# Acid-Base Equilibria Essay

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Acid-base equilibria Define pH pH = -log10[H+] no units Define Ka with units For acid HA [H+][A-] Acid dissociation constant Ka = [HA] moldm-3 Temperature dependent, Ka varies with different bases Define Kw with units Ionic product of water Kw = [H3O+] [OH−] mol2dm-6 Temperature dependent, Define pKa pKa = −log10 Ka Define pKw pKw = −log10 Kw Understand the terms ‘strong’ and ‘weak’ as applied to acids and bases Difference between acid and acidic solution: • HCl acts as an acid when it dissolves in water and donates its proton to the water molecule • Solution is acidic because concentration of H3O+ ions is greater than conc of OH- ions Strong/weak acid – fully/partially ionised Strong/weak base – fully/partially dissociate Conc No of moles of acid dissolved in a given vol of water The greater the value of Ka the stronger the acid HA Strength of acid, measured by equil constant (which determines the POE) HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) Equil lies so far to RHS that acid is considered to be completely ionised(strong acid) Recall the Brønsted-Lowry theory and use it to identify acids-bases Identify acid-base conjugate pairs and relate them by means of suitable equations Students are not expected to recall the Lewis theory of acids and bases Monoprotic/monobasic acid Acids that have a single proton to donate/accept Diprotic/dibasic acid Acids that have a two protons to donate/accept Acid Proton donor Base Proton acceptor Acid 1 Base 2 Acid 2 Base 1 H2SO4 + H2O H3O+ + HSO4- • Ionisation of H2SO4 in water occurs in 2 stages • HSO4- is conjugate base of H2SO4 but conjugate acid of SO42- Acid 3 Base 2 Acid 2 Base 3 HSO4- + H2O H3O+ + SO42- Calculate the pH of solutions of strong acids and strong bases and of weak acids given Ka