Types of Reactions


Types of Reactions: - Library

Aqueous Phase Reactions• Strong vs. weak electrolytes:– Strong: strongly conductive, due to:• high concentration, [ ], of ions present– Weak: weakly conducting, due to :• relatively few ions, i.e., low [ ] of ions– Non-electrolyte: no conductivity• the substance dissolves but does not break intoions. (because it is not an ionic compound)• Acids and Bases– Acid: any substance which increases [H + ] / [H 3 O + ](hydronium ion) when dissolved in water.– OR proton donor in a reaction2010 72010 8Strong vs. weak electrolytes:• Acids and Bases– Base: any substance that increases [OH - ], hydroxideion, when dissolved in H 2 O.– OR the proton acceptor in a reaction.• Conductivity and Acids/Bases– Most acids/bases are electrolytesl t– Use the same terms with the same meaning as thoseused for electrolytes.• Very strong - strong - weak - very weak• There are 6 strong acids and 6 strong bases.– All the rest are WEAK.2010 93

Reactions of Nitric Acid with WaterHNO3 (aq)H2O (I)NO3 - (aq)H3O + (aq)Copyright2010 © Houghton Mifflin Company. All rights reserved 4-910The Strong Acids/Bases• Strong bases• Strong acids– HNO 3– KOH–HClO 4–H 2 SO 4–LiOH–NaOH–HCl– Ca(OH) 2–HBr– Sr(OH) 2–HI– Ba(OH) 2If it does not appear on this list it is WEAK!2010 11Acids and Bases• Indicators:– Usually vegetable dyes that change color as[H + ]/[OH - ] changes.• Neutralization reaction:HA + BOH H 2 O + BAacid base water salt• Conductivity generalizations:– Weak and Strong Electrolytes• Salts, if soluble are strong• Most acids and bases are weak (except 6 each)• Most other substances are non-electrolytes!2010 124

15_8Color Changes of Some Acid-Base IndicatorsIndicator namepH range for color change0 2 4 6 8 10 12Methyl violetyellowvioletThymol blue (acidic range)redyellowBromphenol blueMethyl orangeBromcresol greenyellowredyellowblueyellowblueMethyl redredyellowBromthymol blueyellowblueThymol blue (basic range)PhenolphthaleinyellowcolorlessbluepinkAlizarin yellow R2010 yellow 13Copyright © Houghton Mifflin Company. All rights reservedred15-8Molecular vs. Ionic Rxn Equations• Molecular rxn equation:molecular compounds–S 8(s) + 8O 2(g) 8SO 2(g)–CaO (s) + 2HCl (aq) CaCl 2(aq) + H 2 O (l)ionic compounds– Convenient format but not always accurate– Reality: HCl (aq) H + (aq) + Cl - (aq)– and CaCl 2(aq) Ca 2+ (aq) + 2Cl - (aq)2010 14Molecular vs. Ionic Rxn EquationsIonic rxn equation: an eqn. where the actual speciespresent are shown.- molecular eqn.HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l)- ionic eqn. OR total ionic eqn.H + (aq) + Cl- (aq) + Na+ (aq) +OH - (aq) Na + (aq) +Cl - (aq) +H 2 O (l)- net ionic eqn.: an ionic eqn. in which the spectatorions have been removed.- Spectator ions: ions which are unchanged from theright side to the left side of the reaction eqn.H + (aq) +OH - (aq) H 2 O (l) - net ionic equation2010 155

Guidelines for writing Ionic eqn.s fromMolecular eqn.s• Ionic substances and molecular compounds that arestrong electrolytes are written as ions when in solution.HCl (aq) H + (aq) + Cl - (aq) strong acidCaCl 2(aq) Ca 2+ (aq) + 2Cl - (aq) soluble salt• Weak or non-electrolytes or insoluble compounds arewritten as molecular formulas– HC 2 H 3 O 2(aq) - soluble molecular liquid, weak acid(or CH 3 COOH (aq) )– BaSO 4(s) - insoluble ionic compound (salt)2010 16Aqueous Reactions:Ways to Drive Rxns• Net ionic shows what is driving the reaction• Types of rxns:– Gas formation: one product is a slightly solublegas, which escapes the solution.– Weak electrolyte: formation of a soluble but weakor non-electrolytic substance.• E.g. - neutralization: due to the formation of thenon-electrolyte water molecule.– Precipitation rxns• Precipitate: a solid formed in an aqueous soln., the solidforms because it is insoluble in water.2010 17Solving Reaction Prediction ProblemsA. Is it a gas reaction?– Two necessary requirements for YES1. Is one of the 5 gas rxn ions present.• If NO, then NOT a gas reaction.• If YES, then:2Isthesoln 2. soln acidic (4 of the 5 ions)OR basic for the 5th ion only.• If NO, then NOT a gas reaction.– If YES to both, write the gas rxn• Then check the other product to see if weakelectrolyte or precipitate forms, then:• Write the ionic and net ionic eqns.2010 186

