1. Aqueous Reactions © 2009, Prentice-Hall, Inc. Chapter 4 Aqueous Reactions and Solution Stoichiometry Mrs. Debbie Amuso Camden High School Camden, NY Chemistry, The Central Science, 11th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

2. Aqueous Reactions © 2009, Prentice-Hall, Inc. Aqueous Solutions (aq) 1.Solutions consist of two or more substances physically combined in a homogeneous mixture. 2. Aqueous solutions consist of solute(s) dissolved in a solvent (water). 3. Solute = substance dissolved 4. Solvent = substance of greater quantity (usually water)

3. Aqueous Reactions Aqueous Solutions (aq) 5.Properties: Clear or transparent but may be colored, Solute doesn’t settle on standing, Can’t be separated by filtering, Components are not in a definite ratio 6.Example: NaCl(aq)

4. Aqueous Reactions Water: The Universal Solvent 1.Due to its polar nature, water will dissolve ionic substances and most polar substances. 2.“Like dissolves like.” 3.Example: NaCl(s)  Na 1+ (aq) + Cl 1= (aq)  +

5. Aqueous Reactions © 2009, Prentice-Hall, Inc. Dissociation Dissociation is the process that occurs when an ionic substance dissolves in water and the solvent pulls the individual ions from the crystal and solvates them. Recall: Ionic substances contain a metal & nonmetal or polyatomic ion. Examples: NH 4 Cl or KBr

6. Aqueous Reactions © 2009, Prentice-Hall, Inc. Electrolytes vs Nonelectrolytes An electrolyte is a substance that dissociates into ions when dissolved in water. A nonelectrolyte may dissolve in water, but it remains a molecule and does not dissociate into ions.

7. Aqueous Reactions © 2009, Prentice-Hall, Inc. Electrolytes A strong electrolyte dissociates completely when dissolved in water & forms lots of ions. A weak electrolyte only dissociates partially when dissolved in water & only forms a few ions.

8. Aqueous Reactions © 2009, Prentice-Hall, Inc. Electrolytes vs Nonelectrolytes Memorize the Chart Note: 1.Ionic compounds contain a metal and nonmetal and/or polyatomic ion. 2.Molecular (also called covalent) compounds contain only nonmetals. 3.There is a separate list of strong acids you need to memorize. 4.The compound must also be soluble in water – Be able to use the Solubility Table. Note: Strong bases are ionic & also strong electrolytes!!

9. Aqueous Reactions © 2009, Prentice-Hall, Inc. Strong Acids & Bases Memorize!

10. Aqueous Reactions Solubility Guidelines Know how to use this Chart.

11. Aqueous Reactions How to Show © 2009, Prentice-Hall, Inc. Strong Electrolyte: KCl (s)  K 1+ (aq) + Cl 1- (aq) Weak Electrolyte: HF (g)  H 1+ (aq) + F 1- (aq) Nonelectrolyte: C 12 H 22 O 11 (s)  C 12 H 22 O 11 (aq)

12. Aqueous Reactions Check Yourself: 1.Electrolyte or Nonelectrolyte? a) C 6 H 12 O 6 b) MgSO 4 c) LiBr d) NH 3 2.Soluble or Insoluble? a) AgCl b) BaCO 3 c) NaC 2 H 3 O 2 d) Li 2 S Answers: 1 a) NE b) E c) E d) E 2 a) Ins b) Ins c) Sol d) Sol

13. Aqueous Reactions © 2009, Prentice-Hall, Inc. Solution Chemistry It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, or an aqueous solution made of ions). If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

14. Aqueous Reactions © 2009, Prentice-Hall, Inc. Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose.” It appears the ions in the reactant compounds exchange, or transpose, ions. Metathesis is another name for a Double Replacement Reaction. Recall the General form: AB + CD  AD + CB AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

15. Aqueous Reactions © 2009, Prentice-Hall, Inc. Precipitation Reactions A Precipitation Reaction is a metathesis reaction that results in the formation of an insoluble solid (precipitate).

