SCHEME 8.13 Using the curved arrow method to represent the flow of electrons in a proton 

SCHEME 8.14 The structure of an amino acid without charged components, on the left, and its structure in the double ion (zwitterion) state, on the right. 

The historical name of the reaction between an acid and a base is a neutralization reaction. This term means that we are adding to the acid just sufficient base such that each acid molecule transfers a proton to a base molecule, and there remains no excess of either the acid or the base. That the reaction is indeed over can be ascertained by inserting a few drops of some indicator (the one I have in mind is called phenolphthalein, which is colorless in acid and pink in base [see Retouches section 8.R.4]). Imagine now that I placed an unknown quantity of HCl in water, and I ask you to tell me the quantity of acid I used. What you can do is add a drop of the indicator to the acid, and then add a base of known concentration, say NaOH, in a dropwise manner to the solution. At some point, the base would neutralize all the HCl molecules. Then, when you add one more drop of the base, tiny as you wish, the solution will turn pink all of a sudden. The magic of chemistry at its best Based on the quantity of the base needed to reach this endpoint, you can determine the unknown quantity of the acid in solution. 

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