Title: Lesson 10: Salt Hydrolysis Learning Objectives: – Understand why some salts do not form neutral solutions – Predict whether a given salt will form an acidic, neutral or alkaline solution – Complete an experiment to determine any trends in acidity/basicity of salt solutions Refresh a) Explain what is meant by the term buffer solution. b) Calculate the pH of a solution prepared by mixing 50.0 cm3 of 0.200 mol dm–3 CH3COOH(aq) and 50.0 cm3 of 0.100 mol dm–3 NaOH(aq), showing your working. Main Menu Salt Hydrolysis A neutralisation reaction between an acid and a base produces a salt – an ionic compound containing a cation from the parent base and an anion from the parent acid. The cation is the conjugate acid of the parent base. The anion is the conjugate base of the parent acid. However, even though the salts are products of a neutralisation reaction, they do not all form neutral aqueous solutions. Their pH in solution depends on whether and to what extent their ions, which are conjugate acids and bases, react with water and hydrolyse it, releasing H+ or OH- ions. The weaker the acid or base, the stronger the conjugates and vice versa. The relative strengths of the conjugate acid and bases in the salt determines the hydrolysis reaction and so the pH of the salt solution. Main Menu Acid Salt Hydrolysis When you dissolve sodium chloride in water it forms a neutral solution of Na+ and Cl- ions: NaCl(s) Na+(aq) + Cl-(aq) When you dissolve ammonium chloride in water it forms a weakly acidic solution of NH4+ and Cl- ions Why?....discuss Main Menu Why acidic? Ammonium chloride contains the NH4+ ion NH4+ is the conjugate acid of the weak base NH3: NH3 + H2O NH4+ + OH- The NH4+ ion therefore is weakly acidic and will establish the following equilibrium: NH4+ NH3 + H+ Thus, ammonium chloride will form a weakly acidic solution Main Menu Basic Salt Hydrolysis When you dissolve sodium chloride in water it forms a neutral solution of Na+ and Cl- ions: NaCl Na+(aq) + Cl-(aq) When you dissolve sodium ethanoate in water it forms a weakly basic solution of Na+ and CH3COO- ions Why?....discuss Main Menu Why basic? Sodium ethanoate contains the CH3COO- ion CH3COO- is the conjugate base of the weak acid CH3COOH: CH3COOH H+ + CH3COO- The CH3COO- ion therefore is weakly basic and will establish the following equilibrium: CH3COO- + H2O CH3COOH + OH- Thus, sodium ethanoate will form a weakly basic solution Main Menu Salt of strong acid and strong base – no hydrolysis Strong acid and strong base salt with weak conjugate acid and base. Virtually no hydrolysis of ions, the pH is close to neutral. Salt of weak acid and strong base – anion hydrolysis Weak acid strong conjugate base (A-) Hydrolysis occurs Release of OH- causes the pH of the solution to increase Main Menu Salt of strong acid and weak base – cation hydrolysis Weak base strong conjugate acid (M+) Hydrolyses water Release of H+ causes the pH of the solution to decrease. The above cation is a non metal, but it gets a little more complex when the cation is a metal ion. For a metal ion, it must have a high charge density (e.g. Al3+, Fe3+) to carry out hydrolysis Main Menu Salt of weak acid and weak base Both acid and base will form strong conjugates. Hydrolysis carried out. pH depends on the relative Ka and Kb values of the acids and bases involved. Summary pH can be deduced from the relative strengths of the parent acids and bases, as seen below: Main Menu Main Menu Solutions Main Menu Main Menu Recap: Conjugate Acids and Bases A conjugate acid/base pair are two species that differ by a single proton. A conjugate base is a species with has one less proton A conjugate acid is a species with one more proton For example: Hydrochloric acid, HCl HCl is the acid, Cl- is a conjugate base The HCl can donate a proton…it is an acid The Cl- could accept a proton….it is a base Ammonia, NH3 NH3 is the base, NH4+ is its conjugate acid The NH3 can accept a proton….it is a base The NH4+ could donate a proton….it is an acid Main Menu Rules of Thumb Conjugate bases: Conjugate acids: Conjugate bases of weak acids will form basic solutions Conjugate bases of strong acids will form neutral solutions Conjugate acids of weak bases will form acidic solutions Conjugate acids of strong bases will form neutral solutions The final pH of an individual salt will depend on the relative acidity of both the conjugate acid and conjugate base is formed from Main Menu Investigate Plan and conduct an experiment to order the following salts from most to least acidic.You should make a prediction first: Sodium ethanoate, Mr = 82.0 Sodium chloride, Mr = 58.4 Sodium carbonate, Mr = 106 Ammonium chloride, Mr = 53.5 Ammonium carbonate, Mr = 96.1 Ammonium Ethanoate, Mr = 77.1 Plan and conduct an experiment to look at the effect of charge density of the metal ion on acidity.You will have access to: Main Menu Sodium chloride, Mr = 58.5 Potassium chloride, Mr = 74.6 Calcium Chloride, Mr = 111 Magnesium chloride, Mr = 95.2 Aluminium chloride, Mr = 133 Iron (II) chloride, Mr = 127 Iron (III) chloride, Mr = 162 Copper (II) chloride, Mr = 134 Analysis You need to fully explain the results of your experiment, using equations where necessary. You may wish to use data from the data booklet and other sources to help with your explanations. Main Menu Key Points Salts of a weak acid and strong base form basic solutions: Salts of a weak base and a strong acid form acidic solutions: Due to the conjugate acid of the weak base being acidic Salts of a strong acid and strong base form neutral solutions Due to the conjugate base of the weak acid being basic Neither the conjugate base of the acid or the conjugate acid of the base form acid/base equilibria Salts of a weak acid and weak base can be acidic, basic or neutral Depends on the relative basicity of the conjugate base and acidity of the conjugate acid Main Menu