16. Aqueous Reactions Precipitates 1.Precipitates are insoluble solids. 2.Practice: Name the precipitate formed when solutions of barium nitrate and sodium sulfate are mixed. Answer: Barium sulfate

17. Aqueous Reactions Writing Net Ionic Equations 1.Write the balanced Molecular Equation. 2. Identify the product that does not ionize. It will be either: Water - H 2 O A gas - CO 2 or H 2 or O 2, etc. A precipitate – one of the insoluble ionic solids on the Solubility Table 3. Write the Ionic Equation by ionizing everything else. 4. Cross-out Spectator Ions (Those ions that don’t change.) 5. What’s left is the Net Ionic Reaction.

18. Aqueous Reactions Note 1. It is said that a reaction “takes place” or “is spontaneous” or “goes to completion” only if one of the products is water, a gas, or a precipitate. 2. For metathesis reactions, if the products formed are also aqueous, we say the reaction is “not spontaneous” or doesn’t go to competition. © 2009, Prentice-Hall, Inc.

19. Aqueous Reactions © 2009, Prentice-Hall, Inc. Molecular Equation The molecular equation lists the reactants and products in their molecular form. Be sure to include phases. AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq) This rxn is spontaneous because a precipitate forms.

20. Aqueous Reactions © 2009, Prentice-Hall, Inc. Ionic Equation In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

21. Aqueous Reactions © 2009, Prentice-Hall, Inc. Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

22. Aqueous Reactions Practice Completing & Balancing Metathesis Reactions BaCl 2 (aq) + K 2 SO 4 (aq)  Answer: BaCl 2(aq) + K 2 SO 4(aq)  BaSO 4(s) + 2KCl (aq) Fe 2 (SO 4 ) 3 (aq) + LiOH (aq)  Answer: Fe 2 (SO 4 ) 3(aq) + 6LiOH (aq)  2Fe(OH) 3(s) + 3Li 2 SO 4(aq)

23. Aqueous Reactions Write Ionic & Net Ionic Equations too BaCl 2 (aq) + K 2 SO 4 (aq)  Answer: Ba 2+ (aq) + 2Cl - (aq) + 2K + (aq) + SO 4 2- (aq)  BaSO 4(s) + 2K + (aq) + 2Cl - (aq) = spectator ions Net Ionic Rx = Ba 2+ (aq) + SO 4 2- (aq)  BaSO 4(s) Fe 2 (SO 4 ) 3 (aq) + LiOH (aq)  Answer: 2Fe 3+ (aq) + 3SO 4 2- (aq) + 6Li + (aq) + 6OH - (aq)  2Fe(OH) 3(s) + 6Li + (aq) + SO 4 2- (aq) = spectator ions Net Ionic Rx = 2Fe 3+ (aq) + 6OH - (aq)  2Fe(OH) 3(s)

24. Aqueous Reactions © 2009, Prentice-Hall, Inc. Acids Arrhenius defined acids as substances that increase the concentration of H + when dissolved in water. Brønsted and Lowry defined them as proton donors.

25. Aqueous Reactions © 2009, Prentice-Hall, Inc. Acids There are only seven strong acids: Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO 3 ) Sulfuric (H 2 SO 4 ) Chloric (HClO 3 ) Perchloric (HClO 4 ) A common weak acid: –Acetic (HC 2 H 3 O 2 )

26. Aqueous Reactions © 2009, Prentice-Hall, Inc. Bases Arrhenius defined bases as substances that increase the concentration of OH − when dissolved in water. Brønsted and Lowry defined them as proton acceptors.

27. Aqueous Reactions © 2009, Prentice-Hall, Inc. Bases The strong bases are the soluble metal salts of hydroxide ion: Alkali metals Calcium Strontium Barium A common weak base: –Ammonia (NH 3 ) They are: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2

28. Aqueous Reactions © 2009, Prentice-Hall, Inc. Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H + ) to the base.

29. Aqueous Reactions © 2009, Prentice-Hall, Inc. Neutralization Reactions Generally, when solutions of an acid and a base are combined, the products are a salt and water. Neutralization Acid + Base  Salt + Water Example: CH 3 COOH (aq) + NaOH (aq)  CH 3 COONa (aq) + H 2 O (l)

