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Chapter 6

Chapter 6. Chemical Reactions. Setting the Stage – Global Warming. Accepted as a fact after some controversy At least partly due to increase of CO 2 in atmosphere CO 2 (along with other gases) traps heat in the atmosphere by preventing heat loss to space.

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Chapter 6

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  1. Chapter 6 Chemical Reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  2. Setting the Stage – Global Warming • Accepted as a fact after some controversy • At least partly due to increase of CO2 in atmosphere • CO2 (along with other gases) traps heat in the atmosphere by preventing heat loss to space Malone and Dolter - Basic Concepts of Chemistry 9e

  3. Burning Fossil Fuels – Source of Most CO2 Natural gas, coal and oil and all carbon compounds produce CO2 on combustion. Other gases (various pollutants) are potent “greenhouse gases” Malone and Dolter - Basic Concepts of Chemistry 9e

  4. Carbon Cycle • Photosynthesis converts CO2 to carbohydrates (CH2O)n • Carbohydrates undergo reaction with oxygen (combustion) to yield H2O and CO2 • In the past 100-200 years, more combustion occurs than photosynthesis, resulting in an increase in CO2 relative to (CH2O)n Malone and Dolter - Basic Concepts of Chemistry 9e

  5. Reactions • Combustion (reaction with oxygen) is one of a number of possible reactions that chemicals can undergo • Many reactions occur in water, especially since water can hold ionic and polar compounds in solution Malone and Dolter - Basic Concepts of Chemistry 9e

  6. Setting a Goal – Part AThe Representation of Chemical Changes and Three Types of Changes • You will learn to use the symbolic language of chemistry by writing balanced chemical reactions for several identifiable reaction types Malone and Dolter - Basic Concepts of Chemistry 9e

  7. Objectives for Section 6-1 • Write a chemical equation from a word description of the reaction • Balance a simple chemical equation by inspection Malone and Dolter - Basic Concepts of Chemistry 9e

  8. Professor’s Little Joke Old chemistry professors never die, they just fail to react Malone and Dolter - Basic Concepts of Chemistry 9e

  9. 6-1 Chemical Equations • A chemical equation is the representation of a chemical reaction using the symbols of the elements and the formulas of the compounds • The reacting species (starting materials or reactants) are listed on the left, and the products of the reaction are listed on the right Malone and Dolter - Basic Concepts of Chemistry 9e

  10. Example Equation Malone and Dolter - Basic Concepts of Chemistry 9e

  11. Chemical Equations • Chemical equations are the representation of a chemical reaction using the symbols of the elements and the formulas of the compounds • The arrow means “produces” or “yields” • Reactants are shown to the left of the arrow, and products are shown to the right of the arrow • A plus sign is used to indicate that substances combine or react with each other Malone and Dolter - Basic Concepts of Chemistry 9e

  12. Chemical Equations • Symbols commonly used in chemical equations are listed in Table 6-1 (memorize this) • We balance chemical equations because of the Law of the Conservation of Mass • We use coefficients in front of the element or compound formula, which are whole numbers that adjust the amounts of the species • Note that we cannot balance an equation by changing the subscripts of the compound Malone and Dolter - Basic Concepts of Chemistry 9e

  13. Balancing Coefficients • The subscripts of the compound are fixed: they cannot be changed in an equation • The coefficients used should be the smallest whole numbers possible • The coefficient multiplies every number in the formula (e.g. 2 MgCl2 means that there are 2 Mg atoms and 4 Cl atoms) • We will balance equations by inspection Malone and Dolter - Basic Concepts of Chemistry 9e

  14. Balancing Strategies • Consider PbO2 + HCl → PbCl2 + Cl2 + H2O • Balance Pb first, then O, H and finally Cl • Cl last because it is in its free state, Cl2 • Pb first since it is not H or O • O because it is bonded to Pb (in PbO2) • H because it is bonded to O (in H2O) • PbO2 + 4HCl PbCl2 + Cl2 + 2H2O Malone and Dolter - Basic Concepts of Chemistry 9e

  15. Combustion, Combination and Decomposition Reactions • All chemical reactions can be represented by balanced chemical equations • These reactions can also be classified into one of five groups (although other groupings are possible) • Objective for next section is to classify certain chemical reactions as being combustion, combination or decomposition Malone and Dolter - Basic Concepts of Chemistry 9e

  16. Objective for Section 6-2 • Classify certain chemical reactions as being combustion, combination, or decomposition reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  17. 6-2 Combustion, Combination, and Decomposition Reactions Combustion Reactions • Involve the combination of an element or compound with oxygen (as O2) • Usually liberates a lot of heat, and may be accompanied by a flame • Combustion reactions may be incomplete, especially if there is insufficient oxygen. • E.g. 2C8H18(l) + 17O2(g)  16CO(g) + 18H2O(l) Malone and Dolter - Basic Concepts of Chemistry 9e

  18. Combustion Reactions • Balancing combustion reactions involving hydrocarbons is best done by starting with C, then H, followed by O • Consider the complete combustion of ethane. The products are CO2 and H2O • The initial balanced equation isC2H6 (g) + 7/2O2 (g)  2CO2(g) + 3H2O(l) Converting to whole numbers gives: Malone and Dolter - Basic Concepts of Chemistry 9e

