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8.1 EXPLAINING THE PROPERTIES OF ACIDS & BASES SCH4U - Chemistry, Gr. 12, University Prep Mr. Dvorsky
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Common Properties of Acids & Bases PropertyAcidBase TasteSourBitter Texture of SolutionNo characteristic textureSlippery Aqueous Property of Oxides Non-metal oxides form acidic solutions: CO 2(g) + H 2 O (l) H 2 CO 3(aq) Metal oxides form basic solutions: CaO (g) + H 2 O (l) Ca(OH) 2(aq) Reaction with PhenolphthaleinColourlessPink Reaction with litmus paper Blue litmus RedRed litmus Blue Reaction with Metals Acids react with metals above H in the activity series to displace H 2(g) Bases react with certain metals (i.e. Al) to form H 2(g) Reaction with CO 3 2- Form CO 2(g) No reaction Reaction with NH 4 ClNo reactionForm NH 3 Reaction with Fatty AcidsNo reaction React to form soap (saponification reaction) Neutralization Reactions Acid + Base Water + Salt
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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), H 2 SO 4 (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)
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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
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Conjugate Acid-Base Pairs dissociation is an equilibrium reaction because it proceeds in both directions H 2 O 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) base acid conjugate conjugate acid base
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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) Factors: 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
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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
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Calculations that involve strong acids & bases Strong acids/bases (and strong electrolytes) completely dissociates into ions in water [H 3 O] + (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)
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