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Chapter 4 Aqueous Reactions and Solution Stoichiometry

Chapter 4 Aqueous Reactions and Solution Stoichiometry

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Chapter 4 Aqueous Reactions and Solution Stoichiometry

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  1. Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Ch4 Hmwk # 7, 8, 11, 15, 19, 23, 25, 27, 29, 31, 37, 39, 41, 45, 47, 51, 53, 59, 63, 67, 69, 75, 96 Chapter 4Aqueous Reactions and Solution Stoichiometry John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall, Inc.

  2. Aqueous Reactions and Solution Stoichiometry • What is aqueous? • What is an aqueous reaction? • What are some types of aqueous reactions? • Why might these be important? Give examples. • Aqueous reactions cannot take place without water. What do you already know about water that will help us understand aqueous reactions?

  3. Solutions: • Homogeneous mixtures of two or more pure substances. • The solvent is present in greatest abundance. • All other substances are solutes.

  4. Dissociation • When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them. • This process is called dissociation.

  5. Electrolytes • Substances that dissociate into ions when dissolved in water. Example: NaCl • Anonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Example: C12H22O11 Notice, solubility does not imply that something is or is not an electrolyte.

  6. Electrolytes and Nonelectrolytes Soluble ionic compounds tend to be electrolytes.

  7. Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases.

  8. SAMPLE EXERCISE 4.1 Relating Relative Numbers of Anions and Cations to Chemical Formulas The diagram below represents an aqueous solution of one of the following compounds: MgCl2 , KCl, or K2SO4. Which solution does it best represent?

  9. SAMPLE EXERCISE 4.1 Relating Relative Numbers of Anions and Cations to Chemical Formulas The diagram below represents an aqueous solution of one of the following compounds: MgCl2 , KCl, or K2SO4. Which solution does it best represent? Solution Analyze: We are asked to associate the charged spheres in the diagram with ions present in a solution of an ionic substance. Plan: We examine the ionic substances given in the problem to determine the relative numbers and charges of the ions that each contains. We then correlate these charged ionic species with the ones shown in the diagram. Solve: The diagram shows twice as many cations as anions, consistent with the formulation K2SO4. Check: Notice that the total net charge in the diagram is zero, as it must be if it is to represent an ionic substance.

  10. Electrolytes • A strong electrolyte dissociates completely when dissolved in water. • A weak electrolyte only dissociates partially when dissolved in water.

  11. Strong Electrolytes Are… • Strong acids

  12. Strong Electrolytes Are… • Strong acids • Strong bases

  13. Strong Electrolytes Are… • Strong acids • Strong bases • Soluble ionic salts

  14. Precipitation Reactions When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

  15. Metathesis comes from a Greek word that means “to transpose” AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactionsa.k.a. Double Replacement Rxns

  16. Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactions a.k.a. Double Replacement Rxns

  17. Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Metathesis (Exchange) Reactions

  18. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) How did we know that silver chloride was a solid??? We checked the solubility table.

  19. Predict the products of the following reactions • BaCl2(aq) + K2SO4(aq) • Fe2(SO4)3(aq) + LiOH (aq) • Mg(NO3)2(aq) + CaS (aq) • NaCl (aq) + KNO3(aq)

  20. Predict the products of the following reactions • BaCl2(aq) + K2SO4(aq) • → BaSO4(s) + 2KCl (aq) • Fe2(SO4)3(aq) + 6LiOH (aq) • → 2Fe(OH)3 (s) + 3Li2SO4 (aq) • Mg(NO3)2(aq) + CaS (aq) • → MgS (s) + Ca(NO3)2 (aq) • NaCl (aq) + KNO3(aq) • → NaNO3(aq) + KCl (aq) or NR

  21. Solution Chemistry • It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). • If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

  22. The molecular equation lists the reactants and products in their molecular form. AgNO3 (aq) + KCl(aq) AgCl(s) + KNO3 (aq) Molecular Equation

  23. 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) + NO3- (aq) + K+ (aq) + Cl- (aq)  AgCl(s) + K+(aq) + NO3- (aq) Ionic Equation

  24. To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl(s) + K+(aq) + NO3-(aq) Net Ionic Equation

  25. 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. Ag+(aq) + Cl-(aq) AgCl(s) Net Ionic Equation

  26. 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) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl(s) + K+(aq) + NO3-(aq) Net Ionic Equation

  27. Writing Net Ionic Equations • Write a balanced molecular equation. • Dissociate all strong electrolytes. • Cross out anything that remains unchanged from the left side to the right side of the equation. • Write the net ionic equation with the species that remain.

