A2 Chemistry: F325 – Equilibria, Energetics and Elements Acids, Bases and Buffers Plymstock School P.J.McCormack

Learning Outcomes. By the end of the lesson I will be able to.... 29 November, 2018 Learning Outcomes. By the end of the lesson I will be able to.... All.. State the meaning of the terms acid and base Write an equation for an acid reacting with a base and label the conjugate acid and conjugate base. Most.. Write an expression for Kw Explain what the term ionic product of water is Some.. Calculate the pH and hydrogen ion concentration Low High Key Words: ionic product of water, conjugate acid, conjugate base P.J.McCormack

29 November, 2018 Key Terminology Acid Proton (H+) donor Base Proton acceptor Buffer A substance that can resist change in pH (a weak acid and a salt of a weak acid) Ka The acid dissociation constant (shows the extent of the acid dissociation) P.J.McCormack

29 November, 2018 Acid-base Equilibria The Brønsted-Lowry theory of acids and bases defines an acid as a proton donor and it may only function as such when in the presence of a proton acceptor – a base. HCl + NH3 → Cl- + NH4+ acid base conjugate base conjugate acid HCl and Cl- are a conjugate pair. NH3 and NH4+ are also a conjugate pair. P.J.McCormack

Amphoteric Nature of Water 29 November, 2018 Amphoteric Nature of Water HCl can also donate a proton to water: HCl + H2O  H3O+ + Cl- In this case the water is acting as a base and the H3O+ is its conjugate acid. (H2O and H3O+ are a conjugate pair). H3O+ is called the oxonium ion. Water can also act as an acid: NH3 + H2O  OH- + NH4+ The OH- is the conjugate base of the acid water. Water is an amphiprotic solvent because it may either donate or accept protons. P.J.McCormack

H2O(l) + H2O(l) H3O+(aq) + OH-(aq) 29 November, 2018 Ionisation of Water H2O(l) H+(aq) + OH-(aq) or H2O(l) + H2O(l) H3O+(aq) + OH-(aq) The equilibrium expression can be written: Kc = [H+] [OH-] [H2O] P.J.McCormack

29 November, 2018 Ionic Product of Water In this the concentration of the water is constant so the expression can be modified to: Kw = [H+] [OH-] Where Kw is called the ionic product of water. At 298 Kw = 1 x 10-14 mol2 dm-6 P.J.McCormack

29 November, 2018 Ionic Product of Water Since, for every molecule of H2O that splits up, one H+ and one OH- is formed, therefore [OH-] = [H+] or 1 x 10-14 = [H+]2 [H+] = 1 x 10-7 mol dm-3 and [OH-] = 1 x 10-7 mol dm-3 P.J.McCormack

Tell me two things... (for each) that you have done well today that you know now that you didn’t know at the start of the lesson that you could have done better next lesson that you would like to know more about 2

pH = -log10[H+] The pH Scale 29 November, 2018 The pH Scale The acidity of a solution depends on the concentration of H+ ions and is measured on the pH scale. The definition of pH is: pH = -log10[H+] P.J.McCormack

29 November, 2018 The pH Scale pH [H+] /mol dm-3 1 8 1 x 10-8 1 x 10-1 9 1 x 10-9 2 1 x 10-2 10 1 x 10-10 3 1 x 10-3 11 1 x 10-11 4 1 x 10-4 12 1 x 10-12 5 1 x 10-5 13 1 x 10-13 6 1 x 10-6 14 1 x 10-14 7 1 x 10-7   A difference of one pH number means a tenfold difference in acidity. So pH 2 is ten times the acidity of pH 3. P.J.McCormack

Calculating pH pH = -log10[H+] pH = -log10[0.01] = 2.0 29 November, 2018 Calculating pH pH = -log10[H+] If a strong acid has a concentration of 0.01 mol dm-3. The pH is calculated pH = -log10[0.01] = 2.0 P.J.McCormack

29 November, 2018 Calculating pH Calculate the pH of an acid solution in which [H+(aq)] = 8.10 x 10-6 mol dm-3. Answer = 5.09 Answer = 5.09 P.J.McCormack

29 November, 2018 Calculate [H+] from pH [H+(aq)] = 10-pH A strong acid has a pH of 3.4. What is the value of [H+(aq)]? [H+] = 10-3.4 = 3.98 x 10-4 mol dm-3 P.J.McCormack

29 November, 2018 Weak and Strong Acids A strong acid is an acid that fully dissociates HCl → H+ + Cl- A weak acid is one that does not fully dissociates CH3COOH H+ + CH3COO- P.J.McCormack