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Acids and Bases

Acids and Bases. Acid/Base Properties. In the past, we have classified acids and bases according to their observed properties ACIDS BASES Sour taste bitter taste Watery feel slippery feel Reactive with metals can be corrosive Water soluble may be soluble in H 2 O .

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Acids and Bases

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  1. Acids and Bases

  2. Acid/Base Properties • In the past, we have classified acids and bases according to their observed properties ACIDSBASES • Sour taste bitter taste • Watery feel slippery feel • Reactive with metals can be corrosive • Water soluble may be soluble in H2O

  3. Extending Acid/Base Theories • Arrhenius Theory (Svante Arrhenius 1859-1927) • Proposed that when salts dissolve in water, they dissociate completely into ions Acids- dissolve in water to produce H+(aq) ions Eg. HCl, HNO3 , H3PO4 , H2SO4 , HF Bases- dissolve in water to produce OH-(aq) ions Eg. NaOH , Ca(OH)2 , Mg(OH)2 Neutralization- acid + base  water + a salt HCl(aq) + NaOH(aq)  HOH(ℓ) + NaCl(aq)

  4. HELP WANTED Extending Acid/Base Concepts Although Arrhenius theory is very successful and useful for explaining acid/base chemistry, it has limitations WHY is Na2CO3(aq) BASIC?!? Bronsted-Lowry Theory (1923 Johannes Bronsted, Thomas Lowry) • Independently noted that some acid-base neutralization reactions can occur without dissolving ions in water Eg. HCl(g) + NH3(g)  NH4Cl(s) • They concluded that acid/base reactions simply involve the transfer of a hydrogen ion from one molecule to another

  5. Acid Base Arrhenius (grade 11) Bronsted-Lowry Extending Acid/Base Concepts According to Bronsted-Lowry definitions: ACIDS- are proton (H+) donors BASES- are proton (H+) acceptors Comparing theories Produces H+ in solution Produces OH– in solution HCl  H+ + Cl– NaOH  OH– + Na+ H+ donor H+ acceptor HCl+NaOHH2O+NaCl

  6. + H H - H N + H Cl N H H + Cl H H Extending Acid/Base Concepts Bronsted-Lowry theory could be used to explain neutralization reactions that could not be explained by Arrhenius theory. HCl is an acid because it donates H+, NH3 accepts H+ and therefore is the base (NH4+ and Cl– then form an ionic compound) So, Na2CO3 is basic because… CO32-(aq) + H2O  HCO31-(aq) + OH1-(aq)

  7. Strong vs. Weak Electrolytes • DEMO conductivity of acids and bases 1.0 mol/L HCl(aq) and 1.0 mol/L CH3COOH(aq) 1.0 mol/L NaOH(aq) and 1.0 mol/L NH3(aq) • The conductivity of these solutions varies eventhough they are all of the same concentration • Substances that dissolve in water and completely dissociate into ions are called strong electrolytes • Acids/Bases that dissolve in water and completely dissociate into ions are called strong acids/bases

  8. Strong vs. Weak Electrolytes Hydronium ion • Some strong acids and bases: HCl(aq) + H2O(ℓ)  H3O+(aq) + Cl1-(aq) NaOH(s) + H2O(ℓ)  Na1+(aq) + OH1-(aq) fully dissociated!

  9. Strong vs. Weak Electrolytes • Some substances are highly water soluble, yet they do not fully dissociate into ions upon dissolution. These substances are weak electrolytes • Acids and bases that do not fully dissociate into ions upon dissolving are weak acids/bases • When weak acid/bases are dissolved in water, a chemical equilibriumis established in the solution

  10. Strong vs. Weak Electrolytes • Weak acid/base equilibria(CLICK HERE!) CH3COOH(aq) + H2O(ℓ)  H3O+(aq) + CH3COO1-(aq) NH3(aq) + H2O(ℓ)  NH4+(aq) + OH1-(aq) • The reactant sides of these equilibria are favoured, so complete dissociation of the acid/base molecules does not occur.

  11. Oxyacids • Oxyacids are mineral acids which contain oxygen combined with another non-metal • Eg. HNO3(aq), H2SO4(aq), H3PO4(aq) Monoprotic diprotoic triprotic • The relative strength (ability to donate protons) of an oxyacid increases with the number of oxygen atoms that are not bonded to an hydrogen atom in the acid molecule. • Eg. H2SO4(aq) sulfuric acid is stronger than H2SO3(aq)

  12. Oxyacids • The ionization of polyprotic acids can be expressed using more than one chemical equation. Eg. H3PO4(aq)ionization: • H3PO4(aq) + H2O(ℓ) H3O+(aq) + H2PO41-(aq) • H2PO41-(aq) + H2O(ℓ) H3O+(aq) + HPO42-(aq) • HPO42-(aq) + H2O(ℓ) H3O+(aq) + PO43-(aq)

  13. Conjugate Acid/Base Pairs CH3COO1-(aq) is the conjugate base of CH3COOH(aq) H3O+(aq) is the conjugate acid of H2O(ℓ) • In any equilibrium expression involving a weak acid/base more than one Bronsted/Lowry acid and base can be identified CH3COOH(aq) + H2O(ℓ)  H3O+(aq) + CH3COO1-(aq) Conjugate acid/base pair Conjugate acid/base pair- two substances that differ from each other by just one proton (H+).

  14. Conjugate Acid/Base Pairs HCN(l) + H2O  CN–(aq) + H3O+(aq) Forward: HCN is acid, H2O is base HCN(l) + H2OCN–(aq) + H3O+(aq) Reverse: H3O+ is acid, CN– is base • Relative strengths of conjugate acid/base pairs • The conjugate base of a weak acid must be a strong base • The conjugate base of a strong acid must be a weak base • The conjugate acid of a weak base must be strong acid • The conjugate acid of a strong base must be a weak acid Q. Explain whether you would expect a solution of NH4Cl(aq) to be acidic or basic. For more lessons, visit www.chalkbored.com

  15. Amphoterism • Any substance that can behave either as an acid or a base is amphoteric (amphiprotic) Eg. Bicarbonate buffer in blood, HCO31-(aq) HCO31- + OH1- H2O + CO32- (as an acid) HCO31- + H3O+ H2CO3 + H2O (as a base)

  16. Homework p.532 #1, 2 p.549 #2, 3 Read p.532 to 549 for next class

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