## 1.12 Acids and bases

Acids and bases are important in domestic, environmental and industrial contexts. Acidity in aqueous solutions is caused by hydrogen ions and a logarithmic scale, pH, has been devised to measure acidity. Buffer solutions, which can be made from partially neutralised weak acids, resist changes in pH and find many important industrial and biological applications.

Required practical

Investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base.

• Brønsted–Lowry acid–base equilibria in aqueous solution - Acids dissolve in water to give solutions of pH<7: acidity occurs as due to aqueous hydronium ions, H3O+ (aq). Basic solutions contain aqueous hydroxide ions, OH− (aq). Acids neutralise bases. A Brønsted-Lowry acid is a proton donor. A Brønsted-Lowry base is a proton acceptor. Conjugate acid-base pairs are related by the transfer of a proton. Hydrolysis of an acid or base produces a conjugate

• Determining pH -   Determination of pH The term pH was introduced by Sørensen, a Danish chemist, as a way of measuring acidity during the fermentation process. pH = –log10[H+] where [ ] represents the concentration in mol dm-3 In conventional terminology, the log notation is taken as corresponding to log10 The concentration of hydrogen ions in aqueous solution covers a very wide range. Expressing

• The ionic product of water Kw - Water is slightly dissociated. Kw is derived from the equilibrium constant for this dissociation. Kw = [H+][OH–] The value of Kw varies with temperature. The ionic product, Kw = [H+][OH–] may be used to determine the pH of strong bases (click here)   Examples of calculations using Kw

• Weak acids and bases Ka for weak acids - Weak acids and weak bases dissociate only slightly in aqueous solution. Ka is the dissociation constant for a weak acid. pKa = –log10 Ka Examples of calculating pH in weak acids IMPORTANTLY, there is no requirement to repeatedly state the proof for the derivation of the pH in a weak acid calculation. Full credit will be awarded to candidates if you simply

• pH curves titrations and indicators - pH curves Understand typical pH curves for acid–base titrations in all combinations of weak and strong monoprotic acids and bases. Students should be able to: • sketch and explain the shapes of typical pH curves • use pH curves to select an appropriate indicator. A titration curve shows a graphical representation of the neutralisation reaction occurring between an acid and a base.

• Buffer action - Definition A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base. Acidic buffer solutions contain a mixture of a weak acid (e.g. CH3COOH) and the salt of that weak acid (e.g. CH3COONa). Basic buffer solutions contain a mixture of a weak base (e.g. NH3(aq)) and the salt of that weak base (e.g. NH4Cl). Calculating the pH of a