Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chapter 4 Type of Chemical Reactions and Solution Stoichiometric Water, Nature of aqueous solutions, types of electrolytes, dilution. Types of chemical reactions: precipitation, acid-base and oxidation reactions. Stoichiometry of reactions and balancing the chemical equations.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Aqueous Solutions Water is the dissolving medium, or solvent.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Figure 4.1: (Left) The water molecule is polar. (Right) A space-filling model of the water molecule.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Figure 4.2: Polar water molecules interact with the positive and negative ions of a salt assisting in the dissolving process. Cl - Na + Cl - Na + H2OH2O

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Some Properties of Water 4 Water is “bent” or V-shaped. 4 The O-H bonds are covalent. 4 Water is a polar molecule. 4 Hydration occurs when salts dissolve in water.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Figure 4.3: (a) The ethanol molecule contains a polar O—H bond similar to those in the water molecule. (b) The polar water molecule interacts strongly with the polar O—H bond in ethanol. This is a case of "like dissolving like."

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 A Solute 4 dissolves in water (or other “solvent”) 4 changes phase (if different from the solvent) 4 is present in lesser amount (if the same phase as the solvent)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 A Solvent 4 retains its phase (if different from the solute) 4 is present in greater amount (if the same phase as the solute)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 General Rule for dissolution  Like dissolve like  Polar dissolve polar (water dissolve ethanol)  Non-polar dissolve nonpolar (benzene dissolve fat)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure 4.5: When solid NaCl dissolves, the Na+ and Cl- ions are randomly dispersed in the water.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Electrolytes Strong - conduct current efficiently NaCl, HNO 3 Weak - conduct only a small current vinegar, tap water Non - no current flows pure water, sugar solution

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Figure 4.4: Electrical conductivity of aqueous solutions.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 Acids Strong acids -dissociate completely to produce H + in solution hydrochloric and sulfuric acid HCl, H 2 SO 4 Weak acids - dissociate to a slight extent to give H + in solution acetic and formic acid CH 3 COOH, CH 2 O

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Bases Strong bases - react completely with water to give OH  ions. sodium hydroxide Weak bases - react only slightly with water to give OH  ions. ammonia

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Figure 4.6: HCl(aq) is completely ionized.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Figure 4.7: An aqueous solution of sodium hydroxide.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Figure 4.8: Acetic acid (HC 2 H 3 O 2 ) exists in water mostly as undissociated molecules. Only a small percentage of the molecules are ionized.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Molarity Molarity (M) = moles of solute per volume of solution in liters:

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Common Terms of Solution Concentration Stock - routinely used solutions prepared in concentrated form. Concentrated - relatively large ratio of solute to solvent. (5.0 M NaCl) Dilute - relatively small ratio of solute to solvent. (0.01 M NaCl): (MV) initial =(MV) Final

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Figure 4.10: Steps involved in the preparation of a standard aqueous solution.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Figure 4.12: Dilution Procedure (a) A measuring pipet is used to transfer 28.7mL of 17.4 M acetic acid solution to a volumetric flask. (b) Water is added to the flask to the calibration mark. (c) The resulting solution is 1.00 M acetic acid.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Types of Solution Reactions 4 Precipitation reactions AgNO 3 (aq) + NaCl(aq)  AgCl(s) + NaNO 3 (aq) 4 Acid-base reactions NaOH(aq) + HCl(aq)  NaCl(aq) + H 2 O(l) 4 Oxidation-reduction reactions Fe 2 O 3 (s) + Al(s)  Fe(l) + Al 2 O 3 (s)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Simple Rules for Solubility 1.Most nitrate (NO 3  ) salts are soluble. 2.Most alkali (group 1A) salts and NH 4 + are soluble. 3.Most Cl , Br , and I  salts are soluble (NOT Ag +, Pb 2+, Hg 2 2+ ) 4.Most sulfate salts are soluble (NOT BaSO 4, PbSO 4, HgSO 4, CaSO 4 ) 5.Most OH  salts are only slightly soluble (NaOH, KOH are soluble, Ba(OH) 2, Ca(OH) 2 are marginally soluble) 6.Most S 2 , CO 3 2 , CrO 4 2 , PO 4 3  salts are only slightly soluble.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Figure 4.13: When yellow aqueous potassium chromate is added to a colorless barium nitrate solution, yellow barium chromate precipitates.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Describing Reactions in Solution Precipitation 1.Molecular equation (reactants and products as compounds) AgNO 3 (aq) + NaCl(aq)  AgCl(s) + NaNO 3 (aq) 2.Complete ionic equation (all strong electrolytes shown as ions) Ag + (aq) + NO 3  (aq) + Na + (aq) + Cl  (aq) AgCl(s) + Na + (aq) + NO 3  (aq)