  19. Combustion Reactions - Example Strip of magnesium combusting Malone and Dolter - Basic Concepts of Chemistry 9e

  20. Combination Reactions • Often called synthesis reactions • Involves two or more substances combining to form a single substance • Consider:2Mg(s) + O2 (g)  2MgO(s) 2Na(s) + Cl2(g)  2NaCl(s) NH3(g) + HCl(g)  NH4Cl(s) Malone and Dolter - Basic Concepts of Chemistry 9e

  21. Decomposition Reactions • The reverse of a combination (or synthesis) reaction • One substance breaking apart into two or more substances • Consider:H2CO3(aq)  H2O(l) + CO2(g) N2O4(g)  2NO2(g) Malone and Dolter - Basic Concepts of Chemistry 9e

  22. Setting a Goal - Part BIons In Water And How They React • You will learn how ions are formed in aqueous solution and how these ions interact in two important types of chemical reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  23. Objective for Section 6-3 • Write the ions formed when ionic compounds or acids dissolve in water Malone and Dolter - Basic Concepts of Chemistry 9e

  24. 6-3 The Formation of Ions in Water • When we consider a substance dissolving in another substance, we define the solute as the substance in lesser amount, and the solvent as the substance in greater amount • Solubility is the relative degree to which a substance dissolves in a solvent • Soluble substances dissolve to a considerable degree • Insoluble substances do not dissolve at all or only to a small degree Malone and Dolter - Basic Concepts of Chemistry 9e

  25. Dissolution of Ionic Compounds • When ionic compounds dissolve in water, they dissociate into separate anions and cations: NaCl(s)  Na+(aq) + Cl-(aq) • See Figure 6-4 • The ions are often highly solvated. Ion solvation is an energetically favorable process; the energy lost more than compensates for the energy needed to break down the crystal lattice H2O Malone and Dolter - Basic Concepts of Chemistry 9e

  26. Dissolution of Ionic Compounds - Example Na+ Cl- Cl- Na+ Cl- Na+ Solvated ions Sodium chloride crystal lattice (NaCl(s)) Na+Cl- Water molecules Malone and Dolter - Basic Concepts of Chemistry 9e

  27. Dissolution of Polyatomic Salts • It is important to be able to recognize polyatomic ions, since they do not dissociate into simple ions H2OCa(ClO4)2(s) → Ca2+(aq) + 2 ClO4-(aq) • The salts do dissociate into anions and cations, but the polyatomic ions remain intact Malone and Dolter - Basic Concepts of Chemistry 9e

  28. Dissolution of Acids • Acids are so named because when they dissolve, they ionize to yield H+and the associated anionHCl(aq)  H+(aq) + Cl-(aq) • or HCl(g) + aq  H+(aq) + Cl-(aq) Malone and Dolter - Basic Concepts of Chemistry 9e

  29. Acids • Strong acids completely dissociate into ions upon dissolution in water • Examples are HX (X = Cl-, Br-, I-), H2SO4, HNO3, HClO4 • Weak acids dissolve, but do not completely dissociate into H+ and anions • Examples are HF, HCN, HOCl, HCO2H and CH3CO2H (+ most organiccarboxylic acids) Malone and Dolter - Basic Concepts of Chemistry 9e

  30. Acids – Indicator Test The indicator phenolphthalein is pink in alkaline solution and colorless in acid solution Malone and Dolter - Basic Concepts of Chemistry 9e

  31. Objective for Section 6-4 • Given the activity series, complete several single-replacement reactions as balanced molecular, total ionic, and net ionic equations Malone and Dolter - Basic Concepts of Chemistry 9e

  32. 6-4 Single-Replacement Reactions • An element replaces another in a compoundZn(s) + CuCl2(aq)  ZnCl2(aq) + Cu(s) • This particular form of the equation is termed the molecular equation, where all reactants and products are shown as neutral compounds Malone and Dolter - Basic Concepts of Chemistry 9e

  33. Reaction of Zn with Cu2+ • Immersing a strip of Zn in a solution containing Cu2+ causes Cu metal to form a coat on the Zn strip • See Figure 6-5. Malone and Dolter - Basic Concepts of Chemistry 9e

  34. Ionic Equations • The total ionic equation separates all of the anions and cations Zn(s) + Cu2+(aq) + 2Cl-(aq)  Zn2+ (aq) + 2Cl-(aq) + Cu(s) • Spectator ions (such as the Cl- ions) are those that are in the same state on both sides of the equation Malone and Dolter - Basic Concepts of Chemistry 9e

  35. Net Ionic Equation • The net ionic equation leaves out the spectator ionsThe complete ionic equation:Zn(s) + Cu2+(aq) + 2Cl-(aq)  Zn2+(aq) + 2Cl-(aq) + Cu(s)The net ionic equation: • Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) Malone and Dolter - Basic Concepts of Chemistry 9e

