net ionic equation for citric acid and sodium citrate in solution

Net ionic equation for citric acid and sodium citrate in solution. Also a possible function for these two chemicals in the soda when an acid or base is added to the soda.

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Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid), HOOC-CH2-C(OH)(COOH)-CH2-COOH is a triprotic acid, and like all triprotic acids, it ionises in a stepwise manner. ie:

C6H8O7 (aq) + H2O (l) <---> C6H7O7- (aq) + H3O+ (aq)

C6H7O7- (aq) + H2O (l) <---> C6H6O72- (aq) + H3O+ (aq)

C6H6O72- (aq) + H2O (l) <---> C6H5O73- (aq) + H3O+ (aq)

All three equilibria lie to the left, each further left than the one before - so, not many citric acidmolecules undergo the first ioinisation, and virtually none the third. In fact, many questions treat it as if it were a monoprotic acid - which is wrong, but you should recognise that it can happen, especially in assessments and trials, which may be written by people with not as much chemistry knowledge as would be desirable.

Sodium citrate salt is completely ionised in solution

(Na+)3C6H5O73- (s) + aq -----> 3Na+(aq) + C6H5O73- (aq)

So the salt is able to mop up any H+ ions added by converting C6H5O73- (aq) to C6H6O72- (aq) and to C6H7O7- (aq) and to C6H8O7. Add ing OH- ions will cause more C6H8O7 to ionise as in the first equation. Because of the poor ionisation of this acid, there will always be plenty of citric acid molecules to provide hydrogen ions to remove the OH- ions.

'Chemistry is not just the study of matter; Chemistry is the study which matters!' - Kingchemist

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