30. Aqueous Reactions © 2009, Prentice-Hall, Inc. Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H 2 O (l) H + ( aq ) + Cl - ( aq ) + Na + ( aq ) + OH - ( aq )  Na + ( aq ) + Cl - ( aq ) + H 2 O ( l ) H + (aq) + OH - (aq)  H 2 O (l)

31. Aqueous Reactions © 2009, Prentice-Hall, Inc. Gas-Forming Reactions Memorize: When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water. CaCO 3 (s) + HCl (aq)  CaCl 2 (aq) + CO 2 (g) + H 2 O (l) NaHCO 3 (aq) + HBr (aq)  NaBr (aq) + CO 2 (g) + H 2 O (l)

32. Aqueous Reactions © 2009, Prentice-Hall, Inc. Gas-Forming Reactions Memorize: Similarly, when a sulfite reacts with an acid, the products are a salt, sulfur dioxide, and water. NO  SrSO 3 (s) + 2 HI (aq)  SrI 2 (aq) + H 2 SO 3 (aq) YES  SrSO 3 (s) + 2 HI (aq)  SrI 2 (aq) + SO 2 (g) + H 2 O (l)

33. Aqueous Reactions © 2009, Prentice-Hall, Inc. Gas-Forming Reactions This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! But just as in the previous examples, a gas is formed as a product of this reaction. Na 2 S (aq) + H 2 SO 4 (aq)  Na 2 SO 4 (aq) + H 2 S (g)

34. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. A reduction occurs when an atom or ion gains electrons. One cannot occur without the other.

35. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation-Reduction Reactions The substance oxidized is called the reducing agent. The substance reduced is called the oxidizing agent.

36. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation-Reduction Reactions The substance oxidized experiences an increase in oxidation number. The substance reduced experiences a decrease in oxidation number.

37. Aqueous Reactions © 2009, Prentice-Hall, Inc. Oxidation Numbers To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity. If the oxidation numbers change, a redox rxn has occurred.

38. Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 1.Elements in their elemental form have an oxidation number of zero. Mg 0 (s) + 2HCl (aq)  MgCl 2 (aq) + H 2 0 (g) 2. The oxidation number of a monatomic ion is the same as its charge. Mg 0 (s) + 2H +1 Cl -1 (aq)  Mg +2 Cl -1 2 (aq) + H 2 0 (g) Note: Mg 0 is oxidized (its charge increased) 2H 1+ is reduced (its charge decreased)

39. Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 3. The sum of the oxidation numbers in a neutral compound is 0. FeCl 3 (Fe = +3 & each Cl = -1) FeCl 2 (Fe = +2 & each Cl = -1) 4. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion. Sulfate Ion = -2, so for SO 4 2- (S = +6 and each O = -2 which adds up to -2)

40. Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 5. Nonmetals tend to have negative oxidation numbers, although there are some exceptions.  Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1. O 2 (Here oxygen is 0) MgO (Here Mg is +2 and O is - 2) Na 2 O 2 (In peroxides, Na is +1 and O is -1)

41. Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 5. Nonmetals tend to have negative oxidation numbers, although there are some exceptions.  Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal. HCl (Here H is +1 and Cl is -1) NH 3 (Here N is -3 and H is still +1) NaH (Here Na is +1 and H is -1)

42. Aqueous Reactions © 2009, Prentice-Hall, Inc. Rules for Assigning Oxidation Numbers 5. Nonmetals tend to have negative oxidation numbers, although there are some exceptions.  Fluorine always has an oxidation number of −1. NaF (Na is +1 and F is -1)  The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions. KClO 3 (K is +1, Cl is +5 and each O is -2)

43. Aqueous Reactions Redox Reactions 1. Be able to identify which substance is oxidized and which substance is reduced. 0 0 +1 -2 Example: H 2 + O 2  H 2 O (H 2 is oxidized because its charge increases) (O 2 is reduced because its charge decreases) 2. Metathesis Rxn are not redox because the charges don’t change. © 2009, Prentice-Hall, Inc.

44. Aqueous Reactions © 2009, Prentice-Hall, Inc. Displacement Reactions In displacement reactions, ions oxidize an element. The ions, then, are reduced.

