ACIDS & BASES How does pH value affect our environment? Recall Questions What is an acid? What is a base/alkali? •A substance which produces hydrogen ions (protons) when dissolves in water. •A base refers to substances like metal oxides and metal hydroxides. •A substance which reacts with acid to form salt and water only •An alkali is a soluble base which in solution produces hydroxide ions. •Most bases are insoluble in water. 3 soluble bases are NaO/NaOH,KO/KOH,CaO/Ca(OH)2 Recall Questions What causes acidity? •It is the hydrogen ions that give an acid its acidic properties when they dissolve in water and dissociate into ions. E.g. HCl gas is a covalent compound. When dissolves in water, it forms HCl acid which dissolciate to form ions. Recall Questions What is basicity(proticity)? Basicity • refers to the no.of H atoms in one molecule of acid that acn be repleced by a metal. •refers to the no. of H+ that can be replaced by one molecule of that acid. E.g. HCl (monobasic),H2SO4(dibasci),H3PO4(tribasic) Bronsted-Lowry theory An acid is defined as a molecule or ion that acts as a proton donor (H+). A base is defned as a molecule or ion that acts as a proton acceptor (H+). HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) Hydrogen chloride gas dissolved in water HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) The equation can be split into (i) HCl(aq) Cl-(aq) acid conjugate base (ii) H2O(l) + H+(aq) base + H+(aq) H3O+(aq) conjugate acid Ammonia gas dissolved in water NH3(g) + H2O(l) NH4+(aq) + OH-(aq) What does ammonia and water act as in the reaction above? Competition between acid/base and its conjugate (i) HCl(g) + H2O(l) acid base (ii) CH3COOH(l) + H2O(l) acid (i) (ii) base H3O+(aq) + Cl-(aq) conjugate acid conjugate base H3O+ (aq) + CH3OOO-(aq) conjugate acid conjugate base Water is a much stronger base than chloride ion and has a stronger tendency to accept proton.The equilibrium shifts more to the right. Ethanote ion is a much stronger base than water molecule. The equilbrium shifts to the left. Gas-phase acid-base reaction HCl(g) + NH3(g) NH4Cl(s) The Bonsted-Lowry model can be extended to gasphase acid-base reaction. It involves the transfer of hydrogen ion from hydrogen chloride to ammonia. (i) HCl(g) + H2O(l) acid base conjugate acid (ii) CH3COOH(l) + H2O(l) acid H3O+(aq) + Cl-(aq) base conjugate base H3O+ (aq) + CH3OOO-(aq) conjugate acid conjugate base Strong acids produce relatively weak conjugate bases in aqueous solutions. Weak acids produce relatvely strong conjugate acids in aqueous solutions. Common acids & conjugate bases in order of strengths Acids that have single proton to donate – monoprotic (monobasic). E.g. HCl(aq), HNO3(aq), HNO2(aq) Acids that have 2 protons to donate – diprotic E.g. H2SO4(aq), H2SO3(aq), H2CO3(aq) H3PO4(aq) is triprotic. Lewis theory A Lewis acid is defined as a substance that can accept a pair of electrons from another atom to form a dative (coordinate) covalent bond. A Lewis base is defined as a substance that can donate a pair of electrons to another atom to form a dative (coordinate) covalent bond. B: H+ +BH Examples Reaction between ammonia and proton H3N: H+ +NH4 Reaction between a water molecule and proton H2O: H+ H3O+ Lewis bonding In complex ions formed by transition metals The 6 water molecules, each donate a lone air electrons from oxygen of their water molecules to the empty 3d orbitals of iron. What does each water molecule and iron(III) ion act as in the reaction above? Properties of acids and bases Construct a concept map to show the physical and chemical properties of acids and bases Preparation of Salts Construct a concept map to show a summary of the preparation of salts. Strong and weak acids and bases Strong acid When strong acid dissolves, virtually all acid molecules react with the water to produce hydronium ions In general for a strong acid HA HA + H2O(l) H3O+(aq) + A-(aq) 0% 100% Examples : HCl, H2SO4,HNO3, HClO4 or HA H+(aq) + A-(aq) 0% 100% Strong and weak acids and bases Weak acid When a weak acid dissolves in water, only a small % of its molecules (typically 1%) react with water molecules to release hydrogen or hydronium ions. The equilibrium lies on the lefthand side of the equation. HA + H2O(l) 99% H3O+(aq) + A-(aq) 1% or HA H+(aq) + A-(aq) 99% 1% Examples : CH3COOH, aqueous carbon dioxide Distinguish between strong and weak acids 0.1 mol dm-3 HCl(aq) 0.1 mol dm-3 CH3COOH (aq) [H+(aq)] 0.1 mol dm-3 - 0.0013 mol dm-3 pH 1.00 2.87 Electrical conductivity high low Relative rate of reaction with magnesium fast slow Relative rate of reaction with calcium carbonate fast slow Base on the information above, how do we distinguish betwee strong and weak acids of the same concentration (e.g. HCl and CH3COOH)? How to distinguish between strong and weak acids? A weak acid has a lower concentration of hydrogen ions and hence a higher pH than a stronger acid of the same concentration. A weak acid, because of its lower concentration of hydrogen ions, will have much poorer electrical conductor than a stronger acid of the same concentration. Weak acids react more slowly with reactive metals, metal oxides, metal carbonates and metal hydrogencarbonates than strong acids of the same concentration. Strong and weak acids can also be distnguished by measuring and comparing their enthalpies of meutralisation. What is the difference between the strength (strong and weak) and the concentrated (concentrated or dilute)? Strong and weak acids and bases Strong base A strong base undergoes almost 100% dissociation/ionisation when in dilute aqueous solution. BOH B+(aq) + OH-(aq) 0% 100% Examples : NaOH, KOH, Ba(OH)2 Strong and weak acids and bases Weak base All bases are weak except the hydroxides of groups 1 and 2. Weak bases are composed of molecules that react with water molecules to release hydoxide ions. In general for a weak molecular base, BOH BOH + (aq) B+(aq) + OH-(aq) The equiibrium lies on the left side of the equation. Examples : aqueous ammonia, ethylamine, caffeine, bases of nuclei acids The pH indicator scale that measures the strength of an acid and alkali. pH of a substance is measured when it is dissolved in water. pH stands for “power of hydrogen” [H+] = 1 x 10-n moldm-3 ( n = pH number) The pH Scale pH probe and meter An accurate method of measuring pH value. A pH probe is dipped into the solution being tested and the pH value is then read directly from the meter.