  28. Writing Net Ionic Equations Write the net ionic equation for each reaction • Pb(NO3)2(aq) + KI (aq) • Li2S (aq) + BaBr2(aq) • NH4Cl (aq) + Sr(OH)2(aq)

  29. Writing Net Ionic Equations Write the net ionic equation for each reaction • Pb(NO3)2(aq) + 2KI (aq) • → PbI2(s) + 2K(NO3) (aq) Pb2+ + 2I-→ PbI2(s) • Li2S (aq) + BaBr2(aq) • → LiBr (aq) + BaS (aq) 2Li+(aq) + S-2(aq) + Ba2+ (aq) + 2Br-(aq) → 2Li+(aq) + 2Br-(aq) + Ba2+ (aq) + S-2(aq) Notice ….all spectator ions. They all cancel and therefore, no reaction occurs. • 2NH4Cl (aq) + Sr(OH)2(aq) • → SrCl2 (aq) + 2NH4OH (aq) 2NH4+(aq) + 2Cl-(aq) + Sr2+(aq) + 2OH-(aq) → Sr2+(aq) + 2Cl-(aq) + 2NH4+(aq) + 2OH-(aq) Notice ….all spectator ions. They all cancel and therefore, no reaction occurs.

  30. Acids: • Substances that increase the concentration of H+ when dissolved in water (Arrhenius). • Proton donors (Brønsted–Lowry).

  31. Acids There are only seven strong acids: • Hydrochloric (HCl) • Hydrobromic (HBr) • Hydroiodic (HI) • Nitric (HNO3) • Sulfuric (H2SO4) • Chloric (HClO3) • Perchloric (HClO4) These substances are consider strong acids because they do not just dissolve in water, they will completely dissociate.

  32. Bases: • Substances that increase the concentration of OH− when dissolved in water (Arrhenius). • Proton acceptors (Brønsted–Lowry).

  33. Bases The strong bases are the soluble salts of hydroxide ion: • Alkali metals • Calcium • Strontium • Barium

  34. Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H+) to the base.

  35. Generally, when solutions of an acid and a base are combined, the products are a salt and water. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) Neutralization Reactions

  36. When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) H+ (aq)+ Cl- (aq)+ Na+ (aq) + OH-(aq) Na+ (aq)+ Cl- (aq)+ H2O (l) Neutralization Reactions

  37. When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) H+ (aq)+ Cl- (aq)+ Na+ (aq) + OH-(aq) Na+ (aq)+ Cl- (aq)+ H2O (l) H+ (aq)+ Cl- (aq)+ Na+ (aq) + OH- (aq) Na+ (aq) + Cl- (aq) + H2O (l) Neutralization Reactions H+ (aq) + OH- (aq) → H2O (l)

  38. Neutralization ReactionsWhat does this have to do with you??? What happens when you have a stomach ache? What do you do about it? Why?

  39. Neutralization ReactionsWhat does this have to do with you??? Many antacids contain magnesium hydroxide. It undergoes a neutralization reaction with stomach acid. What acid constitutes the largest quantity of stomach acid? Write a net ionic equation for the neutralization reaction between magnesium hydroxide and hydrochloric acid. Mg(OH)2 (s) + H+(aq)→ Mg2+(aq) + H2O (l)

  40. Neutralization Reactions Observe the reaction between Milk of Magnesia, Mg(OH)2, and HCl. Can you overdoes on antacid?? What do you think would happen?

  41. These metathesis reactions do not give the product expected. The expected product decomposes to give a gaseous product (CO2 or SO2). CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l) NaHCO3 (aq) + HBr (aq) NaBr (aq)+ CO2 (g) + H2O (l) SrSO3 (s) + 2 HI(aq) SrI2 (aq) + SO2 (g) + H2O (l) Gas-Forming Reactions

  42. This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! Just as in the previous examples, a gas is formed as a product of this reaction: Na2S (aq) + H2SO4 (aq) Na2SO4 (aq) + H2S (g) Gas-Forming Reactions

  43. A Carbonated Beverage is Another Example How is a beverage carbonated? What happens to the dissolved gas? H2O (l) + CO2(g)↔ H2CO3(aq) H2O (l) + CO2(g)↔ 2H+ (aq) + CO3-2(aq)

  44. Oxidation-Reduction Reactions • An oxidation occurs when an atom or ion loses electrons. (It becomes more positive) • A reduction occurs when an atom or ion gains electrons. (It becomes more negative)

  45. Oxidation-Reduction Reactions One cannot occur without the other.

  46. 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.

  47. Oxidation Numbers • Elements in their elemental form have an oxidation number of 0. • The oxidation number of a monatomic ion is the same as its charge.

  48. Oxidation Numbers • Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. • Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1. • Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.