Glycine, , exists in aqueous solution predominantly as the zwitterion . The molecule…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Glycine, NH2CH2COOH, exists in aqueous solution predominantly as the zwitterion +NH3CH2COO−. The molecule therefore contains both a basic amino group and an acidic carboxyl group. Consider the following two proton-transfer reactions that can occur when glycine solution is titrated sequentially with a strong acid and then with a strong base.
Reaction I (acid addition): +NH3CH2COO−(aq)+H3O+(aq)→+NH3CH2COOH(aq)+H2O(l)
Reaction II (base addition): +NH3CH2COO−(aq)+OH−(aq)→NH2CH2COO−(aq)+H2O(l)
Answer the following questions.
(a) For Reaction I, identify both conjugate acid–base pairs and clearly label the acid and the base in each pair.
(b) For Reaction II, identify both conjugate acid–base pairs and clearly label the acid and the base in each pair.
(c) Glycine can thus act as an amphoteric species. Using the conjugate pairs from parts (a) and (b), construct an argument that links glycine’s molecular structure to its ability to buffer solutions against additions of either strong acid or strong base.
(d) Suppose equal moles of H3O+ and OH− are added simultaneously to a solution containing only the zwitterion. Predict whether the pH will rise, fall, or remain approximately constant, and justify your prediction with reference to the conjugate pairs present.