45. Aqueous Reactions © 2009, Prentice-Hall, Inc. Displacement Reactions In this reaction, silver ions oxidize copper metal. Cu(s) + 2AgNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2Ag(s) Net Ionic Rx: Cu(s) + 2Ag + (aq)  Cu 2+ (aq) + 2Ag(s) NO 3 1- is the spectator ion

46. Aqueous Reactions © 2009, Prentice-Hall, Inc. Displacement Reactions The reverse reaction, however, does not occur. Cu 2+ (aq) + 2 Ag (s)  Cu (s) + 2 Ag + (aq) x

47. Aqueous Reactions © 2009, Prentice-Hall, Inc. Activity Series Rule: A metal will replace (reduce) any metal below it on the table. Given: A + BC  AC + B The reaction will take place only if A is more reactive (above) B on the table! If so, we say the reaction is spontaneous.

48. Aqueous Reactions © 2009, Prentice-Hall, Inc. Molarity Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. Molarity is one way to measure the concentration of a solution. moles of solute volume of solution in liters Molarity (M) = M = mol/L ( Also helpful: mol = mass/gfm)

49. Aqueous Reactions © 2009, Prentice-Hall, Inc. Mixing a Solution To create a solution of a known molarity, one weighs out a known mass (and, therefore, number of moles) of the solute. The solute is added to a volumetric flask, and solvent is added to the line on the neck of the flask. mass = (Molarity) (gfm) (V liters ) mass = (M) (gfm) (V L )

50. Aqueous Reactions © 2009, Prentice-Hall, Inc. Solution Dilution One can also dilute a more concentrated solution by –Using a pipet to deliver a volume of the solution to a new volumetric flask, and –Adding solvent to the line on the neck of the new flask.

51. Aqueous Reactions © 2009, Prentice-Hall, Inc. Solution Dilution The molarity of the new solution can be determined from the equation M 1  V 1 = M 2  V 2, where M 1 and M 2 are the molarity of the concentrated and dilute solutions, respectively, and V 2 and V 2 are the volumes of the two solutions.

52. Aqueous Reactions © 2009, Prentice-Hall, Inc. Titration Titration is an analytical technique in which one can calculate the concentration of a solute in a solution.

53. Aqueous Reactions © 2009, Prentice-Hall, Inc. Acid-Base Titration Use: (M a )(V a ) = (M b )(V b ) Shortcut: Diprotic Acid Like H 2 SO 4 : 2 (M a )(V a ) = (M b )(V b ) Dihydroxyl Base Like Ca(OH) 2 : (M a )(V a ) = 2 (M b )(V b )

54. Aqueous Reactions © 2009, Prentice-Hall, Inc. Using Molarities in Stoichiometric Calculations

55. Aqueous Reactions Stoichiometric Calculation Steps Step 1) Complete & balance equation using coefficients. Step 2) Identify Given & Unknown. Step 3) Convert Given into moles using either #moles = mass / GFM -or- #moles = (Molarity)(V L ) Step 4) Use mole ratio to find moles of Unknown. #moles given = #moles unk Coeff given Coeff unk Step 5) Convert moles of ‘unknown’ to answer using either mass = (#moles) (GFM) –or- Molarity = #moles / V L © 2009, Prentice-Hall, Inc.

56. Aqueous Reactions Example: How many grams of NaOH(s) are needed to neutralize exactly 50.0 mL of 2.00 M H 2 SO 4 ? Step 1) 2 NaOH (s) + H 2 SO 4(aq)  Na 2 SO 4(aq) + 2H 2 O (l) Step 2) Given = 50.0mL of 2.00M H 2 SO 4 Unknown = mass of NaOH Step 3) Convert Given into moles using #moles = (Molarity)(V L ) #moles H 2 SO 4 = (2.00M)(0.0500L) = 0.100 moles H 2 SO 4 Step 4) Use mole ratio to find moles of Unknown. #moles given = #moles unk 0.100 = x x = 0.200 mol Coeff given Coeff unk 1 2 Step 5) Convert moles of ‘unk’ to answer using mass = (#moles) (GFM) mass = (0.200moles)(40.0g/mol) = 8.00 grams NaOH © 2009, Prentice-Hall, Inc.