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Describing Reactions in Solution (continued) 3.Net ionic equation (show only components that actually reacts) Ag + (aq) + Cl  (aq)  AgCl(s) Na + and NO 3  are spectator ions.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Acid-Base Reactions An acid produces H + ions in water A base produces OH - ions in water An acid is a proton donor A base is a proton acceptor Arrhenius’s concept Bronsted & Lowrys’ concept

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Performing Calculations for Acid-Base Reactions 1.List initial species and predict reaction. 2.Write balanced net ionic reaction. 3.Calculate moles of reactants. 4.Determine limiting reactant. 5.Calculate moles of required reactant/product. 6.Convert to grams or volume, as required. Remember: n H+ = n OH- (MV) H+ = (MV) OH-

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Neutralization Reaction acid + base salt + water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 OH + + Cl - + Na + + OH - Na + + Cl - + H 2 O H + + OH - H 2 O 4.3

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Key Titration Terms Titrant - solution of known concentration used in titration Analyte - substance being analyzed Equivalence point - enough titrant added to react exactly with the analyte Endpoint - the indicator changes color so you can tell the equivalence point has been reached. movie

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 Oxidation-Reduction Reactions (electron transfer reactions) 2Mg (s) + O 2 (g) 2MgO (s) 2Mg 2Mg e - O 2 + 4e - 2O 2- Oxidation half-reaction (lose e - ) Reduction half-reaction (gain e - ) 2Mg + O 2 + 4e - 2Mg O e - 2Mg + O 2 2MgO

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Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Redox Reactions Many practical or everyday examples of redox reactions: –Corrosion of iron (rust formation) –Forest fire –Charcoal grill –Natural gas burning –Batteries –Production of Al metal from Al2O3 (alumina) –Metabolic processes combustion

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Rules for Assigning Oxidation States 1. Oxidation state of an atom in an element = 0 2. Oxidation state of monatomic element = charge 3. Oxygen =  2 in covalent compounds (except in peroxides where it =  1) 4. H = +1 in covalent compounds 5. Fluorine =  1 in compounds 6. Sum of oxidation states = 0 in compounds Sum of oxidation states = charge of the ion

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 36 Zn (s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu (s) Zn is oxidizedZn Zn e - Cu 2+ is reducedCu e - Cu Zn is the reducing agent Cu 2+ is the oxidizing agent 4.4 Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Cu (s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2Ag (s) Cu Cu e - Ag + + 1e - AgAg + is reducedAg + is the oxidizing agent

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 37 NaIO 3 Na = +1 O = -2 3x(-2) ? = 0 I = +5 IF 7 F = -1 7x(-1) + ? = 0 I = +7 K 2 Cr 2 O 7 O = -2K = +1 7x(-2) + 2x(+1) + 2x(?) = 0 Cr = +6 Oxidation numbers of all the elements in the following ? 4.4

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 38 Balancing by Half-Reaction Method 1.Write separate reduction, oxidation reactions. 2.For each half-reaction:  Balance elements (except H, O)  Balance O using H 2 O  Balance H using H +  Balance charge using electrons

