Things to note: acid-base equilibria

Devise an experiment to determine the acid dissociation constant, K_a, for a solution of ethanoic acid, CH₃COOH, of unknown concentration.

Assume that you have access to a pH meter and a solution of sodium hydroxide of similar concentration to the acid.

Include how to determine the K_a from your results.

• titrate ethanoic acid with sodium hydroxide

• use phenolphthalein indicator to find end-point

• then add same volume of acid to mixture at end point

• measure pH of resultant mixture (with pH meter)

• at half neutralisation, pH=pK_a so Ka=10^-pH

Ethanoic acid, CH₃COOH is a monobasic acid.

CH₃COOH + H₂O ⇌ CH₃COO^- + H₃O⁺

Give a reason why only the proton from the carboxylic acid group, and not the methyl group, is donated to the water molecule.

• the loss of the hydrogen from the O-H group is made possible by the delocalisation of charge / stabilisation on the carboxylate ion

In acid-base neutralisation reactions, there is a temperature change.

Comment on the relative enthalpy changes of reaction, using the data from the table and including any relevant equations.

Hydrochloric acid and nitric acid:

• same value for hydrochloric acid and nitric acid as they are strong and completely dissociate into ions in solutions

Sulfuric acid:

• sulfuric acid is diprotic

• so value is almost twice that of hydrochloric acid / nitric acid

Ethanoic acid:

• ethanoic acid is weak / partially dissociated into ions in solution

• some energy is needed to break the O-H bond to release H⁺ ions so enthalpy change of neutralisation is less for a weak acid than a strong acid

Give a possible explanation as to why the value of K_a for glycolic acid is approximately ten times larger than that of ethanoic acid.

• the O of the extra O-H in the 2 position withdraws electrons

• so stabilises the anion (CH₂OHCOO^- ion) and weakens the O-H bond in the acid so the hydrogen ion is more easily lost

Predict, with a reason, whether water is acidic, alkaline or neutral at 310K.

• water is neutral at 310K

• because [H⁺ (aq)]=[OH^- (aq)]

Write the equation for the reaction taking place at the second equivalence point. State symbols are not required.

NaHCO₃ + HCl → NaCl + CO₂ + H₂O

Explain how the solution at point X on the graph can act as a buffer solution.

• (solution at X) contains a large amount / reservoir of carbonate ions and hydrogencarbonate ions

• carbonate ions react with added hydrogen ions

• hydrogencarbonate ions react with added hydroxide ions

One of the systems controlling the pH of the blood is the carbonic acid-hydrogencarbonate buffer system.

Explain how this buffer system helps to control the pH of blood when extra carbon dioxide is present due to strenuous exercise.

• carbon dioxide dissolved in blood forms carbonic acid (and so this concentration increases)

• the equilibrium will shift to the right and produce more H+

• the high concentration of hydrogencarbonate ions suppress the ionisation of carbonic acid (to help control the pH) or the large reservoir of hydrogencarbonate ions combine with the H+ ions (to help control the blood pH)

• the ratio of [HCO₃^-]:[H₂CO₃] remains essentially constant so the pH does not change