8.1 Explaining the Properties of Acids & Bases SCH4U - Chemistry, Gr. 12, University Prep Ms. Papaiconomou & Ms. Lorenowicz

Common Properties of Acids & Bases Property Acid Base Taste Sour Bitter Texture of Solution No characteristic texture Slippery Aqueous Property of Oxides Non-metal oxides form acidic solutions: CO2(g) + H2O(l)  H2CO3(aq) Metal oxides form basic solutions: CaO(g) + H2O(l)  Ca(OH)2(aq) Reaction with Phenolphthalein Colourless Pink Reaction with litmus paper Blue litmus  Red Red litmus  Blue Reaction with Metals Acids react with metals above H in the activity series to displace H2(g) Bases react with certain metals (i.e. Al) to form H2(g) Reaction with CO32- Form CO2(g) No reaction Reaction with NH4Cl Form NH3 Reaction with Fatty Acids React to form soap (saponification reaction) Neutralization Reactions Acid + Base  Water + Salt

Arrhenius Theory of Acids & Bases (Arrhenius, 1887) Acids & bases are defined in terms of their structure and the ions produced when they dissolve in water. Explains acid-base reactions and neutralization. ACID: dissociates in water to form H+(aq) HCl (hydrochloric acid), H2SO4 (sulphuric acid) BASE: dissociates in water to form OH-(aq) NaOH (sodium hydroxide), KOH (potassium hydroxide) LIMITATIONS: hydrogen ion combines with water to form hydronium ion does not explain some bases (i.e. ammonia, salt solutions) does not explain acid-base reactions without water (i.e. gas)

Brønsted-Lowry Theory (Johannes Brønsted & Thomas Lowry, 1923) Recognizes an acid-base reaction as chemical equilibrium, have a forward and reverse reaction involving the transfer of a proton ACID: substance from which a proton can be removed “proton-donor” BASE: substance that can accept a proton “proton-acceptor” proton = nucleus of a hydrogen atom (H+ ion) acid base conjugate conjugate acid base

Conjugate Acid-Base Pairs base acid conjugate conjugate acid base dissociation is an equilibrium reaction because it proceeds in both directions H2O donates a proton in forward rxn  acid OH- accepts a proton in reverse rxn  conjugate base If a substance acts as a proton donor and a proton accepter, it is termed “amphoteric” (i.e. water)

Strong Acids Completely dissociates in water (equilibrium favours products, lies to the right) Binary acids [HX(aq) where X = Cl, Br, I (not F)] Factors: Across period: electronegativity Down a group: bond strength Oxoacids (contain oxygen atoms) where the # [O] > # H (by 2 or more) Number of oxygen atoms Monoprotic only have single H atom that dissociates Polyprotic have more than 1 H atom that dissociates Strength decreases as number of hydrogen atoms that have dissociated increases

Strong Bases Completely dissociates in water (equilibrium favours products, lies to the right) Oxides & Hydroxides of alkali metals (Group 1) and of of alkali earth metals (Group 2) below beryllium [e.g. NaOH sodium hydroxide, MgO magnesium oxide] Factors: Metals with low electronegativity form ionic bonds with oxygen  easily break bond with oxygen which reacts with water to form hydroxide ions

Calculations that involve strong acids & bases Strong acids/bases (and strong electrolytes) completely dissociates into ions in water [H3O]+(aq) is equal to the [strong acid] [OH]-(aq) is equal to the [strong base] You cannot determine the concentrations of ions of weak acids/bases/electrolytes this way because they do not completely dissociate in solution (more next class)

Homework Please re-read Section 8.1(pp.377-387) and answer: p.382 Q.1-4 p.386 Q.5-8 p.387 Q.1-5