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Published byHomer McKenzie Modified over 9 years ago
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The `Inconvenient Truth’ about the Arrhenius model: basic salts EXAMPLE: Na 2 CO 3 (sodium carbonate ) No H + or OH - ….=> salt only CLASSICAL REACTIONs OVERVIEW continued)
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The `Inconvenient Truth’ about the Arrhenius model: basic salts (continued) Turns pink in presence of phenolphthalein gas-forming reaction with HCl, pink disappears experimental results of adding to water: EXAMPLE: Na 2 CO 3 (sodium carbonate) => A base !!!!????? Where’s OH ??? CLASSICAL REACTIONs OVERVIEW(continued)
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The `Inconvenient Truth’ about the Arrhenius model: basic salts (continued) Other examples of `basic’ salts Na 3 PO 4 K 2 SiO 3 NaC 2 H 3 O 2 Basic salts cannot be rationalized using Arrhenius’ acid/base model CLASSICAL REACTIONs OVERVIEW (continued)
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Bronsted to the rescue… Young Bronsted…Swedis h chemist circa 1910… Young James Dean…American actor circa 1955 …(“Rebel Without a Cause,”“East of Eden”, “Giant” ) Winner of the Bronsted look alike contest…. Bronsted a few years after marriage and kids CLASSICAL REACTIONs (continued)
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Bronsted Model of Acids/Bases 3 new concepts (see p. 654) Acid= HF, HCl, HBr HNO 3 HClO 4 H 2 SO 4 H 2 CO 3 H 3 PO 4 No real change from Arrhenius proton donor (=H + ) CLASSICAL REACTIONs OVERVIEW(continued) ARRHENIUS ACIDS = BRONSTED ACIDS New Concept # 1: Follow the protons
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BASE= OH - CO 3 -2,PO 4 -3, C 2 H 3 O 2 -,SiO 3 -2 … proton acceptor CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES: New Concept #1: following the protons (cont.) The set of Bronsted bases includes Arrhenius bases + …. … nearly any negative ion (=anion) that even weakly attracts protons Examples of Bronsted Bases
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How the proton acceptor creates the basic salt effect : CO 3 2- + H-OH.. Bronsted Base is basic in water because it splits water to make OH - ! Hydrolysis reaction CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES: New Concept #2 Hydrolysis…Bronsted bases `split’ water !!! EXAMPLE HCO 3 - + OH -
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Conjugate idea is like going drag… Men=acid Women=base BASE ACID CONJUGATE ACID CONJUGATE BASE Identify `acids’ and bases BRONSTED’S MODEL OF ACIDS/BASES: New Concept #3a: conjugate acid/base pairs
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CO 3 2- + H-OH Acid conjugate base Base conjugate acid Base Acid Conjugate acid Conjugate base CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES: New Concept #3a: conjugate acid/base pairs HCO 3 - + OH - Identify `acid’ and base Identify conjugate acid and base
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PO 4 3- + HCO 3 - HPO 4 2- + CO 3 2- Acid conjugate base Base conjugate acid acid base conjugate base conjugate acid new concept #3a: Conjugates (continued) When acid and base reacts…. EXAMPLE: IDENTIFY ACID, BASE AND THEIR CONJUGATE BASE AND ACID ABOVE
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ACID + BASE CONJUGATE BASE + CONJUGATE ACID Bronsted Model Contrast to Arrhenius Model ACID + BASE SALT + WATER CLASSICAL REACTIONs OVERVIEW(continued) BRONSTED’S MODEL OF ACIDS/BASES: New Concept #3b: spontaneous acid + base reactions produce weaker acids + bases Both products `neutral’
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ARRHENIUS ACID-BASE THEORIES SUMMARIZED BRONSTED ACID =H + DONOR PROTON DONOR BASE=OH - DONORPROTON DONOR ACID + BASE SALT + WATER CONJUGATE ACID + CONJUGATE BASE
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What observation required Bronsted to create a new Acid/Base Theory different than Arrhenius’s model ? A.Many salts formed conjugate acids. B.Spontaneous acid+base reactions only occurred if weaker acids and bases formed. C.H 2 O spontaneously decomposed to OH- and H+ in presence of various salts. D.Many salts without OH - somehow created OH - in water.
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Which part of the Bronsted salt K 3 PO 4 is the proton acceptor ? A.K + B.O 2- C.P 3- D.PO 4 3- E.K 3 PO 4 F.OH -
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What new concept of Bronsted’s most directly explains why many salts are bases? A.The anionic part of salts can hydrolyze water. B.Spontaneous acid/base reactions lead to weaker conjugate acids/bases. C.Bronsted bases are proton acceptors. D.Bronsted acids are proton donors.
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In-class reaction Jeopardy Low heat, precipitates, cation swap metathesis WHAT IS… Na 2 CO 3 for example A Bronsted base What’s my reaction type: HCl + KOH KCl + H 2 O Acid/base neutralization (Arrhenius) Pick the Arrhenius acids H 2 O HNO 3 NaHCO 3 HBr NaCl HNO 3 HBr
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In-class reaction Jeopardy WHAT IS… What’s my reaction type: K 3 PO 4 + H 2 O HK 2 PO 4 + KOH Acid/base hydrolysis (Bronsted) Write my products ? AgNO 3 (aq) + NaCl(aq) ????? AgCl(s) + NaNO 3 (aq) Pick the Bronsted base(s) NaOH HBr NaHCO 3 K 3 PO 4 NaOH NaHCO 3 K 3 PO 4 Pick the Arrhenius bases NaOH
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OXIDATION-REDUCTION (REDOX) REACTIONS see pages text 458-466 REDOX REACTIONS ALL INVOLVE CHANGES IN ELECTRON OWNERSHIP EXAMPLE #1 : Mg o + 2H + Mg 2+ + H 2 0+2 +10 Mg loses 2 e - Each H+ gains 1 e- Losing is oxidation Gaining is reduction
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