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 39 Balancing by Half-Reaction Method (continued) 3.If necessary, multiply by integer to equalize electron count. 4.Add half-reactions. 5.Check that elements and charges are balanced.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 40 Half-Reaction Method - Balancing in Base 1.Balance as in acid. 2.Add OH  that equals H + ions (both sides!) 3.Form water by combining H +, OH . 4.Check elements and charges for balance.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 41 Balancing Redox Equations Example: Balance the following redox reaction: Cr 2 O Fe 2+ Cr 3+ + Fe 3+ (acidic soln) 1) Break into half reactions: Cr 2 O 7 2- Cr 3+ Fe 2+ Fe 3+

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 42 Balancing Redox Equations 2) Balance each half reaction: Cr 2 O 7 2- Cr 3+ Cr 2 O Cr 3+ Cr 2 O Cr H 2 O Cr 2 O H + 2 Cr H 2 O 6 e - + Cr 2 O H + 2 Cr H 2 O

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 43 Balancing Redox Equations 2) Balance each half reaction (cont) Fe 2+ Fe 3+ Fe 2+ Fe e -

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 44 Balancing Redox Reactions 3) Multiply by integer so e - lost = e - gained 6 e - + Cr 2 O H + 2 Cr H 2 O Fe 2+ Fe e - x 6

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 45 Balancing Redox Reactions 3) Multiply by integer so e - lost = e - gained 6 Fe 2+ 6 Fe e - 6 e - + Cr 2 O H + 2 Cr H 2 O 4) Add both half reactions Cr 2 O Fe H + 2 Cr Fe H 2 O

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 46 Balancing Redox Reactions Cr 2 O Fe H + 2 Cr Fe H 2 O 5) Check the equation 2 Cr7 O6 Fe14 H

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 47 Balancing Redox Reactions Procedure for Basic Solutions: –Divide the equation into 2 incomplete half reactions one for oxidation one for reduction

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 48 Balancing Redox Reactions –Balance each half-reaction: balance elements except H and O balance O atoms by adding H 2 O balance H atoms by adding H + add 1 OH - to both sides for every H + added combine H + and OH - on same side to make H 2 O cancel the same # of H 2 O from each side balance charge by adding e - to side with greater overall + charge different

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 49 Balancing Redox Equations –Multiply each half reaction by an integer so that # e - lost = # e - gained –Add the half reactions together. Simply where possible by canceling species appearing on both sides of equation –Check the equation # of atoms total charge on each side

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 50 Balancing Redox Reactions Example: Balance the following redox reaction. NH 3 + ClO - Cl 2 + N 2 H 4 (basic soln) NH 3 N 2 H 4 ClO - Cl 2 1) Break into half reactions:

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 51 Balancing Redox Reactions NH 3 N 2 H 4 2) Balance each half reaction: 2 NH 3 N 2 H 4 2 NH 3 N 2 H H + 2 NH OH - N 2 H H 2 O + 2 OH OH - 2H2O2H2O 2 NH OH - N 2 H H 2 O + 2 e -

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 52 Balancing Redox Reactions 2 ClO - Cl 2 2) Balance each half reaction: 2 ClO - Cl H 2 O 2 ClO H + Cl H 2 O + 4 OH OH - 2 ClO H 2 O Cl H 2 O + 4 OH - 2 ClO H 2 O Cl OH - 2 e ClO H 2 O Cl OH - ClO - Cl 2

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 53 Balancing Redox Reactions 3) Multiply by integer so # e - lost = # e - gained 2 NH OH - N 2 H H 2 O + 2 e - 2 e ClO H 2 O Cl OH - 4) Add both half reactions 2 NH OH - + 2ClO H 2 O N 2 H H 2 O + Cl OH -

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 54 Balancing Redox Reactions 5) Cancel out common species 2 NH OH ClO H 2 O N 2 H H 2 O + Cl OH NH ClO - N 2 H 4 + Cl OH - 6) Check final equation: 2 N6 H2 Cl2 O-2

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 55

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 56