  36. Activity Series • Table 6-2 indicates the common metals in decreasing order of their ability to replace metal ions in aqueous solution • When comparing two metals, the one higher in the table replaces the one lower in the table (e.g. Zn can replace Cu but not Al) Malone and Dolter - Basic Concepts of Chemistry 9e

  37. Metal Corresponding Ion K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Cr3+ Fe2+ Ni2+ Sn2+ Pb2+ H+ Cu2+ Ag+ Au3+ K (most active) Na Ca Mg Al Zn Cr Fe Ni Sn Pb H2 Cu Ag Au (least active) The Activity Series (Table 6-2) Malone and Dolter - Basic Concepts of Chemistry 9e

  38. Activity Series (Examples) • Feasible (spontaneous)Ni(s) + 2 Ag+(aq) → Ni2+(aq) + 2 Ag(s) Mg(s) + Zn2+(aq) → Mg2+(aq) + Zn(s) • Not Feasible (no spontaneous reaction)3Zn(s) + 2Al3+(aq) → 3Zn2+(aq) + 2Al(s) 2Ag(s) + Ni2+(aq) → 2 Ag+(aq) + Ni(s) Malone and Dolter - Basic Concepts of Chemistry 9e

  39. Objectives for Section 6-5 • Given a table of solubility rules, determine whether a specific ionic compound is soluble or insoluble in water • Write balanced molecular, total ionic, and net ionic equations for precipitation and neutralization reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  40. 6-5 Double Replacement Reactions – Precipitation • Where the anions and cations in an ionic equation exchange counterionsFeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq) Precipitates are indicated by (s) or Malone and Dolter - Basic Concepts of Chemistry 9e

  41. What Causes Double Replacements? • There has to be some process to cause the reaction to take place • Precipitation of a solid • Evolution of a gas • Neutralization reaction between an acid and a base • If one of these processes does not occur, there is no double replacement reaction Malone and Dolter - Basic Concepts of Chemistry 9e

  42. Examining Reactions • Molecular equation Pb(NO3)2(aq) + 2 NaI(aq) → PbI2(s) + 2 NaNO3(aq) • Total ionic equationPb2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + 2I-(aq) → PbI2(s) + 2 Na+(aq) + 2 NO3-(aq) • Net ionic equationPb2+(aq) + 2I-(aq) → PbI2(s) Malone and Dolter - Basic Concepts of Chemistry 9e

  43. Types of Double Replacement Reactions • PrecipitationAgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) • Gas evolution2HCl(aq) + Na2CO3(aq)  2NaCl(aq) + CO2(g) • Formation of a molecular compoundHCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq) Malone and Dolter - Basic Concepts of Chemistry 9e

  44. Precipitation Reactions • Formation of a precipitate occurs when solutions of two soluble ionic compounds are mixed to produce an insoluble salt • The two cations exchange anions in such reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  45. Precipitation • Use the solubility rules (Table 6-3) to predict whether two ions combine to form an insoluble salt • Pb(NO3)2(aq) + 2 NaI(aq) → PbI2(s) + 2 NaNO3(aq) • NH4NO3 (aq) + NaI(aq) → no precipitate Malone and Dolter - Basic Concepts of Chemistry 9e

  46. Anion Solubility Rule Mostly soluble Cl-, Br-, I- All cations form soluble compounds, except Ag+, Hg22+, and Pb2+. (PbCl2 and PbBr2 are slightly soluble) NO3-, ClO4-, CH3CO2- (acetate) All cations form soluble compounds. (KClO4 and CH3CO2Ag are slightly soluble) SO42- All cations form soluble compounds, except Pb2+, Ba2+ and Sr2+. (Ca2+ and Ag+ form slightly soluble compounds) Mostly insoluble CO32-, PO43- All cations form insoluble compounds, except Group IA metals and NH4+ S2- All cations form insoluble compounds, except Group IA and Group IIA metals and NH4+ OH- All cations form insoluble compounds, except Group IA metals, Ba2+, Sr2+, and NH4+. [Ca(OH)2 is slightly soluble] Solubility Rules for Some Ionic Compounds (Table 6-3) Malone and Dolter - Basic Concepts of Chemistry 9e

  47. What Happens if all Salts are Soluble? • Mixing aqueous solutions of CuCl2 and KNO3 results in a solution containing all four ions in solution Malone and Dolter - Basic Concepts of Chemistry 9e

  48. Analyzing a Possible Precipitation Reaction • When aqueous solutions of two salts are mixed, consider the solubility of the salts that would result if the anions and cations exchanged partners • For example, if a solution of Na2CO3 is mixed with a solution of CaCl2, you need to consider the solubility of CaCO3 and NaCl • If one of the new salts is insoluble, then a precipitate forms Malone and Dolter - Basic Concepts of Chemistry 9e

  49. Objective for Section 6-6 • Write balanced molecular, total ionic, and net ionic equations for neutralization reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  50. 6-6 Double-Replacement Reactions - Neutralization • Neutralization • In water, an acid is a proton donor • In water, a base is a hydroxide donor • Proton plus hydroxide yields water HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) Malone and Dolter - Basic Concepts of Chemistry 9e

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