The five gas rxn ionsION• S 2-• SO2-3• NO-2Net Ionic equation2H + (aq) + S 2- (aq) H 2 S (g)2H + (aq) + SO2-3 (aq) SO 2(g) + H 2 O (l)2H + (aq)+ 2NO-2 (aq) NO (g) + NO 2(g) + H 2 O (l)• CO 32-2H + (aq) + CO 32-(aq) CO 2(g)+ H 2 O (l)• NH 4+OH - (aq)+ NH 4+(aq) NH 3(g) + H 2 O (l)2010 19Solving Reaction Prediction ProblemsB. Is a weak electrolyte formed?– Write the potential molecular rxn equation– Check products against weak electrolyte list• weak acid or base, water, two others– If one product is a weak electrolyte• Then check the second product and• Write the ionic and net ionic equations.• Weak electrolytes are always written aswhole molecule (aq)2010 20• Strong acids–HClO 4–H 2 SO 4– HNO 3– HCl–HBr–HIWeak Electrolytes• Strong bases–LiOH–NaOH– KOH– Ca(OH) 2– Sr(OH) 2– Ba(OH) 2If it does not appear on this list it is WEAK!Therefore a weak electrolyte.Three others: H 2 O , CdSO 4 and HgCl 22010 217

Solving Reaction Prediction ProblemsC. Is a precipitate formed?– Precipitate: a solid formed in an aqueoussoln., the solid forms because the compoundis insoluble in water.– Use the solubility rules to determine if oneor both of the products is a precipitate.p • Rules 1 - 4 All soluble except ...• Rules 5 - 7 All insoluble except ...– If YES, write the ionic and net ionic eqn.– If NO to all the above questions, ie., no gas,no weak electrolyte and no precipitate, then• NO REACTION (NR)2010 22General StatementIf aqueous solutions of two different compounds are combined, a reaction will occur if any of the followingcan be formed:a. Partially-soluble gas b. Weak electrolyte c. Precipitate (insoluble substance)Reaction will also occur if oxidation/reduction takes place.Partially Soluble Gases – Aqueous Gas forming reactionsThe following form in the presence of an acid and the corresponding anion:SO 2 CO 2 H 2 S NO and NO 2 (both form simultaneously)NH 3 gas forms from the ammonium ion in the presence of a base.Weak ElectrolytesAll weak acids and all weak bases are weak electrolytes. In addition the following are weak electrolytes:CdSO 4 HgCl 2 H 2 OSolubility Rules for Inorganic Compounds1. All compounds containing a Group lA cation or NH 4+ are soluble in H 2 O.2. All compounds containing NO 2- , NO 3- , ClO 3- , ClO 4- , or C 2 H 3 0 2- are soluble in H 2 O, except for AgNO 2 .3. All compounds containing Cl - , Br - , or I - are soluble in H 2 O unless the cation is Ag + , Pb 2+ , or Hg 2+2 .4. All compounds containing SO 2-4 are soluble except PbSO 4 , SrSO 4 , BaSO 4 and Hg 2 SO 4 .5. All compounds containing OH - (or O 2- ) are insoluble except those in which the cation is a Group lA ion,NH 4+ , Ca 2+ , Sr 2+ or Ba 2+ .6. All compounds containing S 2- are insoluble except those in which the cation is a Group lA or IIA ionorNH 4+ .7. All compounds containing CO 2-3 , SO 2-3 , CrO 2-4 , PO 3-4 or AsO 3-2010 4 are insoluble except those in which 23 thecation is a Group lA ion or NH +4Reduction/Oxidation Rxns• Oxidation:1) the loss of electron(s) by a chemical species;2) the gain of oxygen and/or loss of H by a chemicalspecies.• Reduction:1)th the gain of electron(s) )by a chemical species;2) the loss of oxygen and/or gain of H by a chemicalspecies.• Essentially a transfer of electrons between twospecies, so …– Both reduction and oxidation ALWAYS occurtogether.2010 248

Assigning Oxidation Numbers• In elemental form the oxidation # of an atom is 0, zero.• For monatomic ions, the oxidation # is the same as thatof the ion.• Hydrogen and oxygen– Oxygen has an oxidation # of -2 in all compoundsexcept peroxides where it is -1.– Hydrogen has an oxidation # of +1 in all compoundsexcept hydrides where it is -1.• Compounds and polyatomic ions– The sum of all the oxidation #’s in a compoundequals 0, zero.– The sum of all the oxidation #’s in a polyatomic ionequals the charge on the polyatomic ion.2010 25Examples of assigning oxidation #’s0 0elemental forms: Na(s) H 2 (g)+1 -1Ionic compounds/ions: Na + Cl -+1 -12+ -1Peroxides H 2 O 2 Hydrides MgH 2+4 -1Compounds: CCl 4 +5 2-Polyatomic ion: P O3-4? + 42- = 3-CrO 42-?2010 26Molarity(M) and Solution Stoichiometry• Solutions generally consist of a solvent and asolute.– Solvent: the material usually in greatestabundance in the solution.– Solute: the material being ‘dissolved’ by thesolvent.• Molarity:M =moles of soluteliters of solution = molL• To calculate molarity requires:1) moles of solute and2) resulting liters of solution2010 279

Dilution of Solutions• Many materials come in concentrated form.To make useful concentrations requiresdilution with the solvent (usually water).mol x L = moles or M x V = molesL• For a given volume and molarity of solution, themoles of solute are constant.• When diluting the volume changes and thus themolarity, but the moles of solute are unchanged.M 1 V 1 = M 2 V 22010 28Volumetric AnalysisAqueous Stoichiometry• Acid/Base titrations - one exampleacid + base H 2 O + saltHA + BOH H 2 O + BA• Methodology:– Known M of base prepared p (standardized)– Precise volume of acid used (volumetric pipet)– Base indicator used to show equivalence point– Titration to end-point, precise volume of basemeasured (buret).– Molar ratios, molarity and volume used to determinemoles and thus molarity of unknown acid